G Model
ARTICLE IN PRESS
ACTROP-3297; No. of Pages 9
Acta Tropica xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Acta Tropica journal homepage: www.elsevier.com/locate/actatropica
An ecological study of Bithynia snails, the first intermediate host of Opisthorchis viverrini in northeast Thailand Yi-Chen Wang a,∗ , Richard Cheng Yong Ho a , Chen-Chieh Feng a , Jutamas Namsanor b , Paiboon Sithithaworn b a b
Department of Geography, National University of Singapore, 1 Arts Link, Singapore 117570, Singapore Department of Parasitology, Khon Kaen University, Khon Kaen 40002, Thailand
a r t i c l e
i n f o
Article history: Available online xxx Keywords: Liver fluke Disease ecology Opisthorchis viverrini Bithynia siamensis goniomphalos Habitat Thailand
a b s t r a c t Infection with the food-borne trematodiasis, liver fluke Opisthorchis viverrini, is a major public health concern in Southeast Asia. While epidemiology and parasitic incidence in humans are well studied, ecological information on the O. viverrini intermediate hosts remains limited. This study aimed to investigate the factors affecting the distribution and abundance of the first intermediate host, Bithynia siamensis goniomphalos snails. Water quality and snails were sampled in 31 sites in Muang District, Khon Kaen Province, Thailand from June 2012 to January 2013 to characterize the B.s. goniomphalos snail habitats. Species relative abundance and Shannon’s diversity and evenness indices were employed to describe snail compositions and diversities across different habitat types. Statistical analyses were conducted to examine the extent to which the water quality variables and species interactions account for the relative abundance of B.s. goniomphalos snails. The results showed that the freshwater habitats of ponds, streams and rice paddies possessed significantly different abiotic water qualities, with water temperature and pH showing distinct statistical differences (P < 0.05). Different habitats had different snail diversity and species evenness, with high B.s. goniomphalos snail abundance at rice paddy habitats. The differences in snail abundance might be due to the distinct sets of abiotic water qualities associated with each habitat types. The relative abundance of B.s. goniomphalos snails was found to be negatively correlated with that of Filopaludina martensi martensi snails (r = −0.46, P < 0.05), underscoring the possible influence of species interaction on B.s. goniomphalos snail population. Field work observations revealed that rice planting seasons and irrigation could regulate snail population dynamics at rice paddy habitats. This study provides new ecological insights into the factors affecting Bithynia snail distribution and abundance. It bridges the knowledge gap in O. viverrini disease ecology and highlights the potential effect of anthropogenic irrigation practices on B.s. goniomphalos snail ecology. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Food-borne parasitic infections are a major group of neglected diseases with more than 750 million people (>10% of the world’s population) at risk (Keiser and Utzinger, 2009; Sripa, 2012). The infections are still endemic in many parts of the world where poverty persists, especially in Southeast Asia. The most important of these parasites endemic in the Lower Mekong region is Opisthorchis viverrini, a liver fluke that has been identified by the International Agency for Research on Cancer as a cause of cholangiocarcinoma (CCA), a fatal bile duct cancer (IARC, 1994). The life
∗ Corresponding author. Tel.: +65 65166811; fax: +65 67773091. E-mail address: [email protected] (Y.-C. Wang).
cycle of O. viverrini is complex, with snails of the genus Bithynia as the first intermediate hosts, Cyprinid fish as the second intermediate hosts, and humans and fish-eating carnivores, such as cats and dogs, as definitive hosts (Wykoff et al., 1965). Infected fish are the direct route by which O. viverrini is transmitted to human, upon consumption. Human infection is common in the Lower Mekong region because consumption of raw, fermented or inadequately cooked fish dishes is a traditional part of the diet (Sithithaworn et al., 2012; Wang et al., 2013). Although the life cycle of O. viverrini has been known for many decades, the bulk of research to date has focused on collecting epidemiological data from humans. Very little information is available on the ecology of the O. viverrini intermediate hosts. Because the first intermediate snail hosts have highly variable infection rates compared to the second intermediate fish hosts, Bithynia snails are likely to be the key link in the O. viverrini
http://dx.doi.org/10.1016/j.actatropica.2014.02.009 0001-706X/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: Wang, Y.-C., et al., An ecological study of Bithynia snails, the first intermediate host of Opisthorchis viverrini in northeast Thailand. Acta Trop. (2014), http://dx.doi.org/10.1016/j.actatropica.2014.02.009
G Model ACTROP-3297; No. of Pages 9
ARTICLE IN PRESS Y.-C. Wang et al. / Acta Tropica xxx (2014) xxx–xxx
2
life cycle (Petney et al., 2012). This prompts the need for ecological investigations of the factors affecting Bithynia snail distribution and abundance. The distribution and abundance of species depend on both abiotic and biotic factors (MacDonald, 2002). Within freshwater environments, physical and chemical properties of water, such as temperature, oxygen, salinity and acidity, are key abiotic modulators that can affect organisms’ ability to survive (Dodds and Whiles, 2010; Chapin et al., 2011). For Bithynia snails, types of water bodies and water quality have been suggested as important determinants influencing snail distributions (Wang, 2012). Bithynia snails have been found in water environments with lower pH in northern Thailand (Ngern-klun et al., 2006). Conversely, salinity has been suggested to be the determinant for broad-scale distribution of Bithynia siamensis goniomphalos in northeast Thailand (Suwannatrai et al., 2011). While abiotic factors influence where a particular species is able to live, biotic factors such as intra-species and inter-species interactions for limited resources, often determine the species’ success, thereby affecting species abundance. Indeed, species diversity and interaction have been extensively studied for Lyme disease and other helminthic diseases (Ostfeld, 2009; Keesing et al., 2010). O. viverrini literature regarding snail diversity and the correlations between different snail species collected from various freshwater environments is, however, still limited, with the exception of Haruay et al. (2008), in which a negative correlation between the abundance of Filopaludina martensi martensi snails and that of B.s. goniomphalos snails was identified. Nevertheless, the work can be further improved through standardized protocols of snail collections for quantification of biodiversity. The objectives of this study are thus to examine the abiotic and biotic factors that affect the distribution and abundance of the first intermediate O. viverrini host, Bithynia snails. Specifically, this study addresses the following three research questions. First, do different freshwater habitats of Bithynia snails possess different water qualities? Second, do snail species compositions and diversities vary across different freshwater habitats, and do Bithynia snails dominate certain habitat type? Third, how do water quality and the presence of the snail species, F.m. martensi snails, affect Bithynia snail abundance? Understanding the characteristics of Bithynia snail habitats and the factors affecting Bithynia snail abundance will provide insights into O. viverrini disease ecology.
2. Materials and methods
ponds, streams and rice paddies. A total of 31 sampling sites in Muang District feasible for sampling were identified along the edges of the three habitat types, including 10 sites for ponds, 10 sites for streams and 11 sites for rice paddies. Of the 31 sampling sites, 18 were located within Phra Lap and Nai Muang Sub-Districts (hereafter, the Phra Lap region) and 13 were situated within Don Chang and Ban Wa Sub-Districts (hereafter, the Don Chang region) (Fig. 1). Fieldwork was conducted in June 2012, September 2012 and January 2013. Sampling in June was to capture the habitat conditions at the beginning of the rainy season. Sampling in September was to measure the habitat conditions of the wettest period of the rainy season, a time period that also corresponded to the peak in snail reproduction according to Brockelman et al. (1986). Sampling in January was to measure the habitat conditions of the drier and cooler season, a time period reported to have the highest snail densities (Lohachit, 2004–2005). Each site was visited during the three sampling periods. Some sites, however, particularly the rice paddies, were completely dried up due to seasonality and therefore could not be sampled. As such, across all sampling periods, instead of having 93 samples (31 identified sites for three sampling periods), a total of 80 samples were obtained. 2.2. Sample collection Sample collection at each site included water quality measurement and snail sampling. A hand-held Global Positioning System device was used to record the coordinates of the sites. 2.2.1. Water quality measurement Two YSI 556 Multiprobe System water quality meters were used to measure water quality variables, including temperature, salinity, electrical conductivity, total dissolved solids (TDS), dissolved oxygen (DO), and pH. These variables have been considered as possible abiotic factors influencing Bithynia snail distributions and measured in prior studies of other areas (i.e., Lohachit, 2004–2005; Ngern-klun et al., 2006). The meters were submerged into water for 10 min to allow the readings to stabilize before recording. In addition, because O. viverrini eggs from human and animal feces washed into the freshwater environments are the main source of snail infection, a 5-ml water sample was collected per site and poured into a Coliscan Easygel testing kit to measure the level of Escherichia coli (E. coli) contamination. The Colisan Easygel kits were kept on ice during the field work and then transported back to the laboratory for analysis.
2.1. Study area and sampling periods The study area is in Khon Kaen Province, northeast Thailand (Fig. 1) where high human O. viverrini prevalence and opisthorchiasis-associated CCA have been reported (Sriamporn et al., 2004; Sithithaworn et al., 2012). The province, located in the Chi River catchment, has a tropical monsoon climate with distinctive dry and wet seasons. The dry season occurs between November and March; the rainy season is from May to October, with the wetter period occurring between August and September. Average annual rainfall is 1379.1 mm. With approximately 80% of the rain experienced during the rainy season, extensive flooding is often encountered toward the end of the rainy season. Mean minimum and maximum temperatures are 16.6 ◦ C and 35.9 ◦ C, respectively (TMD, 2012). In Thailand, different taxa of Bithynia snails have their almost exclusive geographic distributions; the species found in northeast Thailand is Bithynia siamensis goniomphalos (Sithithaworn et al., 2007). Sampling sites were selected within the province in consideration of the potential Bithynia habitats and site accessibility. The study area consisted of three main Bithynia snail habitat types:
2.2.2. Snail sampling Various methods have been employed for snail sampling, including collection within a fixed time period (e.g., Suwannatrai et al., 2011), the Ekman dredge method (e.g., Haruay et al., 2008), and quadrat sampling (e.g., Lohachit, 2004–2005). To enable quantification and comparison of snail abundance and diversity across different sites and in consideration of the site conditions for sampling, a quadrat sampling (i.e., Lohachit, 2004–2005) was adapted for snail collection in this study. Along the shallow edge of the water body of each sampling site, a 0.5-m square quadrat measuring 0.25 m2 in area was placed four times at every meter interval along a transect to measure a total quadrat area of 1 m2 . All snail species found within the quadrat were collected for subsequent analyses. 2.3. Sample processing and analysis 2.3.1. Water samples for estimating fecal contamination The level of E. coli in the water of each sample site was derived according to manufacturer instructions of the Coliscan Easygel
Please cite this article in press as: Wang, Y.-C., et al., An ecological study of Bithynia snails, the first intermediate host of Opisthorchis viverrini in northeast Thailand. Acta Trop. (2014), http://dx.doi.org/10.1016/j.actatropica.2014.02.009
G Model ACTROP-3297; No. of Pages 9
ARTICLE IN PRESS Y.-C. Wang et al. / Acta Tropica xxx (2014) xxx–xxx
3
Fig. 1. Map of the study area, showing the Sub-Districts of field work, indicated by the gray polygons, in Khon Kaen Province, Thailand.
testing kit (Micrology Laboratories, n.d.). The sampled water with the Easygel kits were first poured into pretreated petri dishes. The dishes were then left in an incubator at 35 ◦ C for 24 h. After incubation, purple colonies were identified as E. coli colonies because of the interactions between the Easygel medium and the enzymes produced by E. coli. Finally, the level of E. coli was quantified as the colony forming units (CFUs) per 100 ml of water using the following formula given that 5 ml of water sample was taken at each site: (Identified E. coli colony counts/5) × 100
2.3.2. Snail samples for assessing species abundance and diversity The collected snails were cleaned and counted according to species. The following parameters and indices were used to describe species abundance and diversity (Spellerberg and Fedor, 2003; Tabbabi et al., 2011). First, relative abundance, defined as the proportion of individuals contributed by a single snail species, was calculated and expressed as a percentage to assess if Bithynia snails dominated certain freshwater habitats. Second, species richness, referring to the total number of species in a particular sampling site, was derived. Third, Shannon’s diversity index (H) was computed to evaluate snail diversity across different habitat types with the following formula: n
H=−
pi ln pi
i=1
where pi was the proportion of individuals in population belonging to the ith species (i.e., relative abundance of species i). A higher value indicated a large number of species with similar abundances,
whereas a lower value indicated low diversity that was dominated by one or a few species (Hill et al., 2005). Fourth, species evenness (E), describing the equality of the distribution of proportions across different species, was computed with the following formula: E = H/ln S where S was the total number of species (i.e., species richness) and H referred to Shannon’s diversity index. The values of E ranged from 0 to 1; values closer to zero represented uneven populations that were dominated by one species, while values closer to 1 represented even populations that comprised of several species with similar abundances (Hillebrand, 2008). In addition to the above analyses, the length and width sizes for all B.s. goniomphalos snails were measured using digital calipers to investigate if the snail sizes varied across different habitat types. Following Brockelman et al. (1986), B.s. goniomphalos snail length above 8 mm was considered as mature snails. The cercariae shedding procedures in Kiatsopit et al. (2012) were followed to determine the O. viverrini infection rate in B.s. goniomphalos snails, and if mature snails were more susceptible to infection. 2.3.3. Statistical analysis To examine the influence of abiotic and biotic factors on the distribution and abundance of B.s. goniomphalos snails, three sets of statistical analyses were conducted. The first set of analysis used basic descriptive statistics to summarize the water quality measurements of the three habitat types, i.e., ponds, streams and rice paddies. The second set of analysis included a series of statistical testing. Levene’s test was used to test the assumption of equal variance before the use of the Analysis of Variance (ANOVA)
Please cite this article in press as: Wang, Y.-C., et al., An ecological study of Bithynia snails, the first intermediate host of Opisthorchis viverrini in northeast Thailand. Acta Trop. (2014), http://dx.doi.org/10.1016/j.actatropica.2014.02.009
G Model
ARTICLE IN PRESS
ACTROP-3297; No. of Pages 9
Y.-C. Wang et al. / Acta Tropica xxx (2014) xxx–xxx
4
Table 1 Variations of water quality across different snail habitats in Khon Kaen Province, Thailand. The P value indicated whether the water quality variable was significantly different across the three habitat types. Variable
Ponds (n = 28) Mean
Temperature (◦ C) DO (mg/l)a Conductivity (mS/cm) TDS (g/l)a Salinity (ppt) pH E. coli (CFUs/100 ml) a b
29.22 5.05 1.49 0.89 0.70 7.43 117.9
Streams (n = 27) Range 23.08–34.22 1.97–8.67 0.27–8.47 0.14–4.99 0.12–4.22 6.23–8.65 0–660
Mean 28.99 4.09 0.65 0.40 0.30 7.29 222.2
Paddies (n = 25) Range 23.76–35.46 0.40–13.01 0.31–2.01 0.17–1.15 0.12–0.88 6.90–8.08 20–780
Mean 32.82 3.12 0.91 0.47 0.39 6.84 183.2
P value Range 28.02–41.14 0.05–8.18 0.24–2.47 0.13–1.30 0.09–1.01 6.20–7.55 0–640
0.1). The stepwise multiple regression analysis suggested that overall, water quality parameters of temperature, DO, salinity and pH were statistically significantly associated with the relative abundance of B.s. goniomphalos snails (P < 0.001, Table 4). Comparison of the stepwise multiple regression analyses for the three sampling periods signaled that the extent to which water quality parameters accounted for the variation in the B.s. goniomphalos snail abundance was generally low, indicated by the low R2 values (Table 4), but was the highest for June 2012 (R2 = 0.407), the beginning of the rainy season. Salinity appeared as an important factor contributing to the variation in the B.s. goniomphalos snail abundance in June when water level was still low for most of the sites, while for September 2012 the wetter period of the rainy season, the water quality parameters did not show a statistically significant relationship in predicting the B.s. goniomphalos snail abundance (P = 0.113). Alternatively, Pearson’s correlation coefficient showed a significant negative relationship between the relative abundance of B.s. goniomphalos snails and that of F.m. martensi snails (r = −0.46, P < 0.001) (Fig. 3). This negative correlation suggested inter-species competition between the two snail species, underscoring the possibility that B.s. goniomphalos snail populations could be affected by species competition. Table 4 The extent to which water quality variables accounted for the variations in B.s. goniomphalos snail abundance across the three sampling periods. Variable selected were based on the stepwise regression results with the minimum Akaike Information Criterion, which indicated the relative goodness of fit of the models.
Fig. 2. Comparison of relative abundances of B.s. goniomphalos snails across habitat types.
Sampling period
Variables selected
R2
P value
All three periods included June 2012 September 2012 January 2013
Temperature, DO, Salinity, pH Temperature, DO, Salinity Temperature, DO Temperature, pH
0.233 0.407 0.149 0.316
0.1). The differences in the findings could be due to two reasons. First, the sampling methods differed. In Suwannatrai et al. (2011), the 5 min timed searches and the Ekman dredge method were used, and only B.s. goniomphalos snails were collected. Conversely, this study employed the quadrat method to collect all the snails within the quadrat, providing a more standardized measurement for relative snail abundance. Second, the spatial scale of analysis varied. The sampling sites of Suwannatrai et al. (2011) spanned across 56 water bodies of the Khorat Basin in northeast Thailand, while the sampling sites of this study were all within a province. As such, the broad-scale analysis of Suwannatrai et al. (2011) included various geologic bedrocks and a wide range of soil surface salt levels, which could have revealed certain relationships that this ‘finer scale’ of analysis could not capture. Salinity is a crucial water quality measure for freshwater snail habitats because it affects the physiological functions of many snail species. Although B.s. goniomphalos snails are more tolerant of higher salinity levels than many other freshwater snails in northeast Thailand (Suwannatrai et al., 2011), caution should be exercised when examining the broad-scale relationship because of the potential species complex within both the Bithynia snail host and the O. viverrini parasite (Wang, 2012).
4. Discussion 4.2. Interactions between snail species 4.1. Water qualities of Bithynia snail habitats The differences in snail abundance (Fig. 2) might be due to the distinct sets of abiotic water qualities associated with each habitat types (Table 1). Compared with Lohachit (2004–2005) and Suwannatrai et al. (2011) that examined environmental factors of B.s. goniomphalos snail habitats, the water quality values measured in this study overlapped with the ranges observed previously, but overall, this study recorded greater water quality values than prior work (Tables 1 and 5). Furthermore, this study recorded the highest values of temperature, DO and pH respectively in the paddy, stream, and pond habitat in the Phra Lap region. The highest temperature of 41.14 ◦ C was recorded at a rice paddy habitat, which could be a combining effect of the shallow water environment of the paddy and the sampling time at approximately 12:30 pm in September 2012. The highest DO reading of 13.01 mg/l was measured in a stream habitat, probably as a result of water flow and mixing. The highest pH of 8.65 was collected from a pond habitat, of which the water was from a nearby water treatment plant of the Khon Kaen city. Lohachit (2004–2005) identified pH as a key limiting factor determining B.s. goniomphalos snail populations. The range and maximum values of pH recorded in this study were similar to those found in Lohachit (2004–2005) (Table 5). In addition, the results from this study identified a negative correlation between pH and the relative abundance of B.s. goniomphalos (r = −0.372, P = 0.001), echoing Lohachit’s findings for the dominance of B.s. goniomphalos snails at lower pH conditions. Likewise, Ngern-klun et al. (2006) found B. funiculata in areas with lower pH in northern Thailand. Indeed, pH indicates the underlying limnological conditions of the freshwater habitat, such as the rate of decomposition for plant materials, subsequently impacting snail fauna (Crowl and Schnell, 1990; Zalizniak et al., 2009). Nevertheless, pH values can have a high degree of variability, in particular with respect to rainfall and flooding events, which should be considered in the environmental context of northeast Thailand. This may explain why pH was included as the variables for the multiple linear regression model of the dry season January 2013 sampling to account for the relative abundance of B.s. goniomphalos when water was limited (Table 4).
The result of this study for the negative correlation between the relative abundance of B.s. goniomphalos and of F.m. martensi snails (r = −0.46, P < 0.001) supported the findings from Haruay et al. (2008) that B.s. goniomphalos snail populations could be influenced by species interactions with F.m. martensi. Where F.m. martensi were more abundant, lower B.s goniomphalos populations were observed (Fig. 3). With F.m. martensi being a larger-sized and stronger food competitor, inter-species competition between the two snail species can have a direct impact on the abundance and distribution of B.s. goniomphalos snails. This can have potential implications on disease management of O. viverrini transmission. It has been suggested that F.m. martensi snails can be used as a biological control agent for Bithynia snails to interrupt the disease transmission cycle (Haruay et al., 2008). The use of biological control agents has been successful in the case of Schistosoma mansoni in the Caribbean area through the introduction of a competitor snail species Melanoides tuberculata that reduced the population of the native intermediate snail host (Petney and Taraschewski, 2011). Nevertheless, using F.m. martensi snails as a biological control agent for Bithynia snails requires careful considerations as F.m. martensi snails have been found to be infected with the metacercariae of the intestinal fluke, Echinostoma spp., in north Thailand (Chantima et al., 2013), as well as in the sampling sites of this study (Wang, unpublished data). One of the important modes of transmission of echinostomiasis is eating raw freshwater snails. Although raw snail consumption is uncommon in the study area, human echinostomisasis is a public health concern in Southeast Asia (Chantima et al., 2013). More research must therefore be done with regards to the wide-spread species introduction because of the unknown impacts to the ecosystems and the potential consequences to public health. 4.3. Effects of land use practice on snail ecology Prior work has suggested that B.s. goniomphalos snails preferred slow flowing water environments and substrates composed of muddier and finer materials (Petney et al., 2012). These characteristics account for rice paddies being their preferred habitat
Please cite this article in press as: Wang, Y.-C., et al., An ecological study of Bithynia snails, the first intermediate host of Opisthorchis viverrini in northeast Thailand. Acta Trop. (2014), http://dx.doi.org/10.1016/j.actatropica.2014.02.009
G Model ACTROP-3297; No. of Pages 9
ARTICLE IN PRESS Y.-C. Wang et al. / Acta Tropica xxx (2014) xxx–xxx
7
Fig. 4. The dynamic environments of the rice paddy habitats, affected by human land use practice with regards to rice planting seasons. Rice paddy fields can be (A) dry, (B) irrigated, (C) fully grown, or (D) burnt. Photos (A) to (C) were taken at the same sampling site in different sampling months. Photo (D) was taken at a different sampling site.
choice, evident in the result of this study that B.s. goniomphalos snails were more dominant in the rice paddy habitats than the stream and the pond habitats (Fig. 2). This finding echoes the results of previous studies in which snails were observed in higher densities and greater dominance in rice paddies (Krueger et al., 2004; Suwannatrai et al., 2011). The finding also highlights potential influences of land use practice on B.s. goniomphalos snail ecology. The surrounding earthen banks of the rice paddies keep water in to increase the standing time for water, thereby providing favorable habitat conditions for the snails (Wang et al., 2011). Converting lowland forests into rice paddies in northeast Thailand has therefore created many suitable habitats for B.s. goniomphalos snails. The proliferation of the first intermediate host of O. viverrini can subsequently contribute to the persistence of O. viverrini transmission. On the other hand, rice paddies are dynamic environments that can be irrigated, fully grown, completely dry, or burnt across the rice planting seasons (Fig. 4). Farmers in northeast Thailand usually plant rice crops twice a year, one starting in January and the other in June (Pers comms January 2013). Following rice harvests, the most common practice to clear the land and eliminate rice straw for next cultivation is burning, especially in irrigated paddy fields (Kanokkanjana and Garivait, 2013). Dead B.s. goniomphalos snails have been found in the rice paddy sampling sites that experienced
open burning (Fig. 4D). The burning practice can impact B.s. goniomphalos snail population and subsequently affect O. viverrini transmission because dead snails cannot continue the O. viverrini life cycle. Open burning of rice fields, however, does not completely eliminate B.s. goniomphalos snails from the rice paddies because snails can burrow into the deep soils (Petney et al., 2012) or be introduced from other nearby habitats through irrigation (Wang et al., 2011). This is evident in some rice paddy sampling sites in this study that in spite of the absence of B.s. goniomphalos snails in June sampling (Table 3), probably as a result of burning or drying up of the paddy fields, mature B.s. goniomphalos snails were found three month later during the September sampling. Land use practice of irrigating rice paddies using stream or pond water during rice planting seasons can potentially account for introducing B.s. goniomphalos snails back to areas where they were absent. This underscores the possible effect of irrigation on snail ecology, particularly in irrigated areas during the dry seasons, because the presence of Bithynia snails and the increase of their population are not directly related to the natural hydrologic input, but regulated by anthropogenic irrigation practices during rice planting seasons. As illustrated in Lohachit (2004–2005), peak snail densities were observed during rice crop planting seasons that vary in time across the year.
Table 5 Comparison of the range of water quality characteristics from this study with prior work in northeast Thailand where live B.s. goniomphalos were found. Reference
Lohachit (2004–2005)
Suwannatrai et al. (2011)
This study
Study area in northeast Thailand
Villages near Ubolratana dam of Khon Kaen Province
Muang District of Khon Kaen Province
Sampling period Temperature (◦ C) DO (mg/l) Conductivity (mS/cm) Salinity (ppt) pH
June 1989–December 1990 18–33 2.0–10.0 0.07–0.65a N.A. 6.3–8.5
Khorat basin, consisting of 12 provinces, including Khon Kaen Province October 2006–August 2009 21.9–38.6 0.01–6.47 0.12–40.2 0.05–22.11 6.02–8.07
a
June 2012–January 2013 23.76–41.14 0.05–13.01 0.24–8.47 0.09–4.22 6.20–8.65
Values for conductivity were converted from micromhos/cm to allow comparison.
Please cite this article in press as: Wang, Y.-C., et al., An ecological study of Bithynia snails, the first intermediate host of Opisthorchis viverrini in northeast Thailand. Acta Trop. (2014), http://dx.doi.org/10.1016/j.actatropica.2014.02.009
G Model ACTROP-3297; No. of Pages 9
ARTICLE IN PRESS Y.-C. Wang et al. / Acta Tropica xxx (2014) xxx–xxx
8
4.4. Future work
5. Conclusions
This study demonstrated that the abiotic water quality factors and biotic species interactions contributed to the distribution and abundance of the first intermediate O. viverrini host, Bithynia snails. Most of the variables examined in this study were statistically significantly associated with the relative abundance of B.s. goniomphalos snails. However, the generally low correlation coefficients and R2 values of the multiple regression outcomes (Table 4) suggested that at least two aspects of research related to water quality deserve future considerations. First, other factors, such as the influence of water hardiness, require scrutiny. Snails are less frequently found in soft water because calcium is a key element that helps to build shells (Dillon, 2000; Glass and Darby, 2009). It is thus desirable to know if calcium is a possible limiting factor on snail abundance and diversity. Second, physical and chemical properties of water are dynamic; continuous efforts on long-term multi-season studies are therefore needed. Equipment designed to continuously log water quality data can be installed in the area of interest. The data can serve as a temporal control against which the readings recorded by the handheld meters can be compared because of the temporal variability of water quality. Comparison of the multiple regression results across the three sampling periods revealed that water quality accounted for the B.s. goniomphalos snail abundance in June and January, the beginning of the rainy and the dry seasons, respectively, but did not significantly contribute to the B.s. goniomphalos snail abundance in September, the wetter period of the rainy season (Table 4). The lack of relationship for the rainy season reflects the difficulty of linking the snails with the environments because high water level and flooding events during the rainy seasons may have facilitated the dispersal of snails. In addition to causing the dispersal of snails and affecting the water quality of the host habitats, floods can inundate rice paddies and human settlements nearby streams, forming one temporary large water body that spatially connects the O. viverrini host habitats. The ‘connected’ host habitats potentially allow for all stages of the O. viverrini life cycle to occur, facilitating disease transmission in areas that are previously not at risk (Wang et al., 2011). This is a cause of concern as fecal contamination after rainy seasons has been found to correlate with snail infection rates (Kaewkes et al., 2012). With rice paddies established as the habitat type where B.s. goniomphalos snails dominate (Fig. 2) and streams recorded the highest mean E. coli concentration among the three habitat types (Table 1), an understanding of how connectivity between different habitats facilitates O. viverrini transmission should be further investigated, especially in the context of flooding during the rainy season. Such an understanding can be useful for land use planning and management to minimize liver fluke persistence. This is especially so when rice paddies beside rivers are completely flooded, resulting in the potential concerns of different hosts to interact. Additionally, recent studies have found that open defecation is still a very common practice by villagers in the rural areas (Phongluxa et al., 2013), and unhygienic defecation occurs mostly while working in the rice paddies (Suwannahitatorn et al., 2013). Proper sanitation should thus also be enforced near rice paddies and the benefits of latrine use should be promoted to prevent the transmission of O. viverrini from humans to snail hosts. Due to the small number of O. viverrini infected B.s. goniomphalos snails found in this study, no conclusive results could be determined with regards to the association between fecal contamination and snail infection. As the O. viverrini prevalence has been suggested to be low in the first intermediate host (Sithithaworn et al., 2007), future research should collect a larger sample of B.s. goniomphalos snails to obtain adequate snail infection data for analyses with environmental variables.
This study examined the abiotic and biotic factors affecting the distribution and abundance of the first intermediate host, B.s. goniomphalos snails, of O. viverrini, a carcinogenic parasite that remains of public health concerns in Southeast Asia. The freshwater habitats of ponds, streams and rice paddies possessed significantly different abiotic water qualities that potentially influenced the distribution and abundance of snail populations. The stepwise multiple regression analyses suggested that overall, the abiotic water quality variables of temperature, DO, salinity and pH were statistically significantly associated with the relative abundance of B.s. goniomphalos snails, but the extent to which the water quality variables accounted for the variation in the B.s. goniomphalos snail abundance varied across the dry and rainy seasons. Different habitats had different snail diversity and species evenness, and B.s. goniomphalos snails were found to be more dominant in the rice paddy habitats than the pond and the stream habitats. The significant negative correlation between the relative abundance of B.s. goniomphalos snails and that of F.m. martensi snails emphasized the biotic influence of species interaction on B.s. goniomphalos snail population. Field work observations highlighted the possible effects of land use practice, in particular, rice cultivation and its associated irrigation activities, on B.s. goniomphalos snail ecology. These findings provided insights into the B.s. goniomphalos snail habitats and the factors affecting B.s. goniomphalos snail abundance, contributing to the understanding of O. viverrini disease ecology.
Acknowledgements The project is supported by the National University of Singapore through the Academic Research Fund (Grant #: R109-000-151112). The authors wish to thank Isaac Taoyang Low, Eugene Zong Xi Chum, Nonglak LaOprom, Opal Pitksakurat, Kultida Kopolrat, Nattaya Watwiengkam, Leon Yan Feng Gaw, and Nadda Kiatsopit for assistance in field work or lab analysis.
References Brockelman, W.Y., Upatham, E.S., Viyanant, V., Ardsungnoen, S., Chantanawat, R., 1986. Field studies on the transmission of the liver fluke, Opisthorchis viverrini, in northeast Thailand: population changes of the snail intermediate host. Int. J. Parasitol. 16, 545–552. Chantima, K., Chai, J.-Y., Wongsawad, C., 2013. Echinostoma revolutum: freshwater snails as the second intermediate hosts in Chiang Mai, Thailand. Korean J. Parasitol. 51, 183–189. Chapin III, F.S., Matson, P.A., Vitousek, P.M., 2011. Principles of Terrestrial Ecology, second ed. Springer, New York. Crowl, T.A., Schnell, G.D., 1990. Factors determining population density and size distributions of freshwater snail in streams: effects of spatial scale. Oikios 59, 359–367. Dillon, R.T., 2000. The Ecology of Freshwater Molluscs. Cambridge University Press, Cambridge. Dodds, W.K., Whiles, M.R., 2010. Freshwater Ecology: Concepts and Environmental Applications of Limnology, second ed. Academic Press, Burlington, MA. Glass, N.H., Darby, P.C., 2009. The effect of calcium and pH on Florida apple snail, Pomacea paludosa (Gastropoda: Ampullariidae), shell growth and crush weight. Aquat. Ecol. 43, 1085–1093. Haruay, S., Tesana, S., Suwannatrai, A., Piratae, S., Kampoosa, P., Thammasiri, C., Duangprompo, W., Kaewkes, S., Tarbsripair, P., Paiboonsak, S., 2008. Effects of surface salt on distribution and density of snails in Family Viviparidae in Khon Kaen province, Thailand, analyses by using geographic information system. KKU Res. J. 8, 40–50. Hill, D., Fasham, M., Tucker, G., Shewry, M., Shaw, P., 2005. Handbook of Biodiversity Methods: Survey, Evaluation and Monitoring. Cambridge University Press, Cambridge. Hillebrand, H., 2008. Dominance. In: Jorgensen, S.E., Fath, B. (Eds.), Encyclopedia of Ecology. Elsevier, Oxford, pp. 938–944. International Agency for Research on Cancer (IARC), 1994. Schistosomes, Liver Flukes and Helicobacter pylori. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, vol. 6. International Agency for Research on Cancer, Lyon, France.
Please cite this article in press as: Wang, Y.-C., et al., An ecological study of Bithynia snails, the first intermediate host of Opisthorchis viverrini in northeast Thailand. Acta Trop. (2014), http://dx.doi.org/10.1016/j.actatropica.2014.02.009
G Model ACTROP-3297; No. of Pages 9
ARTICLE IN PRESS Y.-C. Wang et al. / Acta Tropica xxx (2014) xxx–xxx
Kaewkes, W., Kawkes, S., Tesana, S., Laha, T., Sripa, B., 2012. Fecal bacterial contamination in natural water reservoirs as an indicator of seasonal infection by Opisthorchis viverrini in snail intermediate hosts. Parasitol. Int. 61, 49–51. Kanokkanjana, K., Garivait, S., 2013. Alternative rice straw management practices to reduce field open burning in Thailand. Int. J. Environ. Sci. Dev. 4, 119–123. Keesing, F., Belden, L.K., Daszak, P., Dobson, A., Harvell, C.D., Holt, R.D., Hudson, P., Jolles, A., Jones, K.E., Mitchell, C.E., Myers, S.S., Bogich, T., Ostfeld, R.S., 2010. Impacts of biodiversity on the emergence and transmission of infectious diseases. Nature 468, 647–652. Keiser, J., Utzinger, J., 2009. Food-borne trematodiases. Clin. Microbiol. Rev. 22, 466–483. Kiatsopit, N., Sithithaworn, P., Saijunya, W., Boonmars, T., Tesana, S., Sithithaworn, J., Petney, T.N., Andrews, R.H., 2012. Exceptionally high prevalence of infection of Bithynia siamenesis goniomphalos with Opisthorchis viverrini cercariae in different wetlands in Thailand and Lao PDR. Am. J. Trop. Med. Hyg. 86, 464–469. Krueger, A.M., Irvine, K.N., Prangkio, C., Chaokasad, K., Sukontason, K., Sukontason, K.L., Ngern-klun, R., 2004. Visualizing water quality trends in Chiang Mai rice paddies: possible links between environment and health risks. Middle States Geographer 37, 1–8. Lohachit, C., 2004–2005. Ecological studies of Bithynia siamensis goniomphalos, a snail intermediate host of Opisthorchis viverrini, in Khon Kaen province, northeast Thailand. Malacol. Rev. 37/38, 1–26. MacDonald, G.M., 2002. Biogeography: Space. Time and Life. John Wiley and Sons, New York. Ngern-klun, R., Sukotason, K., Tesana, S., Sripakdee, D., Irvine, K.N., Sukontason, K., 2006. Field investigation of Bithynia funiculata intermediate host of Opisthorchis viverrini in northern Thailand. Southeast Asian J. Trop. Med. Public Health 37, 662–672. Ostfeld, R.S., 2009. Biodiversity loss and the rise of zoonotic pathogens. Clin. Microbiol. Infect. 15 (S1), 40–43. Petney, T.N., Sithithaworn, P., Andrews, R.H., Kiatsopit, N., Tesana, S., Grundy-Warr, C., Ziegler, A.D., 2012. The ecology of the Bithynia first intermediate hosts of Opisthorchis viverrini. Parasitol. Int. 61, 38–45. Petney, T.N., Taraschewski, H., 2011. Water-borne parasitic diseases: hydrology, regional development and control. In: Frimmel, F.H. (Ed.), Water Chemistry and Microbiology, vol. 3 of A Treatise on Water Science. Elsevier, Amsterdam, pp. 303–366. Phongluxa, K., Xayaseng, V., Vonghachack, Y., Akkhavong, K., van Eeuwijk, P., Odermatt, P., 2013. Helminth infection in southern Laos: high prevalence and low awareness. Parasit. Vectors 6, 328.
9
Sithithaworn, P., Andrews, R.H., Nguyen, V.D., Wongsaroj, T., Sinuon, M., Odermatt, P., Nawa, Y., Liang, S., Brindley, P.J., Sripa, B., 2012. The current status of opisthorchiasis and clonorchiasis in the Mekong Basin. Parasitol. Int. 61, 10–16. Sithithaworn, P., Yongvanit, P., Tesana, S., Pairojkul, C., 2007. Liver flukes. In: Darwin, M.K., Bernard, F. (Eds.), Food-borne Parasitic Zoonoses. Springer, Berlin, pp. 3–52. Spellerberg, I.F., Fedor, P.J., 2003. A tribute to Claude Shannon (1916–2001) and a plea for more rigorous use of species diversity and the “Shannon–Wiener” Index. Global Ecol. Biogeogr. 12, 177–183. Sriamporn, S., Pisani, P., Pipitgool, V., Suwanrungruang, K., Kamsa-ard, S., Parkin, D.M., 2004. Prevalence of Opisthorchis viverrini infection and incidence of cholangiocarcinoma in Khon Kaen, northeast Thailand. Trop. Med. Int. Health 9, 588–594. Sripa, B., 2012. Global burden of food-borne trematodiasis. Lancet Infect. Dis. 12, 171–172. Suwannahitatorn, P., Klomjit, S., Naaglor, T., Taamasri, P., Rangsin, R., Leelayoova, S., Mungthin, M., 2013. A follow-up study of Opisthorchis viverrini infection after the implementation of control program in a rural community, central Thailand. Parasit. Vectors 6, 188. Suwannatrai, A., Suwannatrai, K., Haruay, S., Piratae, S., Thammasiri, C., Khampoosa, P., Kulsantiwong, J., Prasopdee, S., Tarbsripair, P., Suwanwerakamtorn, R., Sukchan, S., Boonmars, T., Malone, J.B., Kearney, M.T., Tesana, S., 2011. Effect of soil surface salt on the density and distribution of the snail Bithynia siamensis goniomphalos in northeast Thailand. Geospatial Health 5, 183–190. Tabbabi, A., Ghrab, J., Aoun, K., Ready, P.D., Bouratbine, A., 2011. Habitats of the sandfly vectors of Leishmania tropica and L. major in a mixed focus of cutaneous leishmaniasis in southeast Tunisia. Acta Trop. 119, 131–137. Thai Meteorological Department (TMD), 2012. The Climate of Thailand, available at: http://www.tmd.go.th/en/archive/thailand climate.pdf (accessed February 2013). Wang, Y.-C., 2012. Examining landscape determinants of Opisthorchis viverrini transmission. Ecohealth 9, 328–341. Wang, Y.-C., Feng, C.-C., Sithithaworn, P., Feng, Y., Petney, T.N., 2011. How do snails meet fish? Landscape perspective needed to study parasite prevalence. Ecohealth 8, 258–260. Wang, Y.-C., Feng, C.-C., Sithithaworn, P., 2013. Environmental determinants of Opisthorchis viverrini prevalence in northeast Thailand. Geospatial Health 8 (1), 111–123. Wykoff, D.E., Harinasuta, C., Juttijudata, P., Winn, M.M., 1965. Opisthorchis viverrini in Thailand: the life cycle and comparison with O. felineus. J. Parasitol. 51, 207–214. Zalizniak, L., Kefford, B.J., Nugegoda, D., 2009. Effects of pH on salinity tolerance of selected freshwater invertebrates. Aquat. Ecol. 43, 135–144.
Please cite this article in press as: Wang, Y.-C., et al., An ecological study of Bithynia snails, the first intermediate host of Opisthorchis viverrini in northeast Thailand. Acta Trop. (2014), http://dx.doi.org/10.1016/j.actatropica.2014.02.009
ARTICLE IN PRESS
ACTROP-3297; No. of Pages 9
Acta Tropica xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Acta Tropica journal homepage: www.elsevier.com/locate/actatropica
An ecological study of Bithynia snails, the first intermediate host of Opisthorchis viverrini in northeast Thailand Yi-Chen Wang a,∗ , Richard Cheng Yong Ho a , Chen-Chieh Feng a , Jutamas Namsanor b , Paiboon Sithithaworn b a b
Department of Geography, National University of Singapore, 1 Arts Link, Singapore 117570, Singapore Department of Parasitology, Khon Kaen University, Khon Kaen 40002, Thailand
a r t i c l e
i n f o
Article history: Available online xxx Keywords: Liver fluke Disease ecology Opisthorchis viverrini Bithynia siamensis goniomphalos Habitat Thailand
a b s t r a c t Infection with the food-borne trematodiasis, liver fluke Opisthorchis viverrini, is a major public health concern in Southeast Asia. While epidemiology and parasitic incidence in humans are well studied, ecological information on the O. viverrini intermediate hosts remains limited. This study aimed to investigate the factors affecting the distribution and abundance of the first intermediate host, Bithynia siamensis goniomphalos snails. Water quality and snails were sampled in 31 sites in Muang District, Khon Kaen Province, Thailand from June 2012 to January 2013 to characterize the B.s. goniomphalos snail habitats. Species relative abundance and Shannon’s diversity and evenness indices were employed to describe snail compositions and diversities across different habitat types. Statistical analyses were conducted to examine the extent to which the water quality variables and species interactions account for the relative abundance of B.s. goniomphalos snails. The results showed that the freshwater habitats of ponds, streams and rice paddies possessed significantly different abiotic water qualities, with water temperature and pH showing distinct statistical differences (P < 0.05). Different habitats had different snail diversity and species evenness, with high B.s. goniomphalos snail abundance at rice paddy habitats. The differences in snail abundance might be due to the distinct sets of abiotic water qualities associated with each habitat types. The relative abundance of B.s. goniomphalos snails was found to be negatively correlated with that of Filopaludina martensi martensi snails (r = −0.46, P < 0.05), underscoring the possible influence of species interaction on B.s. goniomphalos snail population. Field work observations revealed that rice planting seasons and irrigation could regulate snail population dynamics at rice paddy habitats. This study provides new ecological insights into the factors affecting Bithynia snail distribution and abundance. It bridges the knowledge gap in O. viverrini disease ecology and highlights the potential effect of anthropogenic irrigation practices on B.s. goniomphalos snail ecology. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Food-borne parasitic infections are a major group of neglected diseases with more than 750 million people (>10% of the world’s population) at risk (Keiser and Utzinger, 2009; Sripa, 2012). The infections are still endemic in many parts of the world where poverty persists, especially in Southeast Asia. The most important of these parasites endemic in the Lower Mekong region is Opisthorchis viverrini, a liver fluke that has been identified by the International Agency for Research on Cancer as a cause of cholangiocarcinoma (CCA), a fatal bile duct cancer (IARC, 1994). The life
∗ Corresponding author. Tel.: +65 65166811; fax: +65 67773091. E-mail address: [email protected] (Y.-C. Wang).
cycle of O. viverrini is complex, with snails of the genus Bithynia as the first intermediate hosts, Cyprinid fish as the second intermediate hosts, and humans and fish-eating carnivores, such as cats and dogs, as definitive hosts (Wykoff et al., 1965). Infected fish are the direct route by which O. viverrini is transmitted to human, upon consumption. Human infection is common in the Lower Mekong region because consumption of raw, fermented or inadequately cooked fish dishes is a traditional part of the diet (Sithithaworn et al., 2012; Wang et al., 2013). Although the life cycle of O. viverrini has been known for many decades, the bulk of research to date has focused on collecting epidemiological data from humans. Very little information is available on the ecology of the O. viverrini intermediate hosts. Because the first intermediate snail hosts have highly variable infection rates compared to the second intermediate fish hosts, Bithynia snails are likely to be the key link in the O. viverrini
http://dx.doi.org/10.1016/j.actatropica.2014.02.009 0001-706X/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: Wang, Y.-C., et al., An ecological study of Bithynia snails, the first intermediate host of Opisthorchis viverrini in northeast Thailand. Acta Trop. (2014), http://dx.doi.org/10.1016/j.actatropica.2014.02.009
G Model ACTROP-3297; No. of Pages 9
ARTICLE IN PRESS Y.-C. Wang et al. / Acta Tropica xxx (2014) xxx–xxx
2
life cycle (Petney et al., 2012). This prompts the need for ecological investigations of the factors affecting Bithynia snail distribution and abundance. The distribution and abundance of species depend on both abiotic and biotic factors (MacDonald, 2002). Within freshwater environments, physical and chemical properties of water, such as temperature, oxygen, salinity and acidity, are key abiotic modulators that can affect organisms’ ability to survive (Dodds and Whiles, 2010; Chapin et al., 2011). For Bithynia snails, types of water bodies and water quality have been suggested as important determinants influencing snail distributions (Wang, 2012). Bithynia snails have been found in water environments with lower pH in northern Thailand (Ngern-klun et al., 2006). Conversely, salinity has been suggested to be the determinant for broad-scale distribution of Bithynia siamensis goniomphalos in northeast Thailand (Suwannatrai et al., 2011). While abiotic factors influence where a particular species is able to live, biotic factors such as intra-species and inter-species interactions for limited resources, often determine the species’ success, thereby affecting species abundance. Indeed, species diversity and interaction have been extensively studied for Lyme disease and other helminthic diseases (Ostfeld, 2009; Keesing et al., 2010). O. viverrini literature regarding snail diversity and the correlations between different snail species collected from various freshwater environments is, however, still limited, with the exception of Haruay et al. (2008), in which a negative correlation between the abundance of Filopaludina martensi martensi snails and that of B.s. goniomphalos snails was identified. Nevertheless, the work can be further improved through standardized protocols of snail collections for quantification of biodiversity. The objectives of this study are thus to examine the abiotic and biotic factors that affect the distribution and abundance of the first intermediate O. viverrini host, Bithynia snails. Specifically, this study addresses the following three research questions. First, do different freshwater habitats of Bithynia snails possess different water qualities? Second, do snail species compositions and diversities vary across different freshwater habitats, and do Bithynia snails dominate certain habitat type? Third, how do water quality and the presence of the snail species, F.m. martensi snails, affect Bithynia snail abundance? Understanding the characteristics of Bithynia snail habitats and the factors affecting Bithynia snail abundance will provide insights into O. viverrini disease ecology.
2. Materials and methods
ponds, streams and rice paddies. A total of 31 sampling sites in Muang District feasible for sampling were identified along the edges of the three habitat types, including 10 sites for ponds, 10 sites for streams and 11 sites for rice paddies. Of the 31 sampling sites, 18 were located within Phra Lap and Nai Muang Sub-Districts (hereafter, the Phra Lap region) and 13 were situated within Don Chang and Ban Wa Sub-Districts (hereafter, the Don Chang region) (Fig. 1). Fieldwork was conducted in June 2012, September 2012 and January 2013. Sampling in June was to capture the habitat conditions at the beginning of the rainy season. Sampling in September was to measure the habitat conditions of the wettest period of the rainy season, a time period that also corresponded to the peak in snail reproduction according to Brockelman et al. (1986). Sampling in January was to measure the habitat conditions of the drier and cooler season, a time period reported to have the highest snail densities (Lohachit, 2004–2005). Each site was visited during the three sampling periods. Some sites, however, particularly the rice paddies, were completely dried up due to seasonality and therefore could not be sampled. As such, across all sampling periods, instead of having 93 samples (31 identified sites for three sampling periods), a total of 80 samples were obtained. 2.2. Sample collection Sample collection at each site included water quality measurement and snail sampling. A hand-held Global Positioning System device was used to record the coordinates of the sites. 2.2.1. Water quality measurement Two YSI 556 Multiprobe System water quality meters were used to measure water quality variables, including temperature, salinity, electrical conductivity, total dissolved solids (TDS), dissolved oxygen (DO), and pH. These variables have been considered as possible abiotic factors influencing Bithynia snail distributions and measured in prior studies of other areas (i.e., Lohachit, 2004–2005; Ngern-klun et al., 2006). The meters were submerged into water for 10 min to allow the readings to stabilize before recording. In addition, because O. viverrini eggs from human and animal feces washed into the freshwater environments are the main source of snail infection, a 5-ml water sample was collected per site and poured into a Coliscan Easygel testing kit to measure the level of Escherichia coli (E. coli) contamination. The Colisan Easygel kits were kept on ice during the field work and then transported back to the laboratory for analysis.
2.1. Study area and sampling periods The study area is in Khon Kaen Province, northeast Thailand (Fig. 1) where high human O. viverrini prevalence and opisthorchiasis-associated CCA have been reported (Sriamporn et al., 2004; Sithithaworn et al., 2012). The province, located in the Chi River catchment, has a tropical monsoon climate with distinctive dry and wet seasons. The dry season occurs between November and March; the rainy season is from May to October, with the wetter period occurring between August and September. Average annual rainfall is 1379.1 mm. With approximately 80% of the rain experienced during the rainy season, extensive flooding is often encountered toward the end of the rainy season. Mean minimum and maximum temperatures are 16.6 ◦ C and 35.9 ◦ C, respectively (TMD, 2012). In Thailand, different taxa of Bithynia snails have their almost exclusive geographic distributions; the species found in northeast Thailand is Bithynia siamensis goniomphalos (Sithithaworn et al., 2007). Sampling sites were selected within the province in consideration of the potential Bithynia habitats and site accessibility. The study area consisted of three main Bithynia snail habitat types:
2.2.2. Snail sampling Various methods have been employed for snail sampling, including collection within a fixed time period (e.g., Suwannatrai et al., 2011), the Ekman dredge method (e.g., Haruay et al., 2008), and quadrat sampling (e.g., Lohachit, 2004–2005). To enable quantification and comparison of snail abundance and diversity across different sites and in consideration of the site conditions for sampling, a quadrat sampling (i.e., Lohachit, 2004–2005) was adapted for snail collection in this study. Along the shallow edge of the water body of each sampling site, a 0.5-m square quadrat measuring 0.25 m2 in area was placed four times at every meter interval along a transect to measure a total quadrat area of 1 m2 . All snail species found within the quadrat were collected for subsequent analyses. 2.3. Sample processing and analysis 2.3.1. Water samples for estimating fecal contamination The level of E. coli in the water of each sample site was derived according to manufacturer instructions of the Coliscan Easygel
Please cite this article in press as: Wang, Y.-C., et al., An ecological study of Bithynia snails, the first intermediate host of Opisthorchis viverrini in northeast Thailand. Acta Trop. (2014), http://dx.doi.org/10.1016/j.actatropica.2014.02.009
G Model ACTROP-3297; No. of Pages 9
ARTICLE IN PRESS Y.-C. Wang et al. / Acta Tropica xxx (2014) xxx–xxx
3
Fig. 1. Map of the study area, showing the Sub-Districts of field work, indicated by the gray polygons, in Khon Kaen Province, Thailand.
testing kit (Micrology Laboratories, n.d.). The sampled water with the Easygel kits were first poured into pretreated petri dishes. The dishes were then left in an incubator at 35 ◦ C for 24 h. After incubation, purple colonies were identified as E. coli colonies because of the interactions between the Easygel medium and the enzymes produced by E. coli. Finally, the level of E. coli was quantified as the colony forming units (CFUs) per 100 ml of water using the following formula given that 5 ml of water sample was taken at each site: (Identified E. coli colony counts/5) × 100
2.3.2. Snail samples for assessing species abundance and diversity The collected snails were cleaned and counted according to species. The following parameters and indices were used to describe species abundance and diversity (Spellerberg and Fedor, 2003; Tabbabi et al., 2011). First, relative abundance, defined as the proportion of individuals contributed by a single snail species, was calculated and expressed as a percentage to assess if Bithynia snails dominated certain freshwater habitats. Second, species richness, referring to the total number of species in a particular sampling site, was derived. Third, Shannon’s diversity index (H) was computed to evaluate snail diversity across different habitat types with the following formula: n
H=−
pi ln pi
i=1
where pi was the proportion of individuals in population belonging to the ith species (i.e., relative abundance of species i). A higher value indicated a large number of species with similar abundances,
whereas a lower value indicated low diversity that was dominated by one or a few species (Hill et al., 2005). Fourth, species evenness (E), describing the equality of the distribution of proportions across different species, was computed with the following formula: E = H/ln S where S was the total number of species (i.e., species richness) and H referred to Shannon’s diversity index. The values of E ranged from 0 to 1; values closer to zero represented uneven populations that were dominated by one species, while values closer to 1 represented even populations that comprised of several species with similar abundances (Hillebrand, 2008). In addition to the above analyses, the length and width sizes for all B.s. goniomphalos snails were measured using digital calipers to investigate if the snail sizes varied across different habitat types. Following Brockelman et al. (1986), B.s. goniomphalos snail length above 8 mm was considered as mature snails. The cercariae shedding procedures in Kiatsopit et al. (2012) were followed to determine the O. viverrini infection rate in B.s. goniomphalos snails, and if mature snails were more susceptible to infection. 2.3.3. Statistical analysis To examine the influence of abiotic and biotic factors on the distribution and abundance of B.s. goniomphalos snails, three sets of statistical analyses were conducted. The first set of analysis used basic descriptive statistics to summarize the water quality measurements of the three habitat types, i.e., ponds, streams and rice paddies. The second set of analysis included a series of statistical testing. Levene’s test was used to test the assumption of equal variance before the use of the Analysis of Variance (ANOVA)
Please cite this article in press as: Wang, Y.-C., et al., An ecological study of Bithynia snails, the first intermediate host of Opisthorchis viverrini in northeast Thailand. Acta Trop. (2014), http://dx.doi.org/10.1016/j.actatropica.2014.02.009
G Model
ARTICLE IN PRESS
ACTROP-3297; No. of Pages 9
Y.-C. Wang et al. / Acta Tropica xxx (2014) xxx–xxx
4
Table 1 Variations of water quality across different snail habitats in Khon Kaen Province, Thailand. The P value indicated whether the water quality variable was significantly different across the three habitat types. Variable
Ponds (n = 28) Mean
Temperature (◦ C) DO (mg/l)a Conductivity (mS/cm) TDS (g/l)a Salinity (ppt) pH E. coli (CFUs/100 ml) a b
29.22 5.05 1.49 0.89 0.70 7.43 117.9
Streams (n = 27) Range 23.08–34.22 1.97–8.67 0.27–8.47 0.14–4.99 0.12–4.22 6.23–8.65 0–660
Mean 28.99 4.09 0.65 0.40 0.30 7.29 222.2
Paddies (n = 25) Range 23.76–35.46 0.40–13.01 0.31–2.01 0.17–1.15 0.12–0.88 6.90–8.08 20–780
Mean 32.82 3.12 0.91 0.47 0.39 6.84 183.2
P value Range 28.02–41.14 0.05–8.18 0.24–2.47 0.13–1.30 0.09–1.01 6.20–7.55 0–640
0.1). The stepwise multiple regression analysis suggested that overall, water quality parameters of temperature, DO, salinity and pH were statistically significantly associated with the relative abundance of B.s. goniomphalos snails (P < 0.001, Table 4). Comparison of the stepwise multiple regression analyses for the three sampling periods signaled that the extent to which water quality parameters accounted for the variation in the B.s. goniomphalos snail abundance was generally low, indicated by the low R2 values (Table 4), but was the highest for June 2012 (R2 = 0.407), the beginning of the rainy season. Salinity appeared as an important factor contributing to the variation in the B.s. goniomphalos snail abundance in June when water level was still low for most of the sites, while for September 2012 the wetter period of the rainy season, the water quality parameters did not show a statistically significant relationship in predicting the B.s. goniomphalos snail abundance (P = 0.113). Alternatively, Pearson’s correlation coefficient showed a significant negative relationship between the relative abundance of B.s. goniomphalos snails and that of F.m. martensi snails (r = −0.46, P < 0.001) (Fig. 3). This negative correlation suggested inter-species competition between the two snail species, underscoring the possibility that B.s. goniomphalos snail populations could be affected by species competition. Table 4 The extent to which water quality variables accounted for the variations in B.s. goniomphalos snail abundance across the three sampling periods. Variable selected were based on the stepwise regression results with the minimum Akaike Information Criterion, which indicated the relative goodness of fit of the models.
Fig. 2. Comparison of relative abundances of B.s. goniomphalos snails across habitat types.
Sampling period
Variables selected
R2
P value
All three periods included June 2012 September 2012 January 2013
Temperature, DO, Salinity, pH Temperature, DO, Salinity Temperature, DO Temperature, pH
0.233 0.407 0.149 0.316
0.1). The differences in the findings could be due to two reasons. First, the sampling methods differed. In Suwannatrai et al. (2011), the 5 min timed searches and the Ekman dredge method were used, and only B.s. goniomphalos snails were collected. Conversely, this study employed the quadrat method to collect all the snails within the quadrat, providing a more standardized measurement for relative snail abundance. Second, the spatial scale of analysis varied. The sampling sites of Suwannatrai et al. (2011) spanned across 56 water bodies of the Khorat Basin in northeast Thailand, while the sampling sites of this study were all within a province. As such, the broad-scale analysis of Suwannatrai et al. (2011) included various geologic bedrocks and a wide range of soil surface salt levels, which could have revealed certain relationships that this ‘finer scale’ of analysis could not capture. Salinity is a crucial water quality measure for freshwater snail habitats because it affects the physiological functions of many snail species. Although B.s. goniomphalos snails are more tolerant of higher salinity levels than many other freshwater snails in northeast Thailand (Suwannatrai et al., 2011), caution should be exercised when examining the broad-scale relationship because of the potential species complex within both the Bithynia snail host and the O. viverrini parasite (Wang, 2012).
4. Discussion 4.2. Interactions between snail species 4.1. Water qualities of Bithynia snail habitats The differences in snail abundance (Fig. 2) might be due to the distinct sets of abiotic water qualities associated with each habitat types (Table 1). Compared with Lohachit (2004–2005) and Suwannatrai et al. (2011) that examined environmental factors of B.s. goniomphalos snail habitats, the water quality values measured in this study overlapped with the ranges observed previously, but overall, this study recorded greater water quality values than prior work (Tables 1 and 5). Furthermore, this study recorded the highest values of temperature, DO and pH respectively in the paddy, stream, and pond habitat in the Phra Lap region. The highest temperature of 41.14 ◦ C was recorded at a rice paddy habitat, which could be a combining effect of the shallow water environment of the paddy and the sampling time at approximately 12:30 pm in September 2012. The highest DO reading of 13.01 mg/l was measured in a stream habitat, probably as a result of water flow and mixing. The highest pH of 8.65 was collected from a pond habitat, of which the water was from a nearby water treatment plant of the Khon Kaen city. Lohachit (2004–2005) identified pH as a key limiting factor determining B.s. goniomphalos snail populations. The range and maximum values of pH recorded in this study were similar to those found in Lohachit (2004–2005) (Table 5). In addition, the results from this study identified a negative correlation between pH and the relative abundance of B.s. goniomphalos (r = −0.372, P = 0.001), echoing Lohachit’s findings for the dominance of B.s. goniomphalos snails at lower pH conditions. Likewise, Ngern-klun et al. (2006) found B. funiculata in areas with lower pH in northern Thailand. Indeed, pH indicates the underlying limnological conditions of the freshwater habitat, such as the rate of decomposition for plant materials, subsequently impacting snail fauna (Crowl and Schnell, 1990; Zalizniak et al., 2009). Nevertheless, pH values can have a high degree of variability, in particular with respect to rainfall and flooding events, which should be considered in the environmental context of northeast Thailand. This may explain why pH was included as the variables for the multiple linear regression model of the dry season January 2013 sampling to account for the relative abundance of B.s. goniomphalos when water was limited (Table 4).
The result of this study for the negative correlation between the relative abundance of B.s. goniomphalos and of F.m. martensi snails (r = −0.46, P < 0.001) supported the findings from Haruay et al. (2008) that B.s. goniomphalos snail populations could be influenced by species interactions with F.m. martensi. Where F.m. martensi were more abundant, lower B.s goniomphalos populations were observed (Fig. 3). With F.m. martensi being a larger-sized and stronger food competitor, inter-species competition between the two snail species can have a direct impact on the abundance and distribution of B.s. goniomphalos snails. This can have potential implications on disease management of O. viverrini transmission. It has been suggested that F.m. martensi snails can be used as a biological control agent for Bithynia snails to interrupt the disease transmission cycle (Haruay et al., 2008). The use of biological control agents has been successful in the case of Schistosoma mansoni in the Caribbean area through the introduction of a competitor snail species Melanoides tuberculata that reduced the population of the native intermediate snail host (Petney and Taraschewski, 2011). Nevertheless, using F.m. martensi snails as a biological control agent for Bithynia snails requires careful considerations as F.m. martensi snails have been found to be infected with the metacercariae of the intestinal fluke, Echinostoma spp., in north Thailand (Chantima et al., 2013), as well as in the sampling sites of this study (Wang, unpublished data). One of the important modes of transmission of echinostomiasis is eating raw freshwater snails. Although raw snail consumption is uncommon in the study area, human echinostomisasis is a public health concern in Southeast Asia (Chantima et al., 2013). More research must therefore be done with regards to the wide-spread species introduction because of the unknown impacts to the ecosystems and the potential consequences to public health. 4.3. Effects of land use practice on snail ecology Prior work has suggested that B.s. goniomphalos snails preferred slow flowing water environments and substrates composed of muddier and finer materials (Petney et al., 2012). These characteristics account for rice paddies being their preferred habitat
Please cite this article in press as: Wang, Y.-C., et al., An ecological study of Bithynia snails, the first intermediate host of Opisthorchis viverrini in northeast Thailand. Acta Trop. (2014), http://dx.doi.org/10.1016/j.actatropica.2014.02.009
G Model ACTROP-3297; No. of Pages 9
ARTICLE IN PRESS Y.-C. Wang et al. / Acta Tropica xxx (2014) xxx–xxx
7
Fig. 4. The dynamic environments of the rice paddy habitats, affected by human land use practice with regards to rice planting seasons. Rice paddy fields can be (A) dry, (B) irrigated, (C) fully grown, or (D) burnt. Photos (A) to (C) were taken at the same sampling site in different sampling months. Photo (D) was taken at a different sampling site.
choice, evident in the result of this study that B.s. goniomphalos snails were more dominant in the rice paddy habitats than the stream and the pond habitats (Fig. 2). This finding echoes the results of previous studies in which snails were observed in higher densities and greater dominance in rice paddies (Krueger et al., 2004; Suwannatrai et al., 2011). The finding also highlights potential influences of land use practice on B.s. goniomphalos snail ecology. The surrounding earthen banks of the rice paddies keep water in to increase the standing time for water, thereby providing favorable habitat conditions for the snails (Wang et al., 2011). Converting lowland forests into rice paddies in northeast Thailand has therefore created many suitable habitats for B.s. goniomphalos snails. The proliferation of the first intermediate host of O. viverrini can subsequently contribute to the persistence of O. viverrini transmission. On the other hand, rice paddies are dynamic environments that can be irrigated, fully grown, completely dry, or burnt across the rice planting seasons (Fig. 4). Farmers in northeast Thailand usually plant rice crops twice a year, one starting in January and the other in June (Pers comms January 2013). Following rice harvests, the most common practice to clear the land and eliminate rice straw for next cultivation is burning, especially in irrigated paddy fields (Kanokkanjana and Garivait, 2013). Dead B.s. goniomphalos snails have been found in the rice paddy sampling sites that experienced
open burning (Fig. 4D). The burning practice can impact B.s. goniomphalos snail population and subsequently affect O. viverrini transmission because dead snails cannot continue the O. viverrini life cycle. Open burning of rice fields, however, does not completely eliminate B.s. goniomphalos snails from the rice paddies because snails can burrow into the deep soils (Petney et al., 2012) or be introduced from other nearby habitats through irrigation (Wang et al., 2011). This is evident in some rice paddy sampling sites in this study that in spite of the absence of B.s. goniomphalos snails in June sampling (Table 3), probably as a result of burning or drying up of the paddy fields, mature B.s. goniomphalos snails were found three month later during the September sampling. Land use practice of irrigating rice paddies using stream or pond water during rice planting seasons can potentially account for introducing B.s. goniomphalos snails back to areas where they were absent. This underscores the possible effect of irrigation on snail ecology, particularly in irrigated areas during the dry seasons, because the presence of Bithynia snails and the increase of their population are not directly related to the natural hydrologic input, but regulated by anthropogenic irrigation practices during rice planting seasons. As illustrated in Lohachit (2004–2005), peak snail densities were observed during rice crop planting seasons that vary in time across the year.
Table 5 Comparison of the range of water quality characteristics from this study with prior work in northeast Thailand where live B.s. goniomphalos were found. Reference
Lohachit (2004–2005)
Suwannatrai et al. (2011)
This study
Study area in northeast Thailand
Villages near Ubolratana dam of Khon Kaen Province
Muang District of Khon Kaen Province
Sampling period Temperature (◦ C) DO (mg/l) Conductivity (mS/cm) Salinity (ppt) pH
June 1989–December 1990 18–33 2.0–10.0 0.07–0.65a N.A. 6.3–8.5
Khorat basin, consisting of 12 provinces, including Khon Kaen Province October 2006–August 2009 21.9–38.6 0.01–6.47 0.12–40.2 0.05–22.11 6.02–8.07
a
June 2012–January 2013 23.76–41.14 0.05–13.01 0.24–8.47 0.09–4.22 6.20–8.65
Values for conductivity were converted from micromhos/cm to allow comparison.
Please cite this article in press as: Wang, Y.-C., et al., An ecological study of Bithynia snails, the first intermediate host of Opisthorchis viverrini in northeast Thailand. Acta Trop. (2014), http://dx.doi.org/10.1016/j.actatropica.2014.02.009
G Model ACTROP-3297; No. of Pages 9
ARTICLE IN PRESS Y.-C. Wang et al. / Acta Tropica xxx (2014) xxx–xxx
8
4.4. Future work
5. Conclusions
This study demonstrated that the abiotic water quality factors and biotic species interactions contributed to the distribution and abundance of the first intermediate O. viverrini host, Bithynia snails. Most of the variables examined in this study were statistically significantly associated with the relative abundance of B.s. goniomphalos snails. However, the generally low correlation coefficients and R2 values of the multiple regression outcomes (Table 4) suggested that at least two aspects of research related to water quality deserve future considerations. First, other factors, such as the influence of water hardiness, require scrutiny. Snails are less frequently found in soft water because calcium is a key element that helps to build shells (Dillon, 2000; Glass and Darby, 2009). It is thus desirable to know if calcium is a possible limiting factor on snail abundance and diversity. Second, physical and chemical properties of water are dynamic; continuous efforts on long-term multi-season studies are therefore needed. Equipment designed to continuously log water quality data can be installed in the area of interest. The data can serve as a temporal control against which the readings recorded by the handheld meters can be compared because of the temporal variability of water quality. Comparison of the multiple regression results across the three sampling periods revealed that water quality accounted for the B.s. goniomphalos snail abundance in June and January, the beginning of the rainy and the dry seasons, respectively, but did not significantly contribute to the B.s. goniomphalos snail abundance in September, the wetter period of the rainy season (Table 4). The lack of relationship for the rainy season reflects the difficulty of linking the snails with the environments because high water level and flooding events during the rainy seasons may have facilitated the dispersal of snails. In addition to causing the dispersal of snails and affecting the water quality of the host habitats, floods can inundate rice paddies and human settlements nearby streams, forming one temporary large water body that spatially connects the O. viverrini host habitats. The ‘connected’ host habitats potentially allow for all stages of the O. viverrini life cycle to occur, facilitating disease transmission in areas that are previously not at risk (Wang et al., 2011). This is a cause of concern as fecal contamination after rainy seasons has been found to correlate with snail infection rates (Kaewkes et al., 2012). With rice paddies established as the habitat type where B.s. goniomphalos snails dominate (Fig. 2) and streams recorded the highest mean E. coli concentration among the three habitat types (Table 1), an understanding of how connectivity between different habitats facilitates O. viverrini transmission should be further investigated, especially in the context of flooding during the rainy season. Such an understanding can be useful for land use planning and management to minimize liver fluke persistence. This is especially so when rice paddies beside rivers are completely flooded, resulting in the potential concerns of different hosts to interact. Additionally, recent studies have found that open defecation is still a very common practice by villagers in the rural areas (Phongluxa et al., 2013), and unhygienic defecation occurs mostly while working in the rice paddies (Suwannahitatorn et al., 2013). Proper sanitation should thus also be enforced near rice paddies and the benefits of latrine use should be promoted to prevent the transmission of O. viverrini from humans to snail hosts. Due to the small number of O. viverrini infected B.s. goniomphalos snails found in this study, no conclusive results could be determined with regards to the association between fecal contamination and snail infection. As the O. viverrini prevalence has been suggested to be low in the first intermediate host (Sithithaworn et al., 2007), future research should collect a larger sample of B.s. goniomphalos snails to obtain adequate snail infection data for analyses with environmental variables.
This study examined the abiotic and biotic factors affecting the distribution and abundance of the first intermediate host, B.s. goniomphalos snails, of O. viverrini, a carcinogenic parasite that remains of public health concerns in Southeast Asia. The freshwater habitats of ponds, streams and rice paddies possessed significantly different abiotic water qualities that potentially influenced the distribution and abundance of snail populations. The stepwise multiple regression analyses suggested that overall, the abiotic water quality variables of temperature, DO, salinity and pH were statistically significantly associated with the relative abundance of B.s. goniomphalos snails, but the extent to which the water quality variables accounted for the variation in the B.s. goniomphalos snail abundance varied across the dry and rainy seasons. Different habitats had different snail diversity and species evenness, and B.s. goniomphalos snails were found to be more dominant in the rice paddy habitats than the pond and the stream habitats. The significant negative correlation between the relative abundance of B.s. goniomphalos snails and that of F.m. martensi snails emphasized the biotic influence of species interaction on B.s. goniomphalos snail population. Field work observations highlighted the possible effects of land use practice, in particular, rice cultivation and its associated irrigation activities, on B.s. goniomphalos snail ecology. These findings provided insights into the B.s. goniomphalos snail habitats and the factors affecting B.s. goniomphalos snail abundance, contributing to the understanding of O. viverrini disease ecology.
Acknowledgements The project is supported by the National University of Singapore through the Academic Research Fund (Grant #: R109-000-151112). The authors wish to thank Isaac Taoyang Low, Eugene Zong Xi Chum, Nonglak LaOprom, Opal Pitksakurat, Kultida Kopolrat, Nattaya Watwiengkam, Leon Yan Feng Gaw, and Nadda Kiatsopit for assistance in field work or lab analysis.
References Brockelman, W.Y., Upatham, E.S., Viyanant, V., Ardsungnoen, S., Chantanawat, R., 1986. Field studies on the transmission of the liver fluke, Opisthorchis viverrini, in northeast Thailand: population changes of the snail intermediate host. Int. J. Parasitol. 16, 545–552. Chantima, K., Chai, J.-Y., Wongsawad, C., 2013. Echinostoma revolutum: freshwater snails as the second intermediate hosts in Chiang Mai, Thailand. Korean J. Parasitol. 51, 183–189. Chapin III, F.S., Matson, P.A., Vitousek, P.M., 2011. Principles of Terrestrial Ecology, second ed. Springer, New York. Crowl, T.A., Schnell, G.D., 1990. Factors determining population density and size distributions of freshwater snail in streams: effects of spatial scale. Oikios 59, 359–367. Dillon, R.T., 2000. The Ecology of Freshwater Molluscs. Cambridge University Press, Cambridge. Dodds, W.K., Whiles, M.R., 2010. Freshwater Ecology: Concepts and Environmental Applications of Limnology, second ed. Academic Press, Burlington, MA. Glass, N.H., Darby, P.C., 2009. The effect of calcium and pH on Florida apple snail, Pomacea paludosa (Gastropoda: Ampullariidae), shell growth and crush weight. Aquat. Ecol. 43, 1085–1093. Haruay, S., Tesana, S., Suwannatrai, A., Piratae, S., Kampoosa, P., Thammasiri, C., Duangprompo, W., Kaewkes, S., Tarbsripair, P., Paiboonsak, S., 2008. Effects of surface salt on distribution and density of snails in Family Viviparidae in Khon Kaen province, Thailand, analyses by using geographic information system. KKU Res. J. 8, 40–50. Hill, D., Fasham, M., Tucker, G., Shewry, M., Shaw, P., 2005. Handbook of Biodiversity Methods: Survey, Evaluation and Monitoring. Cambridge University Press, Cambridge. Hillebrand, H., 2008. Dominance. In: Jorgensen, S.E., Fath, B. (Eds.), Encyclopedia of Ecology. Elsevier, Oxford, pp. 938–944. International Agency for Research on Cancer (IARC), 1994. Schistosomes, Liver Flukes and Helicobacter pylori. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, vol. 6. International Agency for Research on Cancer, Lyon, France.
Please cite this article in press as: Wang, Y.-C., et al., An ecological study of Bithynia snails, the first intermediate host of Opisthorchis viverrini in northeast Thailand. Acta Trop. (2014), http://dx.doi.org/10.1016/j.actatropica.2014.02.009
G Model ACTROP-3297; No. of Pages 9
ARTICLE IN PRESS Y.-C. Wang et al. / Acta Tropica xxx (2014) xxx–xxx
Kaewkes, W., Kawkes, S., Tesana, S., Laha, T., Sripa, B., 2012. Fecal bacterial contamination in natural water reservoirs as an indicator of seasonal infection by Opisthorchis viverrini in snail intermediate hosts. Parasitol. Int. 61, 49–51. Kanokkanjana, K., Garivait, S., 2013. Alternative rice straw management practices to reduce field open burning in Thailand. Int. J. Environ. Sci. Dev. 4, 119–123. Keesing, F., Belden, L.K., Daszak, P., Dobson, A., Harvell, C.D., Holt, R.D., Hudson, P., Jolles, A., Jones, K.E., Mitchell, C.E., Myers, S.S., Bogich, T., Ostfeld, R.S., 2010. Impacts of biodiversity on the emergence and transmission of infectious diseases. Nature 468, 647–652. Keiser, J., Utzinger, J., 2009. Food-borne trematodiases. Clin. Microbiol. Rev. 22, 466–483. Kiatsopit, N., Sithithaworn, P., Saijunya, W., Boonmars, T., Tesana, S., Sithithaworn, J., Petney, T.N., Andrews, R.H., 2012. Exceptionally high prevalence of infection of Bithynia siamenesis goniomphalos with Opisthorchis viverrini cercariae in different wetlands in Thailand and Lao PDR. Am. J. Trop. Med. Hyg. 86, 464–469. Krueger, A.M., Irvine, K.N., Prangkio, C., Chaokasad, K., Sukontason, K., Sukontason, K.L., Ngern-klun, R., 2004. Visualizing water quality trends in Chiang Mai rice paddies: possible links between environment and health risks. Middle States Geographer 37, 1–8. Lohachit, C., 2004–2005. Ecological studies of Bithynia siamensis goniomphalos, a snail intermediate host of Opisthorchis viverrini, in Khon Kaen province, northeast Thailand. Malacol. Rev. 37/38, 1–26. MacDonald, G.M., 2002. Biogeography: Space. Time and Life. John Wiley and Sons, New York. Ngern-klun, R., Sukotason, K., Tesana, S., Sripakdee, D., Irvine, K.N., Sukontason, K., 2006. Field investigation of Bithynia funiculata intermediate host of Opisthorchis viverrini in northern Thailand. Southeast Asian J. Trop. Med. Public Health 37, 662–672. Ostfeld, R.S., 2009. Biodiversity loss and the rise of zoonotic pathogens. Clin. Microbiol. Infect. 15 (S1), 40–43. Petney, T.N., Sithithaworn, P., Andrews, R.H., Kiatsopit, N., Tesana, S., Grundy-Warr, C., Ziegler, A.D., 2012. The ecology of the Bithynia first intermediate hosts of Opisthorchis viverrini. Parasitol. Int. 61, 38–45. Petney, T.N., Taraschewski, H., 2011. Water-borne parasitic diseases: hydrology, regional development and control. In: Frimmel, F.H. (Ed.), Water Chemistry and Microbiology, vol. 3 of A Treatise on Water Science. Elsevier, Amsterdam, pp. 303–366. Phongluxa, K., Xayaseng, V., Vonghachack, Y., Akkhavong, K., van Eeuwijk, P., Odermatt, P., 2013. Helminth infection in southern Laos: high prevalence and low awareness. Parasit. Vectors 6, 328.
9
Sithithaworn, P., Andrews, R.H., Nguyen, V.D., Wongsaroj, T., Sinuon, M., Odermatt, P., Nawa, Y., Liang, S., Brindley, P.J., Sripa, B., 2012. The current status of opisthorchiasis and clonorchiasis in the Mekong Basin. Parasitol. Int. 61, 10–16. Sithithaworn, P., Yongvanit, P., Tesana, S., Pairojkul, C., 2007. Liver flukes. In: Darwin, M.K., Bernard, F. (Eds.), Food-borne Parasitic Zoonoses. Springer, Berlin, pp. 3–52. Spellerberg, I.F., Fedor, P.J., 2003. A tribute to Claude Shannon (1916–2001) and a plea for more rigorous use of species diversity and the “Shannon–Wiener” Index. Global Ecol. Biogeogr. 12, 177–183. Sriamporn, S., Pisani, P., Pipitgool, V., Suwanrungruang, K., Kamsa-ard, S., Parkin, D.M., 2004. Prevalence of Opisthorchis viverrini infection and incidence of cholangiocarcinoma in Khon Kaen, northeast Thailand. Trop. Med. Int. Health 9, 588–594. Sripa, B., 2012. Global burden of food-borne trematodiasis. Lancet Infect. Dis. 12, 171–172. Suwannahitatorn, P., Klomjit, S., Naaglor, T., Taamasri, P., Rangsin, R., Leelayoova, S., Mungthin, M., 2013. A follow-up study of Opisthorchis viverrini infection after the implementation of control program in a rural community, central Thailand. Parasit. Vectors 6, 188. Suwannatrai, A., Suwannatrai, K., Haruay, S., Piratae, S., Thammasiri, C., Khampoosa, P., Kulsantiwong, J., Prasopdee, S., Tarbsripair, P., Suwanwerakamtorn, R., Sukchan, S., Boonmars, T., Malone, J.B., Kearney, M.T., Tesana, S., 2011. Effect of soil surface salt on the density and distribution of the snail Bithynia siamensis goniomphalos in northeast Thailand. Geospatial Health 5, 183–190. Tabbabi, A., Ghrab, J., Aoun, K., Ready, P.D., Bouratbine, A., 2011. Habitats of the sandfly vectors of Leishmania tropica and L. major in a mixed focus of cutaneous leishmaniasis in southeast Tunisia. Acta Trop. 119, 131–137. Thai Meteorological Department (TMD), 2012. The Climate of Thailand, available at: http://www.tmd.go.th/en/archive/thailand climate.pdf (accessed February 2013). Wang, Y.-C., 2012. Examining landscape determinants of Opisthorchis viverrini transmission. Ecohealth 9, 328–341. Wang, Y.-C., Feng, C.-C., Sithithaworn, P., Feng, Y., Petney, T.N., 2011. How do snails meet fish? Landscape perspective needed to study parasite prevalence. Ecohealth 8, 258–260. Wang, Y.-C., Feng, C.-C., Sithithaworn, P., 2013. Environmental determinants of Opisthorchis viverrini prevalence in northeast Thailand. Geospatial Health 8 (1), 111–123. Wykoff, D.E., Harinasuta, C., Juttijudata, P., Winn, M.M., 1965. Opisthorchis viverrini in Thailand: the life cycle and comparison with O. felineus. J. Parasitol. 51, 207–214. Zalizniak, L., Kefford, B.J., Nugegoda, D., 2009. Effects of pH on salinity tolerance of selected freshwater invertebrates. Aquat. Ecol. 43, 135–144.
Please cite this article in press as: Wang, Y.-C., et al., An ecological study of Bithynia snails, the first intermediate host of Opisthorchis viverrini in northeast Thailand. Acta Trop. (2014), http://dx.doi.org/10.1016/j.actatropica.2014.02.009
