Medical and Veterinary Entomology (1992) 6, 325-334

Biting and resting behaviour of anophelines in western Venezuela and implications for control of malaria transmission Y A S M I N R U B I O - P A L I S and C . F. CURTIS*

Division de Investigaciones, Escuela de Malariologid y Saneamiento Ambiental, Ministerio de Sanidad y Asistencia Social, Maracay, Venezuela, and *Department of Medical Parasitology, London School of Hygiene and Tropical Medicine, Keppel Street, London WClE 7HT, U.K.

Abstract. A study was conducted in three villages in western Venezuela between February 1988 and October 1989 to determine the biting and resting activity of anophelines in relation to human habits, rainfall and fenitrothion spraying of houses. Mosquitoes were collected landing on humans inside and outside experimental huts throughout the night. Only three mosquitoes were found resting in the huts in the morning, but 2470 were collected resting on vegetation in the early morning. The collections yielded eleven species of anopheline, the most abundant being Anopheles nuneztovari, comprising over 75% of the total anophelines collected, followed by An.triannufatus, Andbitarsis s.1. and A n .oswaldoi . The four most abundant species showed different die1 patterns of biting. For An.nuneztovari the peak of activity was close to midnight indoors and outdoors, for An.triannufatus between 19.00 and 20.00 hours outdoors, for An.aZbitarsis mainly before midnight indoors and outdoors and for An.oswaldoi outdoors at 19.00 hours, with an additional smaller peak indoors at midnight. Most of the human population use bednets, go to bed before 22.00 hours and are therefore most exposed to mosquitoes that bite outdoors early in the night. Fenitrothion house-spraying failed to prevent large mosquito populations developing in the wet season, presumably because of their exophilic resting habits. The possible advantages of impregnation of existing bednets with pyrethroids, and provision of nets for people who do not have them, are discussed. Key words. Anopheles nuneztovari, An.albitarsis, A n .triannulatus, A n .oswaldoi, malaria, vector control, mosquito biting cycle, Venezuela.

introduction To control malaria in the endemic area of western Venezuela, houses were sprayed regularly with DDT from 1945 onwards. Since 1984-85 fenitrothion has been used instead, in an attempt to achieve better control of Anopheles (Nyssorhynchus) nuneztovari Gabalddn which, until recently, was the only malaria vector implicated in the area (Rubio-Palis et al., 1992). The National Control Programme changed insecticide because An.nuneztovari is an exophilic mosquito which was thought to make Correspondence: Dr Yasmin Rubio-Palis, Apartado 2064, Maracay 2101-A, Venezuela.

insufficient contact with DDT deposits to be killed (even though it is not physiologically resistant to this insecticide), whereas fenitrothion has a fumigant effect lasting about 6 weeks (Caraballo, 1987) and so was considered to be the insecticide of choice to intercept mosquitoes coming indoors for a short time to bite. Despite the regular house-spraying with residual insecticide in this area, malaria transmission has persisted. At present, the failure of conventional control measures is attributed mainly to the exophilic habit of Anmmeztovari. Hitherto, little effort has been made to assemble a coherent picture of the biology, ecology and ethology of malaria vector(s) in western Venezuela. Information on the anopheline fauna is scanty, mainly based on spot surveys from 325

326

Yasniin Rirhio-Palis arid C. F. Citrris

which the numbers of mosquitoes collected and dissected were small, and referred only to Aiz.rzurieztoi*ari. In order to study the mosquito behaviour in relation to human habits, a small area in western Venezuela was jelected to conduct a longitudinal study.

Materials and Methods Srirdy area. The study area is located on the southern slopes of the Andes near the Venezuelan border with Colombia, encompassing parts of Barinas, Tachira and Apure states (around 7"31'N, 71"41'W), and covers an area o f approximately 760km' (Fig. 1). Altitude ranges from 100 t o 300m. This area has an annual rainfall of 3otM)--40()0rnm. a mean temperature of 24°C and 83% relatike humidity (VAF, 1989). The area is classified as wet tropical woodland (Ewell& Madrid, 1968).The human population is about 8o(MJ, and the economy is based on cattle rearing, growing crops (mainly vegetables and plantains). river fisheries and forest exploitation. All thesz

activities involve the invasion of previously unoccupied land, deforestation and continous ecological changes. Many migrant workers from Colombia come annually to the study area during harvest of the main crops, Epidemiologically, the area is characterized by high malaria receptivity and vulnerability (Bruce-Chwatt, 1985), since the local vectorial capacity and the actual number of imported cases entering the area are high. The malaria incidence per 1000 population was 3.7 in 198Y (API = 3.7), with 98.4% of the cases being Plasmodium i!ivax- (Grassi & Feletti) (DER, 1989). Three typical villages were selected for intensive study: Jahillos (7"33'33"N, 71"33'44"W), Guaquitas (7"32'h"N, 71"SO' IO'W) and Caiio Lindo de Piscuri (7"33'33'", 71'51 '3o"W).Malaria transmission occurs in these villages throughout the year, despite regular house-spraying with D D T or fenitrothion. In each village an experimental hut was built, similar to the temporary houses of the people. It was arranged that routine spraying of insecticide would not be carried out in the experimental huts, in order to avoid interference with

CARIBBEAN

SEA

Fig. 1. 1,ocation of the study area in western Venezuela. Names o f states enclosed by rectanglc arc thosc where Anopheles nuneitovari has been recordcd i n Venezucla (Sutil. 1976).

Anophelines in western Venezuela

327

Venezuelan Air Force weather station located approximately 16 km from the study site.

the study objective to understand the natural behaviour and bionomics of malaria vectors. Mosquito catches. Landing catches of anopheline mosquitoes were carried out on human bait for 12 h between sunset and sunrise (19.00-07.00 hours), two nights per week per village, from February 1988 to October 1989. Collections were made inside and outside of the experimental huts by a team of six catchers with two supervisors. Catchers worked in pairs for shifts of 4 h , one catcher outdoors about 3 m from the hut and one indoors, rotated each night between shifts indoors and outdoors. Catchers sat on stools with their legs exposed below the knees. Mosquitoes were collected with mouth aspirators and placed in paper cups, a new cup being started every hour. Indoor-resting mosquitoes were searched for inside the experimental huts and collected with a mechanical aspirator (Haushem’s Machine Works, New Jersey, U.S.A.) at 06.00-06.10 hours. Outdoors, mosquitoes were collected with a large aspirator consisting of a PVC tube (14cm diam., 125cm long) with a fan operated by a 12V rechargeable battery. Mosquitoes were accumulated in the aspirator in removable plastic containers, brought to the laboratory and killed by freezing or with ethyl acetate or chloroform. Species were identified using the keys of Cova Garcia & Sutil (1977) for subgenus Anopheles and of Delgado and Rubio-Palis (in Rubio-Palis, 1991) for subgenus Nyssorhynchus. Twenty female mosquitoes were dissected daily to determine parity by the simplified Polovodova technique (Detinova, 1962), i.e. presence or absence of dilatations on the ovariole stalks. Other information. Questionnaires on people’s habits were given to householders in the three villages in August 1988 (wet season), March 1989 (dry season) and August 1989. In October 1989 a more specific questionnaire was given to 50% of households within 2km of the experimental huts. Climatological data were obtained monthly from the

Results Anopheline abundance and seasonality

A total of 47,588 female anopheline mosquitoes, representing eleven species, was collected in all-night catches indoors and outdoors on human baits in the three villages (Table 1). The four commonest species were An. ( N y s . ) nuneztovari, An. ( N y s . ) triannulatus (Neiva & Pinto), A n (Nys.) albitarsis Arribalzaga serzsu lato and An. ( N y s . ) oswaldoi (Peryassu). The mean numbers caught per person per night are shown in Fig. 2 in relation to monthly rainfall. The most abundant species was An.nuneztovari, comprising over 75% of the total anophelines collected in the three sites, and reaching 92.7% in Caiio Lindo. An. triannulatus was the second most abundant species in Jabillos and Guaquitas but was rarely collected in Cafio Lindo. AnopheLes nuneztovari densities vaned up to 1500-fold between the numbers collected in the dry season (JanuaryApril) and the wet season (May-December). For instance, in Guaquitas only two specimens were collected in April 1989, while 3489 specimens were caught in one night in August 1989. Regression of the log-transformed mean number caught on rainfall showed a stronger positive relationship ( P < 0.01) of the catches of An.nuneztovari, An.triannulatus and An.oswaldoi in Jabillos and Guaquitas with the rainfall during the previous month than with the rainfall in the month in which the catches were made ( P < 0.05). In Cafio Lindo there was no significant relationship between rainfall and mean numbers of these species. For An.albitarsi.7 s.1. there was no significant relationship between numbers and rainfall in the month of the catch. However, the relationship with the rainfall during the previous month

Table 1. Anophelines collected out-of-doors and indoors on human baits in western Venezuela, August 1988 to October 1989. Jabillos

Cafio Lindo

Guaquitas

Spccies

out

In

out

In

out

An.nuneztovari An. triannulatus An.aIbitarsi3 s.1. A n . oswaldoi Amstrodel A n . rangeli An. neornaculipalpus A n . benarrochi An.pseudopunctipennis An.punctimacu1a An. mediopunctatus

4324 1437 447 242 65 69 43 3 0 2 0

5653 204 357 164 65 80 21 1 0 0 0

4505 26 142 59

4501 13 139 76 47 62 1 4 4 2

8084 686 760 222 1I3 113 10 10 0 1 1

38450 3767 2979 1108

1

11383 1401 1134 345 219 192 17 3 1 2 3

Total

6632

6545

4861

4850

14700

10000

47588

60 54 5 3 5 1 1

In

Total

S69

570 97 24 Iff 8 6

I088

lSS9

1888

MEAN NUMBER CAUGHl

MONTHS

RAINFALL

MONTHS

1880

MONTIiS

1989

Control Programme.

Fig. 2. Catches of the four commoncst Anopheles spccics in relation to rainfall and fenitrothion house spraying in Cano Lindo, June 1988 to October 1989 (top row), Guaquitas, August 1YX8 to October 198') (middle row), and Jabillos, February 1988 to October 1989 (bottom row). Arrows indicate the months when spraying as camed out by the National Malaria

A n . n u n e z t oval-i

Anophelines in western Venezuela was significant in all three villages (P

Biting and resting behaviour of anophelines in western Venezuela and implications for control of malaria transmission.

A study was conducted in three villages in western Venezuela between February 1988 and October 1989 to determine the biting and resting activity of an...
838KB Sizes 0 Downloads 0 Views