New maps reveal first global estimate of anthrax risk

Newly published maps reveal, for the first time, where anthrax poses global risks to people, livestock and wildlife. Popularly viewed as a frightening airborne agent of bioterrorism, the bacteria that causes anthrax infections naturally occurs in the soil on every continent and some islands.

The maps, published today in Nature Microbiology, are the result of 15 years of data collection covering 70 countries compiled by Emerging Pathogens Institute associate research professor Jason Blackburn and his colleagues. Until now, the geographic distribution of anthrax has not been mapped globally.

“Our main purpose was to describe where anthrax occurs, or is likely to occur, across the globe, and to illustrate sub-national areas where surveillance is necessary,” Blackburn says. “Anthrax is a disease that affects both animals and humans, and it is most commonly associated with rural and agricultural communities which contend with it nearly worldwide. Our maps will help countries and health authorities focus on specific anthrax-prone areas to target control and surveillance.”

Blackburn, who is also an associate professor in the University of Florida’s geography department, and his team mapped known occurrences and suitable habitat of Bacillus anthracis, the bacteria that causes anthrax, as a proxy for disease risk. They then overlaid this with models showing where people, livestock and susceptible wildlife occur.

Co-first authors Colin Carlson, an environment initiative postdoctoral researcher at Georgetown University, and Ian Kracalik, a Centers for Disease Control epidemic intelligence service officer (and former doctoral student of Blackburn’s) led the modeling portion of the project.

The team found that 1.83 billion people inhabit anthrax risk areas, but they trimmed this down to 63.8 million workers who engage in agricultural occupations which place them at heightened risk of contracting the disease. Likewise, they mapped livestock by species type and found that anthrax-vulnerable areas contain 1.1 billion livestock.

“This tells medical clinicians and veterinarians that if they are inside a predicted zone, anthrax should be on their routine diagnostic list, or their annual vaccination list, respectively,” Blackburn says. “The best way to protect people is to routinely vaccinate livestock.”

Though an effective livestock vaccine exists, it must be given yearly and the maps will help authorities prioritize using it where there is the greatest livestock-people interface. The pathogen can persist in a nearly-indestructible spore form in soils for several years to decades, which means that sustained vaccination campaigns are necessary in some regions even when there are no known outbreaks.

“There's some incredible patterns in our results,” Carlson says, “such as how the legacy of Soviet vaccination campaigns persists to today. But there's also some very visible impacts of poverty in places where anthrax is hyper-endemic, and I hope our work draws more focus towards that.”

Areas of sub-Saharan Africa, and south and east Asia tend to have vaccinations rates ranging from >1% to 6% — but the researchers found that these regions disproportionately account for 48.5 million rural poor livestock keepers. “Internationally, we need to do a better job of getting the vaccine to high-risk areas,” Blackburn says.

Between 20,000 to 100,000 people report anthrax annually around the world, and most occurrences are in poor or rural areas. It is rare for someone in the U.S. to contract it, due in part to high rates of livestock vaccination in disease-prone areas. But these numbers are incomplete because many subclinical cases go unreported for various reasons, including: stigma, fear of quarantine or lost income, or lack of an accurate diagnosis. Reported figures are even weaker worldwide for livestock and wildlife.

The research has implications for wildlife of conservation priority, such as wood bison and saiga antelope, as well as economically-important game animals such as white-tailed deer. It is not feasible to vaccinate wildlife, Blackburn notes, “But knowing where anthrax is likely to occur can inform hunters, as well as livestock growers, who have close proximity to this wildlife, to prevent spillover events in either direction.”

Outbreaks occur when hoofed mammals consume the soil-dwelling B. anthracis, which sticks to grasses or browse. In the U.S., wildlife and livestock outbreaks tend to occur between May and October.

People most commonly acquire B. anthracis through skin lesions when they handle or butcher infected carcasses. Eating infected meat can also cause illness and lead to deadlier outcomes. How sick someone becomes is determined by how they were exposed.

Blackburn’s future work will examine the genetic diversity of B. anthracis worldwide and the geographic factors associated with specific genetic strains. “This species can be found in many different environmental conditions,” Blackburn says. “Knowing how the strains are distributed, and how effective the vaccine is against different strains, will help us to further refine our risk maps.”

DeLene Beeland May 13, 2019