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

Abstract

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 ova