Medical and Veterinary Entomology (1992) 6, 371-378
Seasonal abundance and biting behaviour of Anopheles punctulatus and An.koliensis in Malaita Province, Solomon Islands, and a trial of permethrin impregnated bednets against malaria transmission W . A . S A M A R A W I C K R E M A , A . D . PARKINSON, N . K E R E and 0 . G A L 0 Malaria Control Project, Honiara, Solomon Islands Abstract. Seasonal abundance of the malaria vectors Anopheles punctulatus Donitz and An. koliensis Owen in Bilimanu, an isolated inland village with forty-two houses in Malaita Province of the Solomon Islands, was monitored over 28 months by means of all-night landing/biting catches at one site during June 1985 to September 1987. Totals of 1250An.punctulutus and 141A n .koliensis were collected, the latter being the largest number of this species ever caught at any locality in the Solomons. Bednets impregnated with permethrin 0.5 g/m2 were introduced in December 1986 to be used at night by all 190 villagers for protection against malaria vectors. Bioassay tests with An.punctulutus blood-fed females exposed under nets for 10min resulted in 100% mortality up to 50 weeks post-impregnation. For An.punctulatus, the main vector species, the mean catch (indoors + outdoors) per man hour was 2.9 (range 0.7-13.2) before a cyclone on 19 May 1986, and, 0.66 (0.2-2.7) after the cyclone. The vector survival rates were usually high before the cyclone, but erratically lower thereafter for An.punctu1atu.s. An. koliensis disappeared after the cyclone. Both An.punctulutus and An. koliensis consistently showed higher rates of biting man indoors than outdoors and their die1 biting cycle showed a peak around midnight. Outdoors, the parous proportion of An.punctulatus was twice the nulliparous, and nearly so indoors. Following intervention with permethrin-treated bednets, the mean catch of An.punctulutus fell to 0.35 per man-hour (monthly range 0-1.5). The Plusmodium falcipurum malaria infection rate reduced from 10% pre-intervention to zero in September 1987, 9 months after intervention, and then rose again. With re-impregnation of bednets in November 1987, the P.fulciparum infection rate was again reduced by March 1988. No effect of treated bednets was observed on the prevalence of P. v i v a malaria.
Key words. Anopheles punctulatus, Anopheles koliensis, parous rates, biting cycles, permethrin, bednets, malaria vectors, malaria control, Solomon Islands.
Introduction The vectors of malaria in the Solomon Islands are three members of the Anopheles punctulatus complex, namely Correspondence: Dr W. A. Samarawickrema, 30/A Kandawatta Road, Nugegoda, Sri Lanka.
Anopheles farauti Laveran, An.punctulatus Donitz and An. koliensis Owen. Indoor residual spraying with DDT for malaria control has been in operation in the country since 1971 (Taylor, 1975). Mosquito collections on human bait in several villages of Guadalcanal and Malaita (Fig. 1A) during the last 10 Yean detected an abundance of An.farauti but the absence of An.punctulutus and An. koliensis. 37 1
W . A. Samarawickrema et al.
Fig. 1. ( A ) Outline map of part of the Solomon Islands showing the location of Bilimanu in relation to Auki in Malaita and Honiara in Guadalcanal. (B) Sketchmap of Bilimanu village showing the three areas of housing.
Macgregor ( l m ) . Slooff (1972) and Taylor (1975) observed that, while the latter two species were suppressed under the sustained pressure of DDT applications, An.farauti populations remained and continued to transmit malaria by avoiding sprayed surfaces and by feeding and resting outdoors. The ongoing status of An.punctuLatus complex mosquitoes as vectors of Bancroftian filariasis in the Solomon Islands is also a matter of concern (Webber & Southgate, 1981). In addition to the predominance of An.farauri, from 1980 onwards small numbers of An.punctulutus, sensu stricto, were taken in man-biting catches in some inland villages of C~uadalcanal.I t was found that some of these villages were newly established and had never been sprayed, while others had not been sprayed for years.
A mass blood survey in twenty-one indicator villages in Malaita Province in 1985, prior to the initiation of a mass anti-malaria drug administration programme, revealed crude parasite rates ranging from zero to 39% generally, with the highest rate of 67.8% in one remote inland village. DDT spraying had never been camed out in this village and, in a preliminary all-night biting catch there, only An.punctuLati~s was taken. This village, named Bilimanu, was taken up for vector studies in June 1985. After 18 months surveillence of the vector population, during which period the village was under mass drug administration, permethrin impregnated bednets were introduced for use by all the people in order for us to evaluate their effect on vector abundance and malaria transmission.
Anopheles in the Solomon Islands Materials and Methods
Study area. Bilimanu lies on the banks of Kwarea river, 5 km from the eastern coastline in the northern third of Malaita island, about 30km north of Auki (Fig. 1A). Access was by foot over rough undulating terrain of bush, shrubland and food gardens. The village had three sections, each roughly 200m from the next (Fig. 1B): six houses in the first section where one entered the village, thirty-one in the second and five in the third. Most houses were on stilts, with their floors made of strips of wood or bamboo about 1-1.5m above the ground. The walls and roofs were of sago palm leaf and doors made of wood. A population census of Bilimanu in 1985 showed 190 people living in the village. They grew yams, banana and vegetables in their food gardens closeby. No medical services were available any closer than Auki for diagnosis and treatment of malaria patients, although this project introduced quarterly mass blood surveys, microscopic diagnosis and chemotherapy, vide infra. Residual spraying of houses with DDT had never been undertaken at Bilimanu, due to refusal by the people. Anti-malaria mass drug administration (MDA), started throughout Malaita province in 1985 and was conducted during AprilJune each year, except 1986 when MDA was interrupted by Cyclone Namu which hit the country on 19 May 1986. No comparable village with a high parasite rate and high An.punctulatus abundance could be found near Bilimanu. Hence the trial was conducted without a control village. Following the damage from Cyclone Namu, starting in September 1987, the houses in Bilimanu were shifted in stages to higher ground, 75-150m away from the river. Vector surveillence. All-night collections of mosquitoes biting/landing on human bait were undertaken indoors, in a bedroom of a house at Bilimanu, and outdoors near shrubs and trees 50m from the house, at fixed sampling sites in the first section of the village. Collections commenced shortly before dusk and concluded shortly after dawn. The team of four collectors worked two at a time, one indoors and one outdoors, for a period of 3 h before being replaced by the other pair, following the procedures described by Haddow (1954). At least one catch was carried out monthly, totalling thirty-one catches: fifteen during June 1985 to May 1986 before Cyclone Namu, five during July to December 1986 and eleven during January to September 1987 after intervention with impregnated bednets. Hourly catches of mosquitoes from the two sites were placed in separate paper cups, humidified with moist lint and provided with sugar water on cotton wool pads. The following morning, mosquitoes were identified in the laboratory. Female Anopheles were dissected to read the parity of individuals from the degree of tracheolar coiling in their ovaries (Detinova, 1962), in order to estimate the mean probability of daily survival. Each hourly catch was processed separately and, when numerous, only 40-60% of specimens were dissected. Species identification was based on the criteria determined by Rozeboom & Knight (1946) as given in the keys of Belkin (1962).
To plot the die1 biting cycle, mosquito densities were expressed as the Williams’ mean of hourly catches using the rationale log(n 1) to avoid zero numbers (Williams, 1937). The daily survival rates were calculated from parous rates using the method of Davidson (1954) and Macdonald (1957). Intervention. Following 19 months of surveillance of the vector population during June 1985 to December 1986, permethrin impregnated bednets were given to the 190 residents of Bilimanu on 22 December 1986. A total of 145 bednets, impregnated with permethrin (ICI trademark ‘Ambush 50% EC) at the dosage of 0.5 g/m2 recommended by Schreck & Self (1985), were distributed. The people were instructed not to wash the nets until reimpregnation in November 1987. The purpose of the trial, the benefits from the use of treated bednets and their care were explained to the villagers by national radio programmes and by the health education and entomological staff visiting Bilimanu. Evaluation. Parasitological: a mass blood survey was carried out in the village on 20 December 1986, 2 days prior to intervention. Thick and thin blood-films on the same slide were made from finger prick of each individual and stained with Giemsa. Each thick film was examined for 5 min, covering approximately 100 fields, and classified as positive or negative for malaria parasites. Plasmodium falciparum, P . vivax, P.malariae, mixed infections and gametocyte positives were identified from the thin films. Mass blood surveys were repeated at 3-monthly intervals. Following each survey all slide positive cases were treated with chloroquine using the standard antimalaria treatment schedule. Entomological: contact bioassay tests of mosquito susceptibility to permethrin were carried out weekly on impregnated bednets issued to the villagers and with an untreated net in the laboratory as control. About ten to fifteen bloodfed Anopheles females were introduced into each net and exposed for lOmin, removed to paper cups and their mortality recorded after 24 h.
During the period June 1985 to September 1987, a grand total of 1430 females of the An.punctulatus complex were taken in all-night catches on human bait. Of these, 486 (34%) were caught outdoors, comprising 16 An.farauti, 411 An.punctulatus and 59 An.koliensis. The majority were collected indoors, comprising 23 An.farauti, 839 An.punctulatus and 82 An. koliensis. No other mosquito species were taken during these catches.
An.punctulatus base-line data The monthly density of An.punctulatus collected at Bilimanu, expressed as the Williams’ mean of the catch per man-hour indoors and outdoors, rose from 1.0 in June 1985 to a peak of 13.2 in September, declined to
W . A . Samaruwickrema et nl
a minimum of 0.7 in December 1985 and then ranged between 1.0 and 3.6 during January-May 1986 (Table 1). The ratio of biting outdoors:indoors was 1:1.93 overall before the cyclone. After the cyclone Ari.punctulutus densities were consistently reduced to a mean of 0.68 (range 0.2- 1.3) per man-hour during July-December 1986, with a ratio of 1:4.2 outdoors:indoors. The rainfall pattern had no obvious influence on the fluctuations in adult population density of Atz.putzctu1aru.s (Fig. 2). The probability of daily survival of An.purrctulatus, estimated from the observed parous rates (Table I ) , was generally high at 89-98% during the months before the cyclone, except for 58% in September 1985 and January 1986 when the population included upsurges of newly emerged nuiiipars (cf. Fig. 2). The die1 cycle of biting activity showed the typical Table 1. Entomological indices o f An.pimctulatirs and Ati. k o l i e n ~ i i n Biliinaiiu villagc. based on all-night landing collections: 12 man-hours indoors and 12 man-hours outdoors. p = Probability of survi\al through one day.
Atiopheles trend of peak activity towards midnight, both indoors and outdoors. This pattern of biting activity remained similar after the cyclone (Fig. 3). Biting cycles of 594 nulliparous and 1021 parous An.punctuZutus read from 594 specimens were assessed before the cyclone. The peak of biting by parous females tended to be earlier than that of the nullipars (Fig. 4).
A 11. kolietisis
This species was scarce throughout the study. Only 141 females were taken in Bilimanu during 28 months. It was first encountered when there was a high abundance of An.pzrnctulutus, from June to September 1985 (Table I), and it disappeared after the cyclone. The biting cycle of Arz.koliensi.9 peaked after midnight (Fig. S ) , later than that of An.punctulatus, with the majority of biting indoors (Fig. 5 ) .
Mean no./ Parous p man-hour rate
Mean no./ Parous man-hour rate p
Junc July Aug. Sepr. Oct. Nov. Dec.
0.925 0.0 0.896 0.3 0.910 0.3
3.9 1.6 0.7
0.792 0.719 0.754 0.198 0.731 0.770 0.750
0.583 0.901 0.917 0.909
O.n 0.0 0.1
3.6 1.0 1.2 1.9 2.7'
0.190 0.792 0.950 0.761 0.698
0.57s 0.925 0.983 0.913 0.887
1.7 0.2 0.1 0.0 0.0
0.6 0.8 0.2 0.4 0.8 1.3
0.5(x) 0.667 1 .Ooo 0.500 0.313 0.615
0.794 0.0 0.873 0.0 1.ooo 0.0 0.794 0.0 0.679 0.0 0.851 0.0
0.8 0.8 1.5
0.350 0.889 0.610
0.705 0.0 0.961 0.0 0.848 0.1 0.0 0.0 0.0 0.0 0.500 0.0 0.0
1.0 9.0 2.7 13.2
Jan. Feb. March April Ma) Junc July Aug. Sept. Oct. Not. Dcc 1087
Jan Fcb. March Apnl Ma! June July Aug. Scpr.
0.0 0.0 0.0 0.4 0.0
* Collection before the cyclone on 19 Ma) 1986 Distnbution o f permethnn-impregnated bednets on 22 December 1W6
The results of mass blood surveys of the villagers before and after intervention are shown separately for P.fakiparum and P.vivux in children aged under 10 years, adolescents of 10-16 years and people aged over 16 years (Fig. 6). Initially, in February 1985, infection rates were 32.0% for P.falcipururn and 35.8% for P. vivux, falling by December 1986 to 10.2% P.fakiparum and 7.4% P.vivax attributed largely to the M D A programme. After the intervention with permethrin impregnated bednets in December 1986, a further reduction in the P.falcipari~minfection rate was observed to zero in September 1987, prior to reinipregnation of bednets in November 1987. Evenso, the infection rate rose by December 1987 and declined again by March 1988. The prevalence of P.vivux showed no clear trends during this period (Fig. 6).
Eiitornologicul impact of permethritz-treated bednets
The density of An.punctuZutus (Table 1, Fig. 2) fell to a mean of 0.35 per man-hour (monthly range 0-1.5) following the introduction of impregnated bednets on 22 December 1986. The initial drop both indoors and outdoors in January was followed by an upsurge in February 1987. N o change was discernible in the biting cycle of An.punctulatus indoors or outdoors in the presence of permethrin-treated bednets (Fig. 3). Surprisingly, the impact appeared to be greater o n An.putictulatu.9 outdoors where the density fell to zero during March-July, whereas the biting rate indoors rose t o almost forty bites per mannight in March before dropping to 0.01 in May-June 1987. Perhaps the relatively few surviving mosquitoes tended to bite more avidly indoors when excited by permethrin. Bioassay tests o n fifty-two bednets used by the villagers showed 100% kill of An.punctulatus when tested upto 5 0 weeks after impregnation. In view of this long residual
Anopheles in the Solomon Islands
Fig. 2. An.punctututus densities at Bilimanu, January 1985 to September 1987, expressed as number of bites per man per night outdoors. XX , and indoors. 0-0 (upper box), in relation to monthly rainfall (lower box).
efficacy, the reimpregnation of bednets was not carried out until November 1987, 11 months after the previous treatment. All usable nets were washed and dried before retreatment and worn out nets were replaced with new impregnated ones.
Vector surveys carried out by the Malaria Eradication Project during the period 1962-71 (Macgregor, 1968; Slooff, 1968; Taylor, 1975) showed an abundance of An.punctuZutus in Guadalcanal but not in Isobel, Malaita and Makira provinces. On the other hand, An.ko1iensi.s
was reported to be widespread in nearly all of the larger Solomon Islands except for New Georgia and Russell islands, with An.farauti even more widespread among the smaller islands. As part of the Global Malaria Eradication Programme, DDT residual spraying at a rate of 2g/m2, repeated at 6-monthly intervals, was launched in Guadalcanal in October 1962 and extended to other provinces (Macgregor, 1968). By 1973 the entire country, except the vector-free islands, was covered by the campaign (Taylor, 1975). Following house-spraying operations, Macgregor ( 1968) found that the population density of all three vector species was suppressed: both An.punctulatus and An. koliensis had disappeared after two applications. Slooff (1968, 1972)
W. A . Samarawickrema et ul. ~
I I C
7 - A
3 2 c
I I I
Fig. 4. Die1 biting cycle of nulliparous (N) and parous (P) Ar.puncfu~atusoutdoors (52N. 12SP) and indoors (126N. 291P) at Bilimanu, June 1985 to May 1986.
Fig. 3. Did cycles of biting activity of An.punctuiorus outdoors and indoors at Bilimanu: June 1985 to May 1986. before the c!clone (343 outdoors, 678 indoors): - - -, July-Dccernbcr 1086. prc-intervention (16 outdoors. 63 indoors); - (January-September 1987. post-intcrvention (33 ouidoors, 60 indoors) ~
found no mosquitoes resting indoors following D D T spraying and hence discontinued the daytime searches inside houses. He also noted that, while An.kolierisis disappeared under the pressure of D D T in the Solomon Islands. this species withstood spraying operations in other countries o f the region. In Makira province during 1971-73, before the spraying operations. Taylor (1975) collected 584 An. kolierzsis from fifty-four outdoor catches, plus 1097 specimens from 60 indoor catches, together with fifty-one An.punctulutus from cightccn catches outdoors plus 109 specimens from twent? catches indoors. These data constituted the largest numbers of An. kolierzsis previously reported from Solomon Islands. They also showed a high degree of endophagy and midnight peak biting by An.koliensi.s.The 141 An.ko1ierz.sr.s
-- INDOOH - OUTDOOR r - 7
Fig. 5. Die1 biting cycle of An.koliensis Bilimanu (62 outdoors. 72 indoors), June 1985 to May 1986.
taken at Bilimanu in our study, meagre though the total, was the largest number of this species recorded at one site in the Solomon Islands since the outset of the continuing country-wide spraying programme. Such a number of An. koliensis has not been previously recorded from Malaita. Routine entomological surveillence in the Malaria
Anopheles in the Solomon Islands
P. folciparum 0
20 10- 16 Years
Fig. 6. Plasmodium fakiparum and P.vivax quarterly infection rates in the human population at Bilimanu before (20 December 1986) and after intervention (quarterly surveys in March, June, September, December 1987 and March 1988). (A) Infection rates of the total population. (B) P.fulciparum infection rates in the three categories: under 10 years, 10-16 years, over 16 years. (C) P.vivav infection rates in the three categories.
Control Programme during the period 1983-88 was confined to Guadalcanal, Malaita and Central Province. During these activities small numbers of An.punctu1atus were found in some inland villages in Guadalcanal, where DDT had not been sprayed for one reason or another, and in newly built villages. We found the largest numbers of An.punctuZutus at Bilimanu in Malaita, where the people had refused to have their houses sprayed. The results presented here on seasonal abundance, parous rates and biting behaviour of An.punctulatus represent the first such study of this species in the Solomon islands, whereas An.punctulatus, s.s., has been investigated more thoroughly in Papua New Guinea (Charlwood et al., 1986). Bednets impregnated with fast-acting pyrethroid insecticides have been widely tested recently for malaria
vector control and other control activities. Studies from several countries, in the laboratory, in experimental huts and in village scale trials were reviewed by Curtis (1989), Rozendaal (1989) and Rozendaal & Curtis (1989) and permethrin-impregnated bednets have been used operationally since 1988 by the Malaria Control Project in the Solomon Islands (W.H.O., 1989). Working in Papua New Guinea, Charlwood & Dagoro (1987) suggested that impregnated bednets may prove more effective against An.punctulatus, due to its endophily (confirmed in our study), than against the more exophilic An.farauti. Despite the small scale of our trial and the wide fluctuations observed in vector densities and parous rates, reduction of the P.fulcipurum infection rate at Bilimanu following the introduction of permethrin im-
121. A . Sumuruwic~kremaet al.
pregnated bednets may be attributed t o the sustained suppression of Ai?.purrctitla/its vectorial capacity by this method o f intervention. Although the P.falciparurn infection rate in Bilimanu was steadily reduced t o zero in the course of 9 months following intervention, it resurged again by 12 months post-intervention, requiring the re-treatment o f bednets. whereafter the prevalence of 1’. fulciparritii was again reduced 4 months post-reimpregnation. Whereas bioassay tests on permethrin-trcated bednets gave consistently for u p t o 50 weeks high mortality-rates of ,4rz./~i~ticti~lutir.~ post-impregnation, the optimum frequency and dosage of bednet treatment remains unclear in terms of cost-efficacy and epideniiological impact. ‘There were two drawbacks in this study. N o other village with the landscape, parasite rate and mosquito fauna similar t o Bilimanu was available t o be used as a control. Therefore it is difficult t o draw definite conclusions from the results obtained. Secondly, Cyclone Namu brought about a drastic change in the ecological picture in Bilimanu shortly before intervention, resulting in the disappearance of An. koliensis and reduced densities of Aii.putzc’tirluru.~for 5 months before the introduction of pernierhrin-impregnated bednets. These entomological events. as well as the unplanned relocation of Bilimanu housing. were conducive t o malaria reduction but complicated the interpretation o f o u r bednet trial. W e concluded. however, that the impact of permethrin-impregnated bednets was sufficiently encouraging t o justify the expansion of their operational use with further evaluation against malaria transmission in the Solomon Islands.
Acknowledgments This study was carried out by the Vector Control unit of the Malaria Control Project, Solomon Islands, funded by the United Nations Development Programme, Fiji, through the World Health Organization, Western Pacific Regional Office in Manila. Dr L. S. Self, former W . H . O . Regional Adviser on Vector Biology and Control introduced ihe use of impregnated bednets as an intervention method t o several countries in the Region including the Solomon Islands and made several visits t o participate in the training of personnel. W e wish to thank the Minister o f Health and Medical Services, Solomon Islands, for his support. W e a r e also gratedful to staff of the Health Education Division of the Ministry and the Entomology team stationed in Auki. Malaita Province. for their assistance in iield and laboratory work.
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