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

Abstract

Bacillus cereus biovar anthracis (Bcbva) is an emergent bacterium closely related to Bacillus anthracis, the etiological agent of anthrax. The latter has a worldwide distribution and usually causes infectious disease in mammals associated with savanna ecosystems. Bcbva was identified in humid tropical forests of Côte d’Ivoire in 2001. Here, we characterize the potential geographic distributions of Bcbva in West Africa and B. anthracis in sub-Saharan Africa using an ecological niche modeling approach. Georeferenced occurrence data for B. anthracis and Bcbva were obtained from public data repositories and the scientific literature. Combinations of temperature, humidity, vegetation greenness, and soils values served as environmental variables in model calibrations. To predict the potential distribution of suitable environments for each pathogen across the study region, parameter values derived from the median of 10 replicates of the best-performing model for each pathogen were used. We found suitable environments predicted for B. anthracis across areas of confirmed and suspected anthrax activity in sub-Saharan Africa, including an east-west corridor from Ethiopia to Sierra Leone in the Sahel region and multiple areas in eastern, central, and southern Africa. The study area for Bcbva was restricted to West and Central Africa to reflect areas that have likely been accessible to Bcbva by dispersal. Model predicted values indicated potential suitable environments within humid forested environments. Background similarity tests in geographic space indicated statistical support to reject the null hypothesis of similarity when comparing environments associated with B. anthracis to those of Bcbva and when comparing humidity values and soils values individually. We failed to reject the null hypothesis of similarity when comparing environments associated with Bcbva to those of B. anthracis, suggesting that additional investigation is needed to provide a more robust characterization of the Bcbva niche. This study represents the first time that the environmental and geographic distribution of Bcbva has been mapped. We document likely differences in ecological niche—and consequently in geographic distribution—between Bcbva and typical B. anthracis, and areas of possible co-occurrence between the two. We provide information crucial to guiding and improving monitoring efforts focused on these pathogens. Bacillus cereus biovar anthracis (Bcbva) is an emergent pathogen causing anthrax in West and Central African countries. It shares multiple bacteriological and genomic characteristics with Bacillus anthracis, the gram-positive bacterium causing anthrax in mammals worldwide. However, Bcbva has only been isolated in tropical humid forested environments in Africa; thus, we attempted to characterize its potential distribution and the differences with B. anthracis using an ecological niche modeling framework. According to our results, both pathogens are using different environments and their distribution looks complementary with B. anthracis occupying savanna-like environments and Bcbva occupying forested areas; further, we detected some level of overlap between the potential geographic distributions of B. anthracis and Bcbva, despite their lack of overlap in their current environmental space. This is the first time the potential distribution of Bcbva is addressed considering all the available spatial information on the pathogen occurrence available as of 2017. Although our Bcbva models are limited due to sample size, this exploration informs on areas that would likely be considered for further investigation of Bcbva outbreaks with special emphasis on areas where Bcbva and B. anthracis could co-exist.