Localized breeding of the Anopheles gambiae complex (Diptera: Culicidae) along the River Gambia, West Africa.

A study was undertaken to identify the major larval habitats of the Anopheles gambiae (Giles) complex in rural Gambia. Mosquito larvae and pupae were sampled along transects and in specific habitats in the central region of the country during the rainy seasons of 1996 and 1997. The sampling showed that the major breeding sites were located on the flooded alluvial soils bordering the river. The largest numbers of larvae were found during September, one month after the peak rains. Polymerase chain reaction analysis of specimens showed that Anopheles melas (Theobald) was the dominant species in the flooded areas (81.5%), followed by A. gambiae sensu stricto (Giles) (18.0%) and A. arabiensis (Patton) (0.5%). By sampling in specific habitats it was evident that A. arabiensis was mainly breeding in rain-fed rice fields along the edge of the alluvial soils. Anopheles melas and A. gambiae s.s. often coexisted but whereas A. melas were found in water with a salinity of up to 72% sea water (25.2 g NaCl l(-1)), A. gambiae s.s. only occurred in water with up to 30% sea water (10.5 g NaCl l(-1)). Anopheles melas larvae were found in association with plant communities dominated by sedges and grasses (Eleocharis sp., Paspalum sp., Sporobolus sp.) and sea-purslane Sesuvium portulacastrum (L.) and the presence of cattle hoof prints, whereas A. gambiae s.s. larvae mainly occurred in association with Paspalum sp. and Eleocharis sp. The study showed that even during the peak rainy season, breeding of the A. gambiae complex is almost entirely restricted to the extensive alluvial areas along the river.

The availability of potential hosts as a determinant of feeding behaviours and malaria transmission by African mosquito populations.

A simple model for the influence of host availability on vector bloodmeal choice is applied to estimate the relative availabilities of humans, cattle and other host populations to malaria vectors in African communities, using published human blood indices and ratios of cattle to humans. Cattle were bitten < 0.01, 0.021 +/- 0.11, 1.61 +/- 0.16 and 1.61 +/- 0.46 times as often as humans by Anopheles funestus, An. gambiae sensu stricto and An. arabiensis in Segera, Tanzania, and An. gambiae sensu lato in The Gambia, respectively. No significant feeding upon host species other than cattle or humans was detected. Even though An. gambiae s.l. in The Gambia were mostly An. gambiae s.s., they were 77 times more likely to choose cattle over humans than An. gambiae s.s. in Tanzania. The model accurately predicted cattle blood indices for the An. arabiensis population in Tanzania (predicted = 0.99 +/- 0.21 x observed + 0.00 +/- 0.10; r2 = 0.66). The potential effect of increased cattle abundance upon malaria transmission intensity was simulated using fitted relative availability parameters and assuming vector emergence rate, feeding cycle length and survivorship were unaffected. The model predicted that increased cattle populations would not affect malaria transmission in Tanzania but could drastically reduce transmission in The Gambia or where An. arabiensis is the dominant vector. We define the availability of a host as the rate at which a typical individual host-seeking vector encounters and feeds upon that host in a single feeding cycle. Mathematical models based on this definition also represent promising tools for quantifying the dependence of vector longevity, feeding cycle length and dispersal upon host availability.

Do untreated bednets protect against malaria?

Bednets are thought to offer little, if any, protection against malaria, unless treated with insecticide. There is also concern that the use of untreated nets will cause people sleeping without nets to receive more mosquito bites, and thus increase the malaria risk for other community members. Regular retreatment of nets is therefore viewed as critical for malaria control. However, despite good uptake of nets, many control programmes in Africa have reported low re-treatment rates. We investigated whether untreated bednets had any protective benefit (in October and November 1996) in The Gambia where nets, although widely used, are mostly untreated. Cross-sectional prevalence surveys were carried out in 48 villages and the risk of malaria parasitaemia was compared in young children sleeping with or without nets. Use of an untreated bednet in good condition was associated with a significantly lower prevalence of Plasmodium falciparum infection (51% protection [95% CI 34-64%], P < 0.001). This finding was only partly explained by differences in wealth between households, and children in the poorest households benefited most from sleeping under an untreated net (62% protection [14-83%], P = 0.018). There was no evidence that mosquitoes were diverted to feed on children sleeping without nets. These findings suggest that an untreated net, provided it is in relatively good condition, can protect against malaria. Control programmes should target the poorest households as they may have the most to gain from using nets.

Effect of passive zooprophylaxis on malaria transmission in The Gambia.

The effect of zooprophylaxis on malaria transmission has not been studied on the African continent despite that the World Health Organization has recommended this intervention method since 1982. The effect of passive zooprophylaxis on malaria vector abundance, mosquito feeding preferences, and infectivity was studied in an area of moderate seasonal transmission in The Gambia. A paired cohort of 204 children <7 yr of age was selected and matched in groups for presence or absence of cattle (Bos taurus) within 20 m of their bedroom. Comparisons were made between mosquitoes collected from the bedrooms of the two groups of children. Other ruminants and equines were present in both groups of compounds. Most of the anopheline mosquitoes (98.5%) collected were Anopheles gambiae sensu lato. There was no difference in the geometric mean number of An. gambiae s.l. mosquitoes caught in houses near or far from cattle. The species composition of the An. gambiae complex was similar in both groups. Blood meal analysis of specimens collected in houses without cattle showed a human blood index (HBI) of 82% for An. Arabiensis (Patton), 56% for An. gambiae sensu stricto (Giles), and 36% for Anopheles Melas (Theobald), indicating that each of these sibling species fed readily on animals. The presence of cattle reduced the HBI of An. arabiensis but did not significantly alter the HBI of An. gambiae s.s. or An. melas. There was no significant difference between the groups in the sporozoite rates of An. gambiae s.l. nor in the estimated malaria transmission risk. These findings suggest that passive zooprophylaxis using cattle does not alter the individual exposure to malaria parasites in The Gambia.

Effect of fly control on trachoma and diarrhoea.

BACKGROUND: Domestic flies are accepted vectors of diarrhoea, but their role in trachoma transmission has never been quantified and no study has shown that fly control decreases the prevalence of trachoma. We assessed the effect of fly control on public health in a pilot study in Gambian villages. METHODS: We studied two pairs of villages--one pair in the 1997 wet season, and one pair in the 1998 dry season. For each pair, deltamethrin was sprayed for 3 months to control flies in one village whilst the other was used as a control. Fly populations were monitored with traps. We surveyed trachoma at baseline and at 3 months, and collected daily data on diarrhoea in children aged between 3 months and 5 years. FINDINGS: Fly control decreased numbers of muscid flies by around 75% in the intervention villages compared with controls. Trachoma prevalence was similar at baseline (wet season, prevalence in intervention village 8.8% vs control 12.2%; dry season, 18.0% vs 16.0%), but after 3 months of fly control there were 75% fewer new cases of trachoma in the intervention villages (wet season 3.7% vs 13.7%; dry season 10.0% vs 18.9%; rate ratio and relative risk of pooled data 0.25 [adjusted 95% CI 0.09-0.64], p=0.003). There was 22% less childhood diarrhoea in the wet season (14% vs 19%, period prevalence ratio 0.78 [0.64-0.95], p=0.01), and 26% less diarrhoea in the dry season (6% vs 8%; 0.74 [0.34-1.59], p=0.60) compared with controls. INTERPRETATION: Muscid flies are important vectors of trachoma and childhood diarrhoea in The Gambia. Deltamethrin spray is effective for fly control and may be useful for reducing trachoma and diarrhoea in some situations, but further research on sustainable fly-control methods is needed.

PIP: The causative agent of trachoma, Chlamydia trachomatis, has been found on flies fed on heavily infected laboratory culture media. Findings are presented from an assessment of the effect of domestic fly control upon the prevalence of trachoma and associated cases of childhood diarrhea in 2 pairs of Gambian villages. 1 pair of villages was studied in the 1997 wet season and the second pair in the 1998 dry season. For each pair, deltamethrin was sprayed for 3 months to control flies in 1 village, while the other village was used as a control. Fly populations were monitored with traps. The prevalence of trachoma was measured at baseline and at 3 months, and data were collected daily on diarrhea in children aged 3 months to 5 years. Fly control decreased the numbers of muscid flies by approximately 75% in the intervention villages compared with controls. While the prevalence of trachoma was similar at baseline between study and control villages, after 3 months of fly control there were 75% fewer new cases of trachoma in the intervention villages. There was 22% and 26% less childhood diarrhea in the wet and dry seasons, respectively, compared with controls. These findings demonstrate that muscid flies are important vectors of trachoma and childhood diarrhea in The Gambia, and that the use of deltamethrin spray can help to reduce the prevalence of both trachoma and associated diarrhea cases.

Permethrin-impregnated bednet effects on resting and feeding behaviour of lymphatic filariasis vector mosquitoes in Kenya.

The impact of permethrin-impregnated bednets on resting and feeding behaviour of mosquito vectors of Wuchereria bancrofti, causing human lymphatic filariasis was studied in six pairs of villages (treated and untreated) before and after intervention. The study villages were in Kwale District, near the coast of Kenya, where Bancroftian filariasis is highly endemic, transmitted by a combination of both anopheline and culicine mosquito vectors. Mosquitoes were collected weekly in each village, indoors (using pyrethrum spray catches) and outdoors (using pit traps) during 3-4 months following the long rainy season. Of the filariasis vector species of mosquitoes collected in 1994 before intervention. 33.6% were members of the Anopheles gambiae complex, 30% were An. funestus and 36.4% were Culex quinquefasciatus. PCR analysis of the An. gambiae complex species collected in 1995 demonstrated that 98.5% were An. gambiae sensu stricto. 1% An. arabiensis and 0.5% An. merus. Introduction of impregnated bednets in 1995 significantly reduced the number of indoor-resting An. gambiae s.l. by 94.6% and An. funestus by 96.7%, but there was no change in the number of Cx quinquefasciatus collected indoors. The number of outdoor-resting An. gambiae s.l. was significantly reduced, whereas densities of An. funestus and Cx quinquefasciatus remained unaffected outdoors. ELISA analysis of mosquito bloodmeals showed a shift from human to animal feeding after the introduction of treated nets. The human blood index (HBI) for indoor resting Cx quinquefasciatus was reduced from 93.1% to 14.4%. Vector potential based on the HBI and mosquito density was estimated to be reduced by 99% for An. gambiae s.l., 98% for An. funestus and 97% for Cx quinquefasciatus and vectorial capacity would be suppressed even more by the impact on the vector survival rates (not measured). These results suggest that permethrin-impregnated bednets give effective personal protection against transmission of W. bancrofti by An. gambiae, An. funestus and Cx quinquefasciatus in East Africa.