Modeling of wind-driven circulation of schistosome larvae in a vegetated side pond

Schistosomiasis, a debilitating parasitic disease of poverty affecting more than 250 million people worldwide, is contracted upon contact with the larval form of the parasite, known as cercaria, emerging from infected freshwater snails, the obligate intermediate host of the parasite. Understanding how infectious larvae can be transported in rivers and irrigation canals is crucial to fine-tune environmental interventions targeting the parasite and its intermediate host. Specifically, lateral cavities along many tropical rivers act as water access points but can also entrap parasitic larvae and provide low-velocity environments for snail-supporting vegetation to flourish, creating potential areas of high schistosomiasis infection. In this paper, the circulation of larvae in a typical transmission site along the Lampsar River in Senegal is modeled under a range of wind and vegetation conditions to better understand how such environmental factors affect their transport. We found that wind direction has a large influence on the distribution and abundance of parasitic larvae at the water access point, whereas increasing wind speed scales velocities but does not affect flow patterns. The area of coverage of vegetation can significantly alter flow magnitudes and circulation patterns for the same wind speed and direction. Increasing vegetation coverage generally leads to an increase in larvae residence time in the side pond, but the relationship is non-monotonic with five regimes of residence time behavior based on vegetation patch radius. The results suggest that there is an optimal patch radius at which larvae residence time and velocity deviations within the side pond are maximized.