Spatial distribution and Schistosomes infection prevalence of Biomphalaria snails along Lake Victoria shoreline of Mbita - Homa-bay county-Kenya

Abstract

Intestinal schistosomiasis caused by Schistosoma mansoni remains a major public health problem responsible for morbidity and mortality in sub-Saharan Africa despite availability of control programs. It is estimated that more than 6 million people in Kenya are suffering from schistosomiasis. Several species of Biomphalaria snails are obligatory intermediate hosts for transmission of S. mansoni. Mbita sub-county in Homa Bay County is one of the regions along the Lake Victoria basin in which schistosoma mansoni infection prevalence in school going children is high at 76.8%, despite the existence of mass chemotherapeutic control programs in primary schools. Identification of exposure sites is therefore necessary in order to integrate and direct intervention to the area. This was a cross sectional study that aimed to determine the abundance of Biomphalaria snails in 16 purposively selected sites, assess the vegetation type associated with population of Biomphalaria snails and determine schistosomes infection prevalence of Biomphalaria snails along the shoreline of Mbita. Sampling of Biomphalaria snails was done once in each of the selected sites using 30 minutes scooping technique. The sampling sites were mapped using geographical global positioning system (GPS). The vegetation types at each sampling location was collected and identified at Kenya Marine and Fisheries Research Institute (KMFRI),-Kisumu. Schistosomes infection of collected snails was determined by their shedding of cercariae at Nagasaki University Laboratory at Mbita. Generated data was analyzed using analysis of variance (ANOVA) and significant differences in mean number of snails collected per site and vegetation types was determined using Tukey’s post hoc test. Chi-square test was used to determine significant differences in proportions of cercariae positive snails between sites. A total of 3135 Biomphalaria sudanica snails were collected. The number of snails collected differed significantly between the 16 sites (F=11.735.df=15; 836: p<0.001). The lowest snail collection was realized at Bau ( M=0.73) and the most at Orundu (M=11.87), with Tukey’s post hoc test indicating that the mean number of snails collected at Orundu differed significantly from all other sites (p<0.001). Significant mean differences (MD) was also observed in number of snails collected per vegetation type (F=7.899. df=5; 846: p<0.001). The mean number of snails collected in Cyprus gracilis was significantly higher than from Enydra fluactuants (MD= 2.032: p< 0.001), Eichhornia crassipes (MD=4.149: p=0.010) and Enydra fluactuants mixed with Eichhornia crassipes (MD=2.516): P=0.010). Eichhornia crassipes alone (MD=4.634: p=0.009) and Enydra fluactuants mixed with Eichhornia crassipes (MD=4.777: p=0.002). However, only 21 (0.67%) of snails sheded human cercariae despite all the 16 sites having human feces contamination except Bau and Kosata, and no significance difference was found in the proportion of snails positive for cercariae between the sites (2 (60)= 64.00; p<0.338). These findings suggest that Cyprus gracilis is the main vegetation type associated with abundance of Biomphalaria snails. Although schistosomes infection prevalence of the snails is low, these 16 sites may still be important exposure sites due to abundance of snail and fecal contamination found at the sites. A molecular technique like PCR is necessary for verification of positive snails with human schistosomes. Cercariometry should be done alongside snail sampling.