Anthrax is an infectious, often fatal disease of wild and domestic animals and humans that is caused by the endospore-forming, soil-borne bacterium Bacillus anthracis. It is primarily a disease in herbivores and sometimes sparks an outbreak in human beings with potentially serious consequences (1). Herbivorous mammals are infected when grazing on infected land, bitten by tabanid flies with contaminated mouthparts or ingesting contaminated feed (2). Herders, livestock farmers, workers in abattoirs, meat and fur processing plants and veterinarians are exposed to the disease as an occupational hazard. Recently, the disease has transposed from industry to agriculture affecting farmers and herdsmen in 87.6% of the human cases (3). Bacillus anthracis, the etiological agent of anthrax, exhibits a bimodal lifestyle consisting of the vegetative and the spore stage (1). Bacteria in the vegetative stage are shed by infected animals and may die rapidly in most environmental conditions. After sporulation from the vegetative cells, the B. anthracis can survive in the soil for decades (4). The bacillus replicates rapidly in the bloodstream to high concentrations and releases toxins resulting in septicemia, which soon kills the host. In soil and vegetation, the spore stage can remain viable and infectious for years until it comes in contact with and enters a new host, where it germinates and begin a new life cycle (4, 5). Human anthrax infections are caused by contact with infected animals or animal products, ingestion of undercooked infected meat; or exposure to processing of contaminated hides, wool, and hair in enclosed spaces (6). Clinically, there are three forms of anthrax namely; cutaneous, gastrointestinal tract and pulmonary (inhalation).
Globally, cutaneous anthrax accounts for over 95% of the human cases. In China, for 97.7% (3). All three types of anthrax are implicitly fatal in wildlife, livestock, and human beings, if not treated promptly (7). Due to B. anthracis’ virulence, tenacious anthrax cases and repetitive outbreaks, concerns have been raised across continents in recent years, e.g., sub-Saharan Africa (8), Asia (9), Europe (10), Australia (11), and the United States of America (12). Also due to its potential use for bioterrorism, anthrax is considered as a global public health threat (13). Effective vaccines have reduced the economic significance of the disease in developed countries where it now occurs sporadically in unvaccinated domestic stock and wildlife populations (14). In the Qinghai Province of China, Anthrax occurs sporadically and all year round. The prevalence of anthrax in Qinghai rose from 0.35/100,000 in 2012 to 1.17/100,000 in 2016. The incidence is gradually increasing as well (15). Qinghai province has recently been identified as one of the potential sources of B. anthracis (16, 17).
Zoonosis of anthrax via livestock has been controlled, but not that via wildlife. The behavior of avian and mammalian scavengers (18), rending the carcass apart, frees a multitude of B. anthracis bacteria. Subsequently, scavengers may disperse the bacteria over a wide range through ingestion and bathing resulting in contamination of water bodies (8). Human beings, livestock, and wildlife will invariably encounter each other or share habitats which would cause a spill back or spillover of the infection. Disease control in livestock through vaccination and intensive surveillance of both livestock and wildlife are essential for prevention (19). However, widespread surveillance is costly, and the vaccination of all animals is unattainable. Therefore, there is a need to concentrate on high risk areas. To identify these, it is expedient to upsurge our understanding of the ecology of the B. anthracis spore.
The mainstay of the rural economy in Qinghai province, including the lake basin is livestock husbandry (20). Livestock mainly includes sheep, goat and yak, but also some horse, cattle and donkey. Livestock numbers per household varied from dozens to more than 1000 (21). In spring or early summer most livestock is transferred to high-altitude pastures (4850 to 4950m a.s.l.) where milk is processed and flocks gain weight. After their return to the homestead (3190 to 3300m a.s.l.) in late summer, fodder (oats) and crop residues are provided as principal feed in addition to stubble grazing and grassy patches near the winter residence (22).
Previous studies found that soil type (cambisol), high pH, high calcium cation concentrations and high soil organic matter were associated with spore germination and maintaining spore viability (8, 23-25) as well as spore adhesion to soil particles (26). Many authors have described the soil types as the key factor in B. anthracis spore germination and maintenance of dormancy (11, 24, 27, 28). Similar studies elsewhere and China have identified climatic variables (11) and human population density as good predictors of B. anthracis spore suitability (16). Therefore, we tested the hypothesis that soil type, climatic variables and human population density are significant predictors of B. anthracis spore suitability in the Qinghai Lake basin. Presence-only modeling algorithms to predict the environmental suitability of B. anthracis spores have been widely used, including maximum entropy (24) and GARP (genetic algorithm for the rule-set prediction) (11). During comparative model studies, MaxEnt outperformed other algorithms (29); Padilla et al., 2017). Therefore, in this study, MaxEnt was used to model B. anthracis spore’s persistence and its spatial distribution (30, 31).