# Ecological influences on host–parasite dynamics among Biomphalaria snails in two schistosomiasis endemic regions of Kenya

## TL;DR

This study explores how ecological factors affect snail infections by Schistosoma mansoni in Kenya, revealing spatial differences and correlations with environmental variables.

## Contribution

The study identifies specific ecological correlates of S. mansoni infection in Biomphalaria snails and provides molecular insights into cercariae diversity in two Kenyan regions.

## Key findings

- Biomphalaria pfeifferi was the most dominant snail species, with infection prevalence varying significantly across sites in the Lake Victoria basin.
- Infection correlated positively with temperature and soil porosity, and negatively with turbidity and salinity in Lake Victoria basin samples.
- Molecular analysis confirmed B. pfeifferi and identified cercariae closely related to Zygocotyle lunata, distinct from schistosomes, in Lake Victoria.

## Abstract

Schistosomiasis is a neglected tropical disease affecting over 240 million people globally, with sub-Saharan Africa bearing the highest burden. In Kenya, transmission of Schistosoma mansoni, the causative agent of intestinal schistosomiasis, remains prevalent in western, coastal, and central regions, particularly in the Mwea Irrigation Scheme and the Lake Victoria basin. The parasite depends on Biomphalaria snails as intermediate hosts, yet ecological determinants influencing infection dynamics remain underexplored. This study examined S. mansoni infection in Biomphalaria snails across two contrasting ecosystems: Lake Victoria and the Mwea rice irrigation scheme in Kenya. Snails, water, and soil samples were collected from the study sites. Water and soil were analyzed for abiotic parameters, including temperature, turbidity, salinity, pH, and soil porosity, while snail infections were confirmed via cercarial shedding and PCR targeting the ITS region. Laboratory-maintained isolates of S. mansoni were passed through baboons and served as positive controls for molecular identification. Biomphalaria pfeifferi was the most dominant species (90.4% of all snails sampled). Infection prevalence among infected snails varied across sites around Lake Victoria basin: Anyanga Beach, Siaya (70.8%, 80/113), Sindo Rangwena, Homabay (20.6%, 7/34), Kasabong, Siaya (16.9%, 12/71), and Kendu Bay, Homabay (16.7%, 3/18), with a chi-squared test confirming a strong site–infection association (χ² = 67.33, df = 3, p < 0.001), indicating significant spatial heterogeneity in transmission risk. Infection correlated positively with temperature (r = 0.72, p < 0.01) and soil porosity (r = 0.65, p < 0.05), and negatively with turbidity (r = −0.63, p < 0.01) and salinity (r = −0.58, p < 0.05) for samples found in areas around Lake Victoria basin. Molecular screening of 272 snail-derived samples using ITS1 primers yielded 113 positives. Sequencing confirmed B. pfeifferi (600 bp) from Mwea Irrigation Scheme and Lake Victoria Basin, forming a monophyletic clade with strong bootstrap support. The 500 bp ITS1 fragment identified S. mansoni in lab-maintained strains and Thiba in Mwea Irrigation Scheme samples, clustering within the S. mansoni clade. Further analysis of cercariae using 18S rDNA revealed ≥98% similarity to Zygocotyle lunata in Lake Victoria sites, forming a well-supported clade distinct from schistosomes.