Soil ingestion, nutrition and the seasonality of anthrax in herbivores of Etosha National Park
Date
2013-01Author
Turner, Wendy C
Imologhome, Peace
Havarua, Zepee
Kaaya, GP
Mpune, John K E
Mpopu, Irven D T
Getz, Wayne M
Type
ArticleLanguage
enMetadata
Show full item recordAbstract
Anthrax, caused by the bacterium Bacillus anthracis, is a seasonally occurring infectious disease
affecting primarily herbivorous wildlife and livestock. The seasonality of anthrax outbreaks varies among
locations, making it difficult to develop a single consistent ecological description of this disease. Over 44
years of mortality surveillance, most anthrax cases in Etosha National Park, Namibia are observed in the
wet season, although elephants have an anthrax mortality peak in the dry season. Focusing on three host
species (plains zebra, Equus quagga; African elephant, Loxodonta africana; and springbok, Antidorcas
marsupialis) occupying the endemic anthrax area of Etosha National Park, Namibia, we tested two
commonly posited causes of anthrax seasonality in herbivores: increased pathogen exposure due to greater
soil contact, and increased host susceptibility due to seasonal nutritional stress. These hypotheses were
assessed using fecal sampling and measurement of the percentage of fecal silicates as an index of soil
ingestion and fecal nitrogen, phosphorus and crude fiber as nutritional indices. Nutritional quality for all
three species was higher in wet than dry seasons. Comparing among wet seasons, nutritional indices
showed either a decline in nutrition with increasing rainfall or no significant pattern. All three species had
greater soil ingestion in the wet season than the dry season. Higher soil contact during the anthrax peak
suggests that anthrax seasonality may in part be due to heightened exposure to B. anthracis in wet seasons,
for zebra and springbok. Elephant anthrax deaths do not correspond with the season of increased soil
ingestion or grazing, suggesting that other behavioral mechanisms may overshadow foraging-based risk
factors for this species. Nutritional stress is unlikely the primary causative factor in wet season anthrax
systems, although nutritional stress sufficient to reduce resistance is difficult to assess non-invasively in
wild herbivores. In contrast, increased soil ingestion may be an important predisposing factor for wet
season anthrax outbreaks. Ultimately, the amount of soil ingested and its importance in the transmission of
soil-borne pathogens will vary based on foraging behaviors, intake rates, grassland structure and on the
likelihood that foraging areas intersect with pathogen aggregations in the environment.
Citation
Ecosphere 4(1):13Publisher
school of biological sciences
Subject
anthraxAntidorcas marsupialis
Bacillus anthracis
Equus quagga
Etosha National Park
Namibia
nitrogen
fecal phosphorus
Loxodonta africana
nutrition
seasonality
soil ingestion