DOI: 10.5061/dryad.zkh1893br
title: Environmental drivers of biseasonal anthrax outbreak dynamics in two multi-host savanna systems

description: Environmental factors are common forces driving infectious disease
 dynamics. We compared inter-annual and seasonal patterns of anthrax
 infections in two multi-host 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 case correlations, and 2) associations between anthrax
 mortalities and environmental factors, specifically rainfall and the
 Normalized Difference Vegetation Index (NDVI). Anthrax cases in Kruger had
 wide inter-annual variation in case numbers, and large outbreaks seemed to
 follow roughly a 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 may imply 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 multi-host
 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.
description: Animal mortality surveillance in Etosha National Park, Namibia
 and Kruger Naitonal Park, South Africa is opportunistic. Anthrax mortality
 in the datasets included anthrax confirmed cases from blood smear
 examination, bacterial culture or molecular diagnosis from blood swabs, as
 well as anthrax suspected cases diagnosed by symptoms (i.e., blood
 exudation) in cases where no samples were collected. We obtained anthrax
 mortality data from 1976 – 2014 in central Etosha and 1990 – 2015 in
 northern Kruger through the Etosha Ecological Institute and Office of the
 State Veterinarian in Kruger. We removed mortalities for which carcass
 discovery involved tracking the locations of scavengers with GPS (Global
 Positioning System) tags if the information was available, to better
 standardize surveillance efforts over time. We also inlcuded data for
 environmental fluctuations, including rainfall and NDVI (Normalized
 Difference Vegetation Index). Rainfall data were recorded at Okaukuejo
 station in central Etosha and Pafuri station in northern Kruger, and NDVI
 data were extracted from the Global Inventory Modeling and Mapping Studies
 (GIMMS) NDVI<sub>3g</sub> dataset. Anthrax case, rainfall, and
 NDVI data were binned into two intervals per month: days 1-15 and day 16
 to the month's end.
description: These files include: 1. Anthrax mortalities
 and environmental variables in bimonthly intervals from central Etosha
 National Park 1976-2014 2. Anthrax mortalities and
 environmental variables in bimonthly intervals from northern Kruger
 National Park 1990-2015 3. Rainfall in bimonthly
 intervals from Pafuri station 1953-1991 4. R code for
 analyses (two scripts)