Schistosomiasis, caused by various trematodes of the genus Schistosoma is one of the five most prevalent parasitic infections worldwide. The transmission cycle of Schistosoma mansoni involves both humans and snails from the genus Biomphalaria as obligatory hosts. S. mansoni is endemic across much of sub-Saharan Africa and, as such, has been the subject of control measures including mass drug administration campaigns targeting the parasite in the human host and molluscicides targeting the snail population. Fluctuations in the aquatic environment and in host snail population size or snail community composition may concentrate or dilute the free-living cercarial and miracidial portions of the schistosome life cycle. Variation in these populations can impact the infection pressure experienced by both snail and human hosts and therefore presents an opportunity to potentially magnify the effect of control measures through seasonal treatment timing. To quantify seasonal variation in snail abundance, we collected absolute and relative snail density, snail community composition, and S. mansoni infection prevalence data bimonthly from a perennial stream in the Kisumu, Kenya area from July 2015 - January 2018. Daily rainfall and temperature data was also obtained for January 2013 -January 2018.
Results/Conclusions
We used time-series analysis, including the evaluation of varying time-lags, to identify the conditions most predictive of host snail density and infection prevalence. There were substantial seasonal fluctuations in the infection prevalence, with high prevalence found in March and September. These fluctuations do not appear to be directly related to shifts in temperature and rainfall but instead to prior snail community composition, with a correlation between high S.mansoni prevalence and prior dominance of Biomphalaria in the snail assemblage. These data were then used to parameterize seasonal forcing of snail population size and infection prevalence in a dynamic model of schistosomiasis. This model allows us to identify the times of year when either mass drug administration or molluscicides alone, or a combination of these control strategies, results in the maximum reduction in the force of infection experienced by the human population.