Identifying Edaphic Factors and Normalized Difference Vegetation Index Metrics Driving Wildlife Mortality From Anthrax in Kenya’s Wildlife Areas

Vincent Obanda,Patrick I Chiyo,David Ndeereh,Edward M Kingori,Olivia W Lwande,Viola A Otieno

doi:10.3389/fevo.2021.643334

Abstract

Anthrax, an acute disease of homeotherms caused by soil-borne Bacillus anthracis is implicated in dramatic declines in wildlife mainly in sub-Saharan Africa. Anthrax outbreaks are often localized in space and time. Therefore, understanding predictors of the spatial and temporal occurrence of anthrax in wildlife areas is useful in supporting early warning and improved response and targeting measures to reduce the impact of epizootic risk on populations. Spatial localization of anthrax is hypothesized to be driven by edaphic factors, while the temporal outbreaks are thought to be driven by extreme weather events including temperature, humidity, rainfall, and drought. Here, we test the role of select edaphic factors and normalized difference vegetation index (NDVI) metrics driven by vegetation structure and climate variability on the spatial and temporal patterns of wildlife mortality from anthrax in key wildlife areas in Kenya over a 20-year period, from 2000 to 2019. There was a positive association between the number of anthrax outbreaks and the total number of months anthrax was reported during the study period and the nitrogen and organic carbon content of the soil in each wildlife area. The monthly occurrence (timing) of anthrax in Lake Nakuru (with the most intense outbreaks) was positively related to the previous month’s spatial heterogeneity in NDVI and monthly NDVI deviation from 20-year monthly means. Generalized linear models revealed that the number of months anthrax was reported in a year (intensity) was positively related to spatial heterogeneity in NDVI, total organic carbon and cation exchange capacity of the soil. These results, examined in the light of experimental studies on anthrax persistence and amplification in the soil enlighten on mechanisms by which these factors are driving anthrax outbreaks and spatial localization.

Highlights

Anthrax is an acute, febrile disease of homeotherms, caused by the gram-positive, non-motile, spore-forming, soil-borne bacteria; Bacillus anthracis and Bacillus cereus biovar anthracis (Koch, 1937; Hoffmaster et al, 2004; Leendertz et al, 2006b)
We examined the influence of normalized difference vegetation index (NDVI) metrics - surrogates for climate variability, vegetation structure, plant diversity, plant growth or phenology and plant cover – on the monthly occurrence and the number of months of anthrax when anthrax mortality was reported each year in selected wildlife areas in Kenya
We found a statistically significant positive relationship between monthly occurrence of anthrax and monthly NDVI deviation from a 20-year mean for each calendar month

Summary

Introduction

Febrile disease of homeotherms, caused by the gram-positive, non-motile, spore-forming, soil-borne bacteria; Bacillus anthracis and Bacillus cereus biovar anthracis (Koch, 1937; Hoffmaster et al, 2004; Leendertz et al, 2006b). Anthrax epizootics have caused significant declines in wildlife species by 10–30% in most outbreaks and by 80–90% in severe outbreaks (Creel et al, 1995; Hugh-Jones and De Vos, 2002; Leendertz et al, 2006a; Clegg et al, 2007; Muoria et al, 2007; Kaitho et al, 2013; Salb et al, 2014; Hoffmann et al, 2017). Areas that B. anthracis is deemed endemic to are characterized by high content of soil calcium, pH (pH ≥ 6.1), moisture, and organic carbon and nitrogen (Joyner et al, 2010; Chikerema et al, 2013; Kracalik et al, 2017; Steenkamp et al, 2018; Romero-Alvarez et al, 2020) and occur at low-lying topography (Dragon and Rennie, 1995; Munang’andu et al, 2012). Seasonally flooded plains and low lying areas are known to be prone to anthrax outbreaks (Dragon and Rennie, 1995; Munang’andu et al, 2012)

Methods

Results

Conclusion