Biomphalaria pfeifferi infected with Schistosoma spp. in Kakamega and Bungoma counties, western Kenya confirms autochthonous transmission of intestinal schistosomiasis

BACKGROUND: Granular mapping conducted in Kakamega and Bungoma counties of western Kenya provided strong evidence of intestinal schistosomiasis among school-age children in this area. However, it was unclear whether the observed infections were due to active transmission. To address this gap, a follow-up malacology survey was conducted to determine the presence of snail vectors and ascertain active transmission. METHODS: Nineteen Wards with ≥10% prevalence of Schistosoma mansoni were selected from Kakamega and Bungoma counties. From these Wards, 42 primary schools with the highest prevalence of S. mansoni were used to identify nearby water bodies with human-water contact activities for sampling snail vectors. Live snails were sampled by experienced collectors using a handheld dip-net scoop in standardized sampling frames for ~30 minutes at each water body. Snails were counted and identified to species-level based on shell morphology. Site locations sampled for snails were mapped using a geographical information system, and the type of water body recorded. All Biomphalaria and Bulinus snails were transported to the laboratory where they were screened for cercariae. Cercariae were identified to basic taxonomic groups using standard identification keys. The relative and mean abundance and prevalence of Schistosoma sp. infection in snails was determined. RESULTS: Out of 4,245 snails collected, 3,341 (78.7%, 95% CI: 77.5-79.9) were putatively identified as Biomphalaria pfeifferi, 88 (2.1%, 95% CI: 1.7-2.5) as Bulinus globosus, 664 (15.6%, 95% CI: 14.6-16.7) as Lymnaea natalensis and 152 (3.6%) as other species. B. pfeifferi were found in 36 out of the 42 primary school regions (85.7%), with the highest number (605 snails) recorded at Indangalasia in Lusheya-Lubinu Ward. A total of 87 (2.6%, 95% CI: 2.07-3.14) B. pfeifferi shed Schistosoma sp. cercariae. The mean abundance of B. pfeifferi was similar between streams (16 ± 35) and swamps/marshes (20 ± 31) (P = 0.356), but was higher compared to rivers (4 ± 10) (P = 0.005). CONCLUSIONS: The high abundance of B. pfeifferi coupled with the observation of field-caught snails shedding cercariae confirms autochthonous schistosomiasis transmission corroborating the reported human infections in Kakamega and Bungoma counties. Incorporation of focal snail control to complement chemotherapy will accelerate interruption of transmission in these areas.