Study finds elk habitat overlaps with potential anthrax reservoir in Montana

Multiple anthrax bacteria are being enveloped by an immune system cell.
Multiple anthrax bacteria (green) are being enveloped by an immune system cell (purple) — Camenzind G. Robinson, Sarah Guilman and Arthur Friedlander, United States Army Medical Research Institute of Infectious Diseases.

Human cases of anthrax are rare in the United States, yet the bacterial spores that cause the disease have the potential to persist in the soil in southwest Montana and other regions where grazing animals can come into contact with them.

In a recent study UF researchers observed significant overlap between areas of southwestern Montana with a high probability of male elk habitat use and areas that are potential reservoirs for Bacillus anthracis, the pathogen that causes anthrax. These findings suggest that there are large regions across the landscape where elk and other susceptible hosts could come into contact with the pathogen.

“Anthrax carcasses can be hard to find, which leads to challenges for anthrax surveillance,” said Lillian Morris, a doctoral student in the Spatial Epidemiology and Ecology Research Laboratory (SEER Lab) at the University of Florida and the first author of the study. “For example, the elk population in our study area is spread out across the region and spends time in a steep, densely forested habitat that is difficult to navigate.”

The study, titled “Elk resource selection and implications for anthrax management in Montana” and published in “The Journal of Wildlife Management,” aims to provide information to help focus surveillance efforts on areas where there is a high probability of overlap between elk habitats and anthrax risk zones.

Anthrax outbreaks in wildlife have been reported frequently in recent years. Previous studies have determined that wildlife surveillance and decontamination of carcasses are the best tactics for preventing the spread of the disease.

“We need to be better at defining surveillance areas,” Jason Blackburn says in a Wildlife Society press release. Blackburn is the senior author in the study and an associate professor of geography at UF’s Emerging Pathogens Institute (EPI). “Anthrax hasn’t been a part of surveillance in the region since it hasn’t been reported there in a long time. Essentially, we’ve stopped looking for it.”

Male elk were of particular concern in the study based on the outcome of a large anthrax outbreak in southwestern Montana in 2008. The disease caused several hundred deaths in a domestic bison herd and upwards of 45 deaths among the free-ranging male elk of a wild elk herd in the region. No female elk were found dead despite a herd of approximately 2200 present during the outbreak.

Disease control in livestock relies on vaccination with an injectable vaccine. However, vaccinating free ranging wildlife is not feasible, Blackburn says, so disease control efforts rely on identifying carcasses from previous anthrax deaths and decontaminating those carcasses via bleaching, burning, or burial. To reduce the risk of disease in wildlife, it is essential to identify where carcasses are most likely and identify how wildlife are using those areas.

“We found overlap between high probability elk habitats and anthrax risk zones in extensive regions across the study area,” Morris said. “These regions of overlap were on federal, state, and private land.”

The bison herd is limited to a single, large private ranch encircled by fences. The elk, in contrast, are not restricted by fences. “Our study found that elk are likely to use private, state, and federal lands,” Morris said.

Anthrax outbreaks are most likely between June and August in Montana. Morris and Blackburn, along with collaborators from Montana Fish, Wildlife and Parks and Turner Enterprises, Inc., used GPS telemetry and environmental data to develop models describing elk habitat selection throughout the region. They then used an ecological niche model to estimate potential anthrax risk zones. B. anthracis has an affinity for moist, low-lying soil, for example, so this was taken into account in the model along with several other factors.

Morris emphasized the need for cooperation between both public and private-sector interests in order to prevent future spread of the disease.

“The pathogen can continue to circulate if all stakeholders aren’t involved in the decontamination of carcasses and appropriate surveillance efforts,” Morris told The Wildlife Society.

This study is part of a larger research project funded by the National Institute of Health Ecology and Evolution of Infectious Diseases program to predict the likelihood of anthrax outbreaks with greater accuracy. In The Wildlife Society press release, Blackburn suggested that with additional research, the SEER lab will improve its ability to predict the geographic distribution of B. anthracis reservoirs and the risk of anthrax to both domestic livestock and wildlife populations.

“In the future, we will create mathematical models using these movement studies as inputs to understand the likelihood and severity of future outbreaks and understand ecological conditions to estimate the intensity and timing of anthrax outbreaks, so we will know where and when to stage intervention,” Blackburn said. “That’s the larger study’s ultimate goal.”

The larger study, titled “Spatio-temporally explicit estimation of R₀ for pathogens with environmentally mediated transmission,” is a collaboration with Drs. Sadie Ryan (UF Geography), Jose Miguel Ponciano (UF Biology), Robert Holt (UF Biology) and Wayne Getz (UC Berkeley).