Modeling the environmental suitability of anthrax in Ghana and estimating populations at risk: Implications for vaccination and control

<div><p>Anthrax is hyper-endemic in West Africa. Despite the effectiveness of livestock vaccines in controlling anthrax, underreporting, logistics, and limited resources makes implementing vaccination campaigns difficult. To better understand the geographic limits of anthrax, elucidate environmental factors related to its occurrence, and identify human and livestock populations at risk, we developed predictive models of the environmental suitability of anthrax in Ghana. We obtained data on the location and date of livestock anthrax from veterinary and outbreak response records in Ghana during 2005–2016, as well as livestock vaccination registers and population estimates of characteristically high-risk groups. To predict the environmental suitability of anthrax, we used an ensemble of random forest (RF) models built using a combination of climatic and environmental factors. From 2005 through the first six months of 2016, there were 67 anthrax outbreaks (851 cases) in livestock; outbreaks showed a seasonal peak during February through April and primarily involved cattle. There was a median of 19,709 vaccine doses [range: 0–175 thousand] administered annually. Results from the RF model suggest a marked ecological divide separating the broad areas of environmental suitability in northern Ghana from the southern part of the country. Increasing alkaline soil pH was associated with a higher probability of anthrax occurrence. We estimated 2.2 (95% CI: 2.0, 2.5) million livestock and 805 (95% CI: 519, 890) thousand low income rural livestock keepers were located in anthrax risk areas. Based on our estimates, the current anthrax vaccination efforts in Ghana cover a fraction of the livestock potentially at risk, thus control efforts should be focused on improving vaccine coverage among high risk groups.</p></div

Cases of anthrax by livestock type in Ghana, 2005–2016.

<p>Cases of anthrax by livestock type in Ghana, 2005–2016.</p

Annual number of livestock anthrax outbreaks in Ghana during January 2005- June 2016.

<p>Annual number of livestock anthrax outbreaks in Ghana during January 2005- June 2016.</p

Environmental and climatic variables used in the random forest models.

<p>Environmental and climatic variables used in the random forest models.</p

Livestock production systems^¥ and the livestock population in the Ghanaian anthrax risk zone.

<p>Livestock production systems<a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0005885#t003fn001" target="_blank">^¥</a> and the livestock population in the Ghanaian anthrax risk zone.</p

Partial dependency plots of environmental variables used in the random forest.

<p>Gray shading represented confidence intervals derived from the model iterations.</p