Seasonality and Ecological Suitability Modelling for Anthrax (Bacillus anthracis) in Western Africa

Abstract

Anthrax is hyper-endemic in West Africa affecting wildlife, livestock and humans. Prediction is difficult due to the lack of accurate outbreak data. However, predicting the risk of infection is important for public health, wildlife conservation and livestock economies. In this study, the seasonality of anthrax outbreaks in West Africa was investigated using climate time series and ecological niche modeling to identify environmental factors related to anthrax occurrence, develop geospatial risk maps and identify seasonal patterns. Outbreak data in livestock, wildlife and humans between 2010 and 2018 were compiled from different sources and analyzed against monthly rates of change in precipitation, normalized difference vegetation index (NDVI) and land surface temperature. Maximum Entropy was used to predict and map the environmental suitability of anthrax occurrence. The findings showed that: (i) Anthrax outbreaks significantly (99%) increased with incremental changes in monthly precipitation and vegetation growth and decremental changes in monthly temperature during January-June. This explains the occurrence of the anthrax peak during the early wet season in West Africa. (ii) Livestock density, precipitation seasonality, NDVI and alkaline soils were the main predictors of anthrax suitability. (iii) Our approach optimized the use of limited and heterogeneous datasets and ecological niche modeling, demonstrating the value of integrated disease notification data and outbreak reports to generate risk maps. Our findings can inform public, animal and environmental health and enhance national and regional One Health disease control strategies.

Keywords: West Africa; anthrax; climate variability; ecological niche modeling; seasonality; time series analysis.

Figure 1

Study area and anthrax outbreak locations before (black triangle) and after (red dots) filtering spatial auto-correlated records (point location data source: EMPRES-i, GIMD and FAO workshop). The dashed polygons are anthrax-affected districts (adm02 level) for which geo-locations of outbreaks are missing (polygon data source: OIE-WHAIS).

Figure 2

Number of anthrax outbreaks by year across the study area (a) and by country (b) between January 2010 and November 2018 (OIE-WHAIS data).

Figure 3

Median number of anthrax outbreaks (orange bar) against (a) median rainfall (blue area), (b) median NDVI (green line) and (c) median temperature (grey line) by month in the study area.

Figure 4

Periodicity of the standardized median number of anthrax outbreaks (z_median_Anthrax; orange bar), rainfall (zMedianRFE; blue area), NDVI (zMedianNDVI; green bars) and temperature (zMedianTemperature; yellow area) by month in the study area. The inset shows that the peak in anthrax outbreaks occurs between April and May, when the temperature decreases while rainfall and NDVI increase.

Figure 5

Standardized median number of anthrax outbreaks (orange line) against (a) first derivative of standardized median precipitation (blue bars) by month and (b) first derivative of standardized median NDVI (green bars) by month. The inset of each chart shows that the number of outbreaks increases with incremental change in precipitation and NDVI during January–June.

Figure 6

(a) The predicted suitability of anthrax presence (average model) and (b) its standard deviation based on 9 selected and uncorrelated variables.