A Multifactorial Investigation of the Role of Water in Shaping Strongyle Parasite Burden Risk in Grevy’s and Plains Zebras

Abstract

The dynamics of host-parasite systems are complex and often shaped by environmental conditions that govern the likelihood of host infection. The Laikipia Plateau, Kenya provides a particularly useful landscape for the study of how environmental conditions shape host burdens of the fecal-oral transmitted strongyle parasite because of the semiannual changes in rainfall that have landscape-wide consequences. While strongyle parasites infect both Grevy’s and Plains zebras, a discrepancy exists in the species’ burdens; specifically, Plains zebras typically exhibit higher parasite loads than Grevy’s zebras (Rubenstein, 2010). Given the extensive negative impacts on fecundity and survival that strongyle infections above a critical threshold can have on zebra health (Rubenstein, 2010), this study sought to understand the underlying interactions that shape this discrepancy in parasite burden by examining how water influences zebra risk of infection on both the host and parasite sides of transmission. Through field and laboratory work conducted at the Mpala Research Centre in Laikipia, Kenya, host fecal egg counts and home range distances to water across species and seasons (wet to dry) were measured in conjunction with habitat usage and the effects of microclimates on strongyle egg survival.Upon synthesis of the data, this study found a strong inverse relationship between host distance to water and parasite burdens. This relationship is attributed to the greater presence of dung found in close proximity to water sources, which in turn is shaped by the spatial distributions of hosts. These spatial distributions are governed by the water requirements of each species. Plains zebras are more water-restricted, meaning they must remain closer to water, while Grevy’s zebras’ ability to survive for longer periods of time without water allow them to range farther from shared water sources (Rubenstein, 2010). Given these differences in water use, Plains zebras are more concentrated on landscapes around water and more likely to graze on infected vegetation in these areas of elevated risk. Season was also found to be an influencing but secondary factor for risk of infection. During the dry season, water availability decreased and the spatial distribution of Plains zebra across the landscape contracted; however, parasite burdens decreased rather than increased. Since strongyle survival and dispersal are dependent on available moisture (Rossanigo and Gruner, 1994), the lack of rain and decrease in available moisture is believed to be responsible for decreased parasite burdens between seasons. Study of strongyle survival across microclimate conditions corroborated the role of parasite survival in shaping host risk of infection by demonstrating the importance of moisture and grass (as a sheltering mechanism) for maximizing the number of strongyle on the landscape. The study concludes that the availability of water throughout the landscape governs host spatial distributions and strongyle survival, shaping the likelihood of transmission and determining infection risk. Given the endangered status of Grevy’s zebras and increasing threat to zebra rangelands, understanding the formative role that water plays in determining host risk of infection and subsequent health is essential in crafting effective conservation policies and programs in the face of changing environmental conditions and resource availability. Finally, the findings of this study can be applied in persuading pastoral herders to cease culling zebras by coupling a non-pharmaceutical parasite reduction strategy found in this study with the facilitative relationship between wildlife and livestock. Given the elevated presence of dung found within 250m of water and the possibility for parasite transmission between wildlife and livestock, pastoral herders should queue their herds outside of this zone and spend as little time in close proximity to water as possible. By reducing infection risk, herders can maximize the weight and milk production of their livestock, which will medically and economically benefit herders and their families.