The causes underlying dynamics in infectious diseases are frequently perplexing. Many different mechanisms may cause pathogens or vectors to vary in their reproduction and survivability in environments. On the other hand, host-dependent causes also include a variety of factors. In response to changes of weather, predation pressure, or food availability, organisms adjust behaviorally and physiologically. Behavioral change in turn alters disease transmissibility  mostly owing to change in the contact frequency between hosts and pathogens. Shifts in host immune function due to factors such as weather, nutrition, or breeding can cause variation in host susceptibility. In addition, reproduction also results in a change of transmission due to a pulse in recruitment of new immunologically naïve host individuals. My PhD research is investigating the contribution to anthrax dynamics in herbivorous wildlife by changes in weather, vegetation, and host movement patterns in Etosha National Park, Namibia and Kruger National Park, South Africa. The study starts with temporal dynamics in the both parks and reveals the spatially anthrax dynamics in Etosha as well.

The first study is an analysis of long-term datasets on rainfall, NDVI, and anthrax mortalities. It has been documented that environmental variation, specifically precipitation and vegetation, are related to anthrax dynamics. However, in many wildlife-anthrax systems, how and why environments are associated to anthrax outbreaks are largely unknown. Revealing how environmental changes relate to outbreaks, and at what time scales, help us potentially pinpoint the transmission mechanisms. Thus, this study analyzes the associations between anthrax outbreaks and rainfall and NDVI for four and five host species in the two parks, respectively.

Though environmental fluctuations are related to anthrax temporal dynamics, environment is mostly likely a distal cause, and the causality may involve animal behavioral change. Herbivores are known to switch their foraging or habitat selection strategies in response to environmental change, leading to different exposure to pathogens over time/seasons. Following the first study exploring associations with environment, my second study investigates the patterns in habitat selection of the zebra population in Etosha across low to high risk habitats, in response to different rainfall and vegetation index.

We collared zebra in Etosha (as described in the previous study), as well as wildebeest in Etosha and impala and kudu in Kruger. They are the most common host species for anthrax in the two parks. We found that the host species in the two parks have really different movement patterns. Meanwhile, the two parks also show different patterns in anthrax dynamics. My last study aims to use simulation to test whether and how host movement and distribution patterns drive disease dynamics.

Ph.D. Student in Wildlife Ecology
University of Wisconsin-Madison

Yen-Hua Huang, D.V.M