Cooperative Fish and Wildlife Research Units Program: Wisconsin Wildlife
Education, Research and Technical Assistance for Managing Our Natural Resources

Seasonality in wildlife infectious diseases is common, however bi-seasonality or bimodal seasonality in infections is less commonly reported. Bi-seasonality may involve multi-host interactions and environmental drivers, with multiple triggers. We explored patterns in anthrax infections in two multi-host systems in southern Africa: Etosha National Park, Namibia, and Kruger National Park, South Africa. We assessed possible transmission mechanisms behind anthrax dynamics, by examining 1) within- and between-species correlations of cases, and 2) associations between anthrax mortalities and environmental factors (rainfall and a remotely-sensed vegetation index). Anthrax cases in Kruger had wide inter-annual variation in intensity, and large outbreaks seemed to follow a largely decadal cycle. In contrast, anthrax outbreaks in Etosha were smaller in magnitude and occurred annually. In Etosha, the host species composition of outbreaks remained consistent over several decades, although plains zebra (Equus quagga) became more dominant. In Kruger, turnover of the main host species occurred, where the previously dominant host species, greater kudu (Tragelaphus strepsiceros), was replaced by impala (Aepyceros melampus), a species that rarely contributed to historical outbreaks. In both parks, anthrax infections showed two seasonal peaks, with each species having only one peak in a year. Anthrax mortalities in host species shared between the two parks peaked in the same season in both systems. Even though the relationships among species may be complex, especially in Kruger, species within the same seasonal (wet or dry) group could have synchronization of anthrax mortalities, implying common or similar transmission mechanisms or infectious sources. We found that with higher vegetation greenness, there were more zebra anthrax mortalities in Etosha, whereas there were fewer elephant cases in both parks. These suggest that host behavioral responses to environmental conditions are important for anthrax transmission. Comparing across species and systems, different seasonal peaks are potentially governed by divergent mechanisms of anthrax transmission and dynamics. This divergence of mechanisms is likely to lead to the multi-host anthrax seasonality and dynamics. This study reveals anthrax dynamics and potentially their driving forces in the two parks, which may help us better understand anthrax transmission worldwide.