Ecological niche modelling of the Bacillus anthracis A1.a sub-lineage in Kazakhstan

Abstract

Background: Bacillus anthracis, the causative agent of anthrax, is a globally distributed zoonotic pathogen that continues to be a veterinary and human health problem in Central Asia. We used a database of anthrax outbreak locations in Kazakhstan and a subset of genotyped isolates to model the geographic distribution and ecological associations of B. anthracis in Kazakhstan. The aims of the study were to test the influence of soil variables on a previous ecological niche based prediction of B. anthracis in Kazakhstan and to determine if a single sub-lineage of B. anthracis occupies a unique ecological niche.

Results: The addition of soil variables to the previously developed ecological niche model did not appreciably alter the limits of the predicted geographic or ecological distribution of B. anthracis in Kazakhstan. The A1.a experiment predicted the sub-lineage to be present over a larger geographic area than did the outbreak based experiment containing multiple lineages. Within the geographic area predicted to be suitable for B. anthracis by all ten best subset models, the A1.a sub-lineage was associated with a wider range of ecological tolerances than the outbreak-soil experiment. Analysis of rule types showed that logit rules predominate in the outbreak-soil experiment and range rules in the A1.a sub-lineage experiment. Random sub-setting of locality points suggests that models of B. anthracis distribution may be sensitive to sample size.

Conclusions: Our analysis supports careful consideration of the taxonomic resolution of data used to create ecological niche models. Further investigations into the environmental affinities of individual lineages and sub-lineages of B. anthracis will be useful in understanding the ecology of the disease at large and small scales. With model based predictions serving as approximations of disease risk, these efforts will improve the efficacy of public health interventions for anthrax prevention and control.

Figure 1

Map of Kazakhstan with anthrax locality data. (A) Locations of genotyped isolates. Inset shows locations of outbreaks used for the full outbreak model. Green points indicate training data for input into GARP and yellow points indicate independent points used to evaluate model accuracy. Shaded area indicates southern polygon used for creating projected models.

Figure 2

Predicted distribution of Bacillus anthracis in Kazakhstan. Predicted distribution of Bacillus anthracis in Kazakhstan based on outbreak data with and without soil variables. (A) Outbreak experiment (excluding soil variables) [26], (B) Outbreak-soil experiment (including soil variables), (C) Differences between distributions predicted by the two experiments.

Figure 3

Predicted geographic distribution of the Bacillus anthracis A1.a sub-lineage. Comparison of predicted geographic distributions of B. anthracis. (A) distribution of B. anthracis predicted by the sub-lineage experiment, (B) difference between predicted distributions of the sub-lineage and the outbreak-soil experiments.

Figure 4

Predicted geographic distribution of B. anthracis based on the large southern outbreak experiment and small southern outbreak experiment. Predicted geographic distribution of B. anthracis based on (A) large southern outbreak experiment and (B) small southern outbreak experiment. (C) Difference between predicted geographic distributions. Green points indicate training data for input into GARP and yellow points indicate independent points used to evaluate model accuracy.

Figure 5

Median ranges of environmental variables predicting B. anthracis presence by the outbreak-soil experiment. O = Outbreak-Soil Experiment; A = A1.a sub-lineage experiment.

Figure 6

Distribution of B. anthracis in ecological space. Predicted distribution of B. anthracis in ecological space based on areas of ten best subset model agreement. Red points = outbreak locations, blue points = A1.a isolate locations, black triangles = A4 locations, black circle = A3.b location.