Abstract:
Environmental factors are common forces driving infectious disease dynamics.
We compared interannual and seasonal patterns of anthrax infections in two
multihost systems in southern Africa: Etosha National Park, Namibia, and
Kruger National Park, South Africa. Using several decades of mortality data
from each system, we assessed possible transmission mechanisms behind
anthrax dynamics, examining (1) within- and between-species temporal case
correlations and (2) associations between anthrax mortalities and environmental
factors, specifically rainfall and the Normalized Difference Vegetation
Index (NDVI), with empirical dynamic modeling. Anthrax cases in Kruger had
wide interannual variation in case numbers, and large outbreaks seemed to
follow a roughly decadal cycle. In contrast, outbreaks in Etosha were smaller
in magnitude and occurred annually. In Etosha, the host species commonly
affected remained consistent over several decades, although plains zebra (Equus quagga) became relatively more dominant. In Kruger, turnover of the
main host species occurred after the 1990s, where the previously dominant
host species, greater kudu (Tragelaphus strepsiceros), was replaced by impala
(Aepyceros melampus). In both parks, anthrax infections showed two seasonal
peaks, with each species having only one peak in a year. Zebra, springbok
(Antidorcas marsupialis), wildebeest (Connochaetes taurinus), and impala
cases peaked in wet seasons, while elephant (Loxodonta africana), kudu, and
buffalo (Syncerus caffer) cases peaked in dry seasons. For common host species
shared between the two parks, anthrax mortalities peaked in the same season
in both systems. Among host species with cases peaking in the same season,
anthrax mortalities were mostly synchronized, which implies similar transmission
mechanisms or shared sources of exposure. Between seasons, outbreaks
in one species may contribute to more cases in another species in the following
season. Higher vegetation greenness was associated with more zebra and
springbok anthrax mortalities in Etosha but fewer elephant cases in Kruger.
These results suggest that host behavioral responses to changing environmental
conditions may affect anthrax transmission risk, with differences in transmission
mechanisms leading to multihost biseasonal outbreaks. This study
reveals the dynamics and potential environmental drivers of anthrax in two savanna systems, providing a better understanding of factors driving
biseasonal dynamics and outbreak variation among locations.