No abstract

Malaria transmission varies from village to village and even from family to family in the same village. The current study was conducted in northern Ethiopia to identify risk factors responsible for such variations in a hypoendemic highland malaria setting: 2114 children aged < 10 years living in 6 villages situated close to small dams at altitudes from 1775 to 2175 m were monitored. Monthly malaria incidence was determined 4 times over a 1-year period during 1997. Incidence results were then analysed by 14 individual and household factors using Poisson multivariate regression. Among 14 factors analysed, use of irrigated land (rate ratio[RR] = 2.68, 95% CI 1.64-4.38), earth roof (RR = 2.15, 95% CI 1.31-3.52), animals sleeping in the house (RR = 1.92, 95% CI 1.29-2.85), windows (RR = 1.84, 95% CI 1.30-2.63), open eaves (RR = 1.85, 95% CI 1.19-2.88), no separate kitchen (RR = 1.57, 95% CI 1.10-2.23), and 1 sleeping room (RR = 1.52, 95% CI 1.05-2.20), were significantly associated with malaria. The proportion of infection among children exposed to one or no risk factor was 2.1%, increasing with the number of risk factors and reaching 29.4% with 5 or more. Further studies are needed to confirm the importance of particular risk factors, possibly leading to simple health education and control measures that could become part of routine control programmes, implemented with inter-sectoral collaboration.

Objective To assess the impact of construction of microdams on the incidence of malaria in nearby communities in terms of possibly increasing peak incidence and prolonging transmission. Design Four quarterly cycles of malaria incidence surveys, each taking 30 days, undertaken in eight at risk communities close to dams paired with eight control villages at similar altitudes but beyond flight range of mosquitoes.

1. Nylon bednets impregnated with different insecticides were evaluated in 1988 against wild adult mosquito populations, mostly Mansonia africana (Theobald) and Anopheles gambiae Giles sensu lato, entering experimental verandah-trap huts in The Gambia. Each bednet had six 10 x 10 cm holes made in the walls to simulate torn conditions and permit female mosquitoes to enter and feed on sleepers. 2. Individual net treatments, determined by gas chromatography of net samples from before and after 12 weeks use of the bednets, were: permethrin 670 +/- 159 and 405 +/- 190 mg/m2 (40% loss), cypermethrin 37 +/- 8 and 16 +/- 9 mg/m2 (57% loss), deltamethrin 10 +/- 7 and 10 +/- 8 mg/m2 (no loss), lambda-cyhalothrin 2.6 +/- 0.9 and 1.6 +/- 0.5 mg/m2 (38% loss), pirimiphos-methyl 4017 +/- 117 and 1160 +/- 319 mg/m2 (71% loss). 3. Washing three times in the traditional manner with local cow-fat soap reduced the initial dosages by about 85% of cypermethrin and lambda-cyhalothrin, 99.8% of pirimiphos-methyl and left no detectable residues of deltamethrin or permethrin. 4. The unwashed permethrin-treated bednet reduced the number of mosquitoes entering a hut by 60% of An.gambiae s.l. and 68% of Mansonia spp. This deterrency was less pronounced with the other insecticides and was lost by washing the bednets. 5. Each insecticide, especially lambda-cyhalothrin and pirimiphosmethyl, caused significant mortality rates of mosquitoes that entered huts with impregnated bednets, and prevented the majority of An. gambiae s.l. and Mansonia females from bloodfeeding. Washing completely removed the efficacy of deltamethrin and permethrin treated bednets, whereas nets treated with cypermethrin, lambda-cyhalothrin or pirimiphos-methyl remained significantly insecticidal after washing. 6. Aerial toxicity from the pirimiphos-methyl treated bednet killed 80% of An.gambiae s.l. confined overnight in the hut at the end of the trial, whereas the pyrethroid-treated bednets gave negligible mortality rates of mosquitoes. 7. Sleepers using the bednets had no medical symptoms significantly associated with any of the treatments. On the contrary, from 216 interviews, 4/10 complaints were associated with the use of untreated nets (P approximately 0.05), perhaps because sleepers were kept awake by mosquitoes and became more aware of any ailments. 8. It is concluded that permethrin tends mainly to deter mosquitoes from house-entry, enhancing personal protection, whereas the other insecticides kill higher proportions of the endophilic mosquitoes, which would give better community protection against malaria transmission.

We investigated local-scale variation in malaria transmission and infection in children within a continuous landscape by retrospective spatial analysis of entomological and clinical data collected during 1988 and 1989 in The Gambia, West Africa. Parasite prevalence was negatively correlated with vector abundance and exposure to malaria parasites in 10 villages where entomological surveillance had been carried out. Variation in bednet use did not explain this finding. Mosquito-breeding habitat was retrospectively mapped using 20-m spatial resolution multispectral SPOT satellite imagery from 1988. From these data we estimated by linear regression the risk of exposure to malaria parasites in 26 villages where clinical surveys of children had been made. As exposure increased, so did parasite prevalence; but at higher levels of exposure, parasite prevalence declined. Our findings demonstrate marked differences in exposure to malaria in villages over distances of less than 2 km from mosquito breeding sites and suggest that there are also large differences in immunity between neighbouring settlements.