Neglected tropical diseases primarily occur in developing countries and have substantial impacts on human health. Specifically, schistosomiasis, caused by trematodes of the family Schistosoma, is one of the top five most prevalent parasitic diseases of humans. These ectothermic trematodes have a free-living cercarial life stage that depends on a set energy reserve to successfully infect humans. Consequently, Schistosoma will likely be affected by global climate change, and thus, there is a growing need to quantify human exposure rates to cercariae as a function of temperature. Mathematical models often estimate exposure by combining cercarial longevity with snail shedding rates, cercarial density, and water volume. However, this approach fails to account for distance cercariae can travel, which must influence the probability of cercariae encountering and infecting human hosts. To address this shortcoming, cercariae of Schistosoma mansoni were used to examine the effects of temperature and viscosity on swimming behavior. A high-speed camera was used to capture videos through an observation cuvette at 10°C, 20°C, 30°C, and 30°C with methylcellulose polymer (MC), which maintains viscosity at 10°C despite the higher temperature. Image J was used to calculate tail beat frequency, defined as one full tail movement, cercarial speed, and distance travelled.
Tail beat frequency doubled with each increase in 10°C. Viscosity had no effect on tail beat frequency. Conversely, cercarial swimming speed doubled with each increase in 10°C until 30°C, but decreased in the MC treatment. Although cercarial swimming speed in the MC treatment did not decrease to levels found at 10°C, total distance travelled was reduced compared to total distance travelled at 30°C. These results indicate that high viscosity at low temperatures partially counteracts tail beat frequency, which ultimately decreases the distance cercariae can travel. That is, low viscosity at warmer water temperatures allows cercariae to travel farther than individuals in colder water temperatures; subsequently, the probability of encountering and infecting a human host increases with temperature. These findings will be combined with temperature-dependent cercarial longevity, snail densities, and snail shedding rates to estimate the temperature-dependent cercarial exposure rates of humans. Ultimately, the parameters measured in this study provide novel insights for quantifying infection risk in countries endemic with schistosomiasis.