The global distribution of Bacillus anthracis and associated anthrax risk to humans, livestock, and wildlife

Abstract

SummaryBacillus anthracis is a spore-forming, Gram-positive bacterium responsible for anthrax, an acute and commonly lethal infection that most significantly affects grazing livestock, wild ungulates and other herbivorous mammals, but also poses a serious threat to human health1, 2. The geographic extent of B. anthracis endemism is still poorly understood, despite multi-decade research on anthrax epizootic and epidemic dynamics around the world3, 4. Several biogeographic studies have focused on modeling environmental suitability for anthrax at local or national scales5–9, but many countries have limited or inadequate surveillance systems, even within known endemic regions. Here we compile an extensive global occurrence dataset for B. anthracis, drawing on confirmed human, livestock, and wildlife anthrax outbreaks. With these records, we use boosted regression trees10, 11 to produce the first map of the global distribution of B. anthracis as a proxy for anthrax risk. Variable contributions to the model support pre-existing hypotheses that environmental suitability for B. anthracis depends most strongly on soil characteristics such as pH that affect spore persistence, and the extent of seasonal fluctuations in vegetation, which plays a key role in transmission for herbivores12, 13. We apply the global model to estimate that 1.83 billion people (95% credible interval: 0.59—4.16 billion) live within regions of anthrax risk, but most of that population faces little occupational exposure to anthrax. More informatively, a global total of 63.8 million rural poor livestock keepers (95% CI: 17.5—168.6 million) and 1.1 billion livestock (95% CI: 0.4—2.3 billion) live within vulnerable regions. Human risk is concentrated in rural areas, and human and livestock vulnerability are both concentrated in rainfed systems throughout arid and temperate land across Eurasia, Africa, and North America. We conclude by mapping where anthrax risk overlaps with vulnerable wild ungulate populations, and therefore could disrupt sensitive conservation efforts for species like bison, pronghorn, and saiga that coincide with anthrax-prone, mixed-agricultural landscapes. Anthrax is a zoonotic disease caused by the Gram-positive bacterium Bacillus anthracis, a generalist soil-transmitted pathogen found on every inhabited continent14, and several islands including Haiti and parts of the Philippines and Indonesia. Worldwide, an estimated 20,000 to 100,000 cases of anthrax occur annually, mostly in poor rural areas15. In clinical presentations of anthrax, case fatality rates are a function of exposure pathway. Respiratory exposure from spore inhalation is important the context of bioterrorism, but is highly uncommon, and accounts for a negligible fraction of the global burden of anthrax cases. Cutaneous exposure to B. anthracis accounts for the majority of human cases worldwide, and typically presents with low mortality; gastrointestinal exposure accounts for the remainder and presents with intermediate to high fatality rates. Cutaneous and gastrointestinal cases of anthrax are most commonly caused by handling and slaughtering infected livestock, or butchering and eating contaminated meat; untreated gastrointestinal cases likely account for most human mortality from anthrax.14–16

Potential distributions of Bacillus anthracis and Bacillus cereus biovar anthracis causing anthrax in Africa

“…The worldwide distribution of B. anthracis is a characteristic derived directly from its ability to form spores that are resistant to diverse environmental conditions [1,4,7,14,15]. The ability of spores to persist for years and even decades in the environment is well-documented, including several reports of spore survival lasting >50 years [1,4,7,[16][17][18].…”

“…The potential distribution of B. anthracis links closely to locations of previous anthrax outbreaks, but understanding of environmental factors leading to emergence and risk across geographic regions remains incomplete. Although B. anthracis has been documented from multiple continents and is considered to have a global distribution [15,17], Bcbva infections have been identified only within sub-Saharan Africa [27,28]. Empirical evidence suggests that Bcbva infections occur in subtropical humid environments, primarily in non-human primates, whereas B. anthracis infections occur more commonly in arid regions, and so far, the pathogen has been found in a broader variety of host species, although susceptibility varies among them; identification of additional Bcbva host species will likely increase with further investigations [7,8,16].…”

Bacillus anthracis Sterne Strain 34f2 Vaccine Antibody Dose Response by Subcutaneous and Oral Administration

“…In North America, anthrax has been documented in white-tailed deer, elk, bison, bears, and other native and exotic species [23,25]. Inconvenient outbreaks in wildlife cause economic, ecological and conservational burdens worldwide [3,26]. Despite the availability of a safe and effective animal vaccine, a recently proposed model for the global distribution and suitability of anthrax found that most livestock and wildlife at risk of exposure are not vaccinated [26].…”

“…Inconvenient outbreaks in wildlife cause economic, ecological and conservational burdens worldwide [3,26]. Despite the availability of a safe and effective animal vaccine, a recently proposed model for the global distribution and suitability of anthrax found that most livestock and wildlife at risk of exposure are not vaccinated [26]. This may be due to the fact that most commercial animal vaccines use the same formulation developed by Max Sterne in 1939 which is outdated, can cause adverse reactions, requires yearly boosters and requires refrigerated storage [27,28].…”

Knowledgeof anthrax infection prevention and its associated factors among livestock owners in Sekotazuria district, Wag-himra zone, northeast, Amhara region Ethiopia: Cross Sectional Study

“…Anthrax is a serious disease of major health or economic importance and it is estimated that about 1.83 billion people live within regions of anthrax risk areas [12]. Literatures reported that adequate knowledge on anthrax prevention is considered as one of the strategy for anthrax prevention in humans [13,14].…”