Mapping of snail intermediate host habitat reveals variability in schistosome and nonschistosome trematode transmission in endemic settings
Authors: Angelo Teckla , Starkloff Naima Camilla , Mahalila Moses Paul , Charles Jenitha , Civitello David James , Kinung’hi Safari 
Published: June 2023

Background

There is growing recognition that mass drug administration must be complemented with environmental interventions to interrupt schistosomiasis transmission. Accurate mapping of snails and schistosome parasite distribution is critical to identify foci of human exposure and prioritize sites for interventions.

Methodology

We conducted longitudinal environmental surveys of snails and schistosomes in 467 waterbodies across 86 villages in northwestern Tanzania to describe spatial and temporal variation in snail and schistosome parasites presence and identify relevant underlying predictors. We conducted time constrained net sampling ofBulinussnails from vegetation, sediments, and floating objects and then examined all collected snails for patent infections.

Principal findings

A total of 43,272Bulinussnails were collected across the three visits to each waterbody spanning November 2020 – August 2021, and we conducted statistical analyses on the latter two visits with more in-depth surveys (25,052 snails). We found patent schistosome infections in 0.87% of snails, 9.8% of waterbodies, and 31% of villages in all six districts. Variance decomposition indicated that variation among waterbodies was associated with variation in snail presence and the prevalence of schistosomes and nonschistosome parasites, whereas variation among villages and districts was not. Snail presence was highest in March-May a period of heavy rains, but otherwise not associated with waterbody characteristics. Waterbodies permitting cattle use had significantly higher prevalence of schistosomes than those permitting only human use. Nonschistosome parasites were more prevalent in June to September the dry season but were not associated with other waterbody characteristics. Waterbody permanence and distance to the nearest primary school were not associated with snails or parasites.

Conclusions/significance

This study revealed substantial variation in snails, schistosome and nonschistosome abundance at local (waterbody) scales, and it suggests links between community-driven water use decisions and schistosome transmission. The identification of local drivers of snail and schistosome abundance level and transmission factors at waterbody scale can complement studies across larger scales to shed light on transmission hotspots and guide the development of targeted interventions for schistosomiasis control.

Author summary

Currently there is increasing need to supplement mass drug administration with environmental interventions by identifying potential sites for human exposure to disrupt schistosomiasis transmission. Here we focus on environmental surveys to determine leading factors for presence and sequential variation in snails and schistosomes. Using a timed sampling strategy, snail vectors were collected and examined for infections. We screened snails collected from November 2020 to August 2021 and found variation among waterbodies in snail presence and infections of schistosomes and nonschistosome parasites. This variation was not observed at village and district levels. High abundance of snails was observed in March to May during heavy rainfall but not connected with waterbody distinctiveness. Waterbodies used by cattle had higher prevalence of schistosomes compared to waterbodies used by humans alone. Nonschistosomes were higher in June to September the dry season but not related to waterbody characteristics. Although chemotherapy reduces schistosomiasis burden, our results suggest that identification of transmission sites at waterbody scale could aid development of targeted interventions for schistosomiasis control.