A Malacology Survey to Map Out Schistosomiasis Transmission Sites on Mageta Island, Siaya County, Western Kenya

Abstract

The persistence of human schistosomiasis in the Lake Victoria region is partly attributed to the presence of snail intermediate hosts in the genera of Biomphalaria and Bulinus. The areas around the Lake Victoria shores provide suitable habitats for the establishment of the snail intermediate hosts. A cross-sectional survey was carried out in August 2020 to map out schistosomiasis transmission sites on Mageta Island, western Kenya, to fill the gap in transmission risk to the Island's residents. A sampling of snails was done using a scoop made up of a stainless-steel sieve with a mesh size of 2 mm by 2 mm from 8 am to 11 am. A long pair of forceps was used to pick snails from sites where the scoop could not be used. The snails were identified to the species level and taken to the Mageta Health Centre for the cercarial shedding process. Cercariae shed by each snail were recorded as either bifurcate or non-bifurcate. Other parameters recorded from each habitat included the presence of human activities, presence, and identification of vegetation, and habitat bottom surfaces, among others (see Appendix 2). A total of 9,779 freshwater snails were collected from 116 habitats on Mageta Island. There were six snail species of medical importance (Biomphalaria sudanica, Bulinus tropicus, Lymnaea natalensis, Biomphalaria pfeifferi, Bulinus nasutus, and Bulinus forskalii) and four of non-medical importance (Melanoides tuberculata, Bellamya spp., Physa acuta, and Pila ovata). The most abundant snails were Bi. sudanica (86.94%; n= 8502) and Bu. tropicus (6.23%; n= 609) and the most abundant habitat types were natural habitats (94.82%; N = 110). Although most snails were found in natural habitats, the odds of finding snails in natural habitat were twice that of finding snails in a manmade habitat (OR = 2.052, 95% CI = 0.237 – 17.776), but without a significant difference (P = 0.514). Most snails were geographically distributed on the northern, eastern, and a few western parts of Mageta Island. Lake shoreline, swamps, and lagoons were classified as natural habitats, while banana plant holes, ditches, and boats were classified as manmade. Habitats classified as natural comprised 94.82% (N = 110) of the habitats and contained 96.32% (n = 9419) of the sampled snails. Manmade habitats comprised 5.17% (N = 6) of habitats and contained 3.68% (n = 360) of the sampled snails. Biomphalaria sudanica was the most abundant snail species (86.94%; n = 8502) followed by Bulinus tropicus (6.23%; n = 609). The mean number of Bi. sudanica (137.061 ± 1.49) was significantly higher (P < 0.05) in habitats with mud bottom surfaces. Biomphalaria pfeifferi and Bu. nasutus were only found in habitats with mud bottom surfaces. The mean number of Bu. tropicus and Bu. forskalii were significantly higher (P < 0.05) on rock and wood bottom surfaces, xiii respectively. Six snail species out of ten sheds come kind of cercariae with only five species shedding bifurcate cercariae. Snails collected from habitats with human activities had the highest prevalence of infection, but the shedding of cercariae was not associated with the presence or absence of human activities in habitats (P = 0.685). A total of only five sites harboring snails infected with bifurcate cercariae were found on Mageta Island. Vegetation played a major role in predicting the presence of snails in habitats on Mageta Island with Eichhornia crassipes (water hyacinth) harboring most snails. Even though there were few schistosomiasis transmission sites on Mageta Island, the mere presence of snail intermediate hosts in habitats around Lake Victoria and schistosomiasis infection being found among some of the residents, indicate that residents of Mageta Island are at an increased risk of schistosomiasis infection. Efforts to control the snail intermediate hosts should be carried out on Mageta Island in combination with studies on the prevalence of schistosomiasis among the residents.