Intrinsic Vulnerability of Human-Water Contact Sites to Contamination with Schistosoma mansoni Ova in an Endemic Focus in Western Kenya

Human intestinal schistosomiasis caused by Schistosoma mansoni occurs in localized foci restricted to specific vector sub population areas. The molluscan vectors, Biomphalaria spp, have a widespread distribution within the lake region and elsewhere in Kenya, but the disease is endemic only in areas with certain physical characteristics and risk factors. A comprehensive study was conducted in Budalangi endemic focus of Western Kenya between May 2006 and June 2008 to determine the bionomics of intestinal schistosomiasis transmission with regards to proximity to various contact sites relative to contamination of ova of the worm in the vector breeding habitats with a view of instituting a deworming programme in primary schools. A survey of sanitation relative to presence, distribution and/or absence of latrines was conducted in the whole study area. The main human contact sites were identified and checked for the possibility of contamination with human stool. Distance of the main human water contact site from sampled homesteads without latrines was estimated and recorded within three categories. Vector snails were sampled from four permanent water habitats using standard procedures. Only 30.5% of the homesteads surveyed had pit latrines (c2; p<0.05) and 70.1% of them were situated less than 100m from various water contact sites. The actual density of vector snails and their mean counts per 10 scoops from the different sites in each type of breeding habitat varied widely (One way ANOVA; Lake Victoria: F = 8.11, df (5, 66), p<0.05; Dam: F = 3.65, df(7, 88), p<0.05; River Nzoia: F = 0.54, df (2, 33), p > 0.05; F = 6.76, df (3, 44), p<0.05) showing that their role in harbouring the vector snails and transmission of intestinal schistosomiasis was variable. However the mean vector snail count from all the different habitats showed no significant difference between them being suggestive of the fact that the four habitats were equally important for the purposes of vector breeding in the study area (One way ANOVA; F= 1.32, df(3, 17); p > 0.051). The study showed that there was a continuous low level of contamination of S. mansoni ova in area among the various other habitats in addition to the lake and that they were equally vulnerable. This information has a bearing on planning and implementing combined mass treatment of people who reside in the study area and vector control programmes in an integrated venture. Keywords: Vulnerability, Contact sites, Contamination, Schistosoma mansoni ov

The physicochemical and environmental factors affecting the distribution of Anopheles merus along the Kenyan coast

Abstract Background Members of the Anopheles gambiae complex are the main transmitters of malaria. Anopheles merus is a member of the complex found along the Kenyan coast because it breeds in saline waters. An entomological study was conducted in Garithe Malindi District, to investigate the physicochemical and environmental factors affecting the distribution of An. merus. Methods Field and laboratory studies were used to investigate the breeding habitats of the subspecies. Mosquito larvae were sampled using standard dipping technique from small pockets of pools, ponds, hoof prints, road drain, wells and mangrove swamps found in Garithe. All 3rd and 4th instars of Anopheles larvae sampled were identified microscopically into species. A representative of Anopheles gambiae complex was then identified to specific sibling species using r-DNA PCR technique. The habitats were characterized based on temperature, conductivity, salinity, dissolved oxygen, total dissolved solids, pH, size, distance to nearest house, canopy coverage, surface debris, presence of algae, emergent plants, turbidity and habitat types. Results A total of 159 morphologically identified late stage instar Anopheles gambiae s.l larvae were selected for r-DNA analysis by PCR. Out of these, 60.4% (n = 96) were Anopheles merus, 8.8% (n = 14) were Anopheles arabiensis, 18.2% (n = 29) were Anopheles gambiae s.s and 12.6% (n = 20) were unknown. Using paired t-test (t (121) = −3.331, P = 0.001) a significantly high proportion of An. merus was observed in all habitats compared to An. arabiensis, and An. gambiae s. s. In habitat characterization, Pearson’s correlation analysis test showed different parameters being associated with the occurrence of An. merus larvae in the different habitats sampled. Six out of the 55 correlation coefficients (10.9%) were statistically significant, suggesting non-random association between some pairs of variables. Those that had a significantly high positive correlation with An. merus included temperature, salinity, conductivity, total dissolved solids and algae. Conclusions Different physicochemical parameters and environmental parameters affect the occurrence of An. merus. In this study, higher temperatures accelerate the growth of the larvae and aids in growth of micro-organisms and algae which are food sources for the larvae. Saline waters favour the growth and development of An. merus larvae; they are also able to develop in a range of saline waters. Conductivity, total dissolved solids and canopy coverage are among the important factors influencing the development and abundance of An. merus larvae in their habitats. Habitat type also influences the abundance of An. merus larvae. They mainly prefer to breed in pools and ponds, but not swamps, hoof prints and wells

The bionomics of <it>Anopheles merus</it> (Diptera: Culicidae) along the Kenyan coast

Abstract Background Anopheles merus, a sibling species of the Anopheles gambiae complex occurs along the East African coast but its biology and role in malaria transmission in this region is poorly understood. We evaluated the blood feeding pattern and the role of this species in malaria transmission in Malindi district, Coastal Kenya. Methods Adult mosquitoes were collected indoors by CDC light traps and Pyrethrum Spray Catch and outdoors by CDC light traps. Anopheles females were identified to species by morphological characteristics and sibling species of An. gambiae complex distinguished by rDNA polymerase chain reaction (PCR). Screening for host blood meal sources and presence or absence of Plasmodium falciparum circumsporozoite proteins was achieved by Enzyme Linked Immunosorbent Assays (ELISA). Results Anopheles merus comprised 77.8% of the 387 Anopheles gambiae s.l adults that were collected. Other sibling species of Anopheles gambiae s.l identified in the study site included An. arabiensis(3.6%), and An. gambiae s.s. (8%). The human blood index for An. merus was 0.12, while the sporozoite rate was 0.3%. Conclusion These findings suggest that An. merus can play a minor role in malaria transmission along the Kenyan Coast and should be a target for vector control which in turn could be applied in designing and implementing mosquito control programmes targeting marsh-breeding mosquitoes; with the ultimate goal being to reduce the transmission of malaria associated with these vectors.</p