Anthrax is a silent killer on the African savanna, quietly knocking off zebras and other herbivores. Now, a team of biologists has observed a way in which anthrax keeps itself in circulation in Etosha National Park, a sprawling wilderness area situated in northwestern Namibia. The carcasses of animals that have died of anthrax fertilize patches of grass, making them particularly attractive to other animals, even though they're laced with the pathogen.
Scientists have long understood the complex life cycle of the soil-borne anthrax pathogen Bacillus anthracis, which starts out as a hardy spore that is able to linger for years in the environment and eventually matures into its active state once it finds its way inside the body of an unsuspecting host. But there are still many unanswered questions about how this pathogen is transmitted and why certain species, namely zebra, are disproportionally affected. In 2007, Wendy Turner, a disease and wildlife ecologist working with Wayne Getz at the University of California, Berkeley, was reviewing her photos of the park and noticed that the vegetation growing around zebra carcasses had an especially lush and green appearance. The idea dawned on Turner, now at the University of Oslo, to investigate the potential role of these carcasses in anthrax transmission.
Her first step was to study the effect of the pathogen on vegetation. In a series of experiments, she added B. anthracis spores to the soil of native grass plantings and showed that the seeds were 50% more likely to germinate than controls, she reported in PLOS Neglected Tropical Diseases in June. This work suggested a symbiotic relationship between savanna grasses and anthrax transmission. But where do the zebras come in?
To study how zebras interact with anthrax-infected grass, Turner and Getz headed to Etosha. She chose the park as her study site because authorities there do not make any effort to contain anthrax; thus, natural transmission routes can be observed. A total of 26 grazing sites were selected across an area of 300 square kilometers. At 13 sites, there were corpses of zebras determined to have died from anthrax, and the other 13 sites served as controls. Motion-sensing camera traps were mounted at each site to capture animal visitations, and researchers later analyzed the photos to see if the animals were grazing or just passing by. Meanwhile, sites were tested annually for the presence of anthrax on vegetation and in the soil. “There has never been a study that looked for B. anthracis on grasses that animals could actually contact through grazing,” Turner says.
A total of 1.2 million photographs were taken over a 3-year period. Careful analysis showed animals were as likely to visit the anthrax-laden sites as the control sites. But they were four times more likely to graze at the anthrax sites, exposing themselves to the deadly pathogen, Turner and colleagues report online today in the Proceedings of the Royal Society B. The strongest preference for munching on infected grass was observed in zebras, which also have the highest incidence of anthrax infection, followed by wildebeests and springboks.
“This paper provides insight into at least a part of the circle of disease transmission,” says Wolfgang Beyer, a molecular epidemiologist at the University of Hohenheim in Stuttgart, Germany. Still, he notes that this work is specific to Etosha and may not explain anthrax outbreaks among grazing animals in other areas of the world like northern Canada, China, Texas, and Turkey. Next, Turner plans to look at animal movement patterns and weather to understand what triggers outbreaks on a wider geographic and ecological level.
*Correction, 1 October, 6:04 p.m.: The story has been corrected to reflect that there were no zebra corpses at the control sites in the study and that the work was done in collaboration with researchers at the University of California, Berkeley.