Can scavengers save zebras from anthrax? A modeling study

Namibia's Etosha National Park (ENP) is home to many different animals such as lions, jackals, hyenas, zebras, elephants, etc. Each year, grazing animals are infected and die from anthrax caused by the bacteria Bacillus anthracis. This increases the number of carcasses in the park, which serve as food for scavengers such as jackals. This study investigates the interplay between anthrax transmission in zebras and the scavenging of zebra carcasses in ENP, using a deterministic mathematical model to describe the population dynamics. We strive to answer the following research questions: Under what conditions can the presence of scavengers control anthrax outbreaks in zebra populations? Does carcass production by anthrax help or hurt scavengers in the long term? Standard qualitative analysis techniques distinguished outcomes (stable equilibria) using reproduction numbers as threshold quantities. We found that, when scavengers feed on anthrax-laden carcasses, the scavengers help the zebras, by eliminating potential infection zones for the zebras. In this way they reduce anthrax's spread by orders of magnitude. We also identify conditions under which the presence of anthrax benefits the scavengers, in terms of death-to-birth ratios for zebras, scavengers and anthrax.