To mitigate the effects of zoonotic diseases on human and animal populations,
it is critical to understand what factors alter transmission dynamics. Here we
assess the risk of exposure to lethal concentrations of the anthrax bacterium,
Bacillus anthracis, for grazing animals in a natural system over time through
different transmission mechanisms. We follow pathogen concentrations at
anthrax carcass sites and waterholes for five years and estimate infection
risk as a function of grass, soil or water intake, age of carcass sites, and
the exposure required for a lethal infection. Grazing, not drinking, seems the
dominant transmission route, and transmission is more probable from grazing at
carcass sites 1–2 years of age. Unlike most studies of virulent pathogens that
are conducted under controlled conditions for extrapolation to real
situations, we evaluate exposure risk under field conditions to estimate the
probability of a lethal dose, showing that not all reservoirs with detectable
pathogens are significant transmission pathways