New maps reveal first global estimate of Anthrax risk

Bacillus anthracis bacteria recovered from a white-tailed deer growing on sheep blood agar
Bacillus anthracis bacteria recovered from a white-tailed deer growing on sheep blood agar

Newly published maps reveal, for the first time, where anthrax poses global risks to people, livestock and wildlife. The maps 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.

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 some of 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 used historical records, news of active outbreaks, existing published maps and original field data to create a new, comprehensive database with GPS coordinates for each record of occurrence. The team then combined the known occurrences of anthrax outbreaks with 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 Ian Kracalik, a Centers for Disease Control epidemic intelligence service officer (and former doctoral student of Blackburn’s, UF ’17), and Colin Carlson, an environment initiative postdoctoral researcher at Georgetown University, led the modeling portion of the project. Kracalik performed his portion of the work while earning a PhD in medical geography in Blackburn’s Spatial Epidemiology and Ecology Research Lab.

Global maps, local insights

Global environmental suitability for Bacillus anthracis. Map A is the most conservative, Map B is the most pragmatic, and Map C is the most liberal estimate for the probability of where B. anthracis occurs. Mapping the pathogen is a proxy for understanding where disease may occur in people, livestock and wildlife. 

“Dr. Blackburn’s anthrax database gave us an incredibly detailed dataset on where we’ve seen anthrax not just in humans but also in animals — in our livestock, and in wildlife,” Carlson says. “We used machine learning to predict where we see anthrax based on habitat traits like alkaline, sandy soils. That lets us say, who lives in areas of anthrax risk, and how protected are they by vaccination? How many people, how many livestock, and how many vulnerable wildlife species?”

By asking these questions of their models, 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.

“The big result that 64 million poor livestock keepers live in anthrax areas is staggering,” Carlson says. “It means we have a lot of work left to do to map anthrax in a lot of places, and to work on interventions.”

The maps will help inform health workers and policymakers. “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.”

Strategic vaccines

Though an effective livestock vaccine exists, it must be given annually 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, south and east Asia tend to have vaccinations rates ranging from >1 percent to 6 percent, 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 our 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.

Reporting figures are even weaker for wildlife and livestock worldwide. While livestock are tended by humans, wildlife are not. Many wildlife may sicken or die, and the diseases is never observed by people; unless there is a headline-grabbing mass die off as sometimes happen with reindeer or antelope.

“Unfortunately, there is a big problem with anthrax reporting in that it skews to people,” Blackburn says. “In most cases, you expect to see more animals infected than people, but in reporting we see more human cases noted. This creates a challenge for understanding where to focus control efforts.”

But several of Blackburn’s prior studies clearly show that vaccinating livestock is protective for people. In 1996, Azerbaijan implemented a compulsory livestock vaccine and their human anthrax cases declined dramatically. In a similar vein, the elimination of a compulsory livestock vaccine in the Eurasian country of Georgia resulted in a subsequent spike in human cases.

The human anthrax vaccine is very costly and requires multiple doses; it tends to be reserved for laboratory personnel and emergency first responders who have bioterrorism roles. It is not appropriate for use as a preventative vaccine in local communities.

Wildlife and anthrax

Senior author Jason Blackburn sampling bacteria from the nasal turbinates of a deer skull during an anthrax investigation. Photo courtesy of SEER Lab.

The new 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 these wildlife to prevent spillover events in either direction.”

It can also help inform land managers of best practices for carcass disposal in anthrax prone areas: burning reduces the pathogen’s spread far more effectively than burying.

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

Flies may also play a role in moving the bacteria around: carrion-feeding flies are thought to pick it up while scavenging infected carcasses, then move it to grasses or browse which are then eaten by hoofed mammals. “Likewise, biting flies may play a role in amplifying transmission by acquiring the pathogen when biting one animal during an active outbreak, then slipping it into subsequent animals through the mechanical injection of a bite,” Blackburn says.

The life cycle of B. anthracis necessitates being eaten and moving through a mammal’s digestive tract where it transitions from the spore form that persists in the environment to vegetative cells, which cause the anthrax infection. After the animal dies, the vegetative cells sporulate and those spores are reintroduced into the soil, where it can reside undisturbed for years. It can then be ingested by another mammal and infection begins anew within their fluid-rich body.

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

Looking forward

Carlson says he hopes that the work will move the needle on people’s perception of anthrax away from the newsy anomalies of thawing permafrost or bioterrorism threats to a more global perspective of connections between human and animal health.

“Anthrax cases in humans are preventable, and tend to be a product of poor, vulnerable folks living in close proximity to the environment,” Carlson says. “Hopefully having a better inventory of that will let us do more to help them, and shift some focus away from flashier emerging diseases to the more neglected ones.”

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.”

By: DeLene Beeland