Immunological Evidence of Variation in Exposure and Immune Response to Bacillus anthracis in Herbivores of Kruger and Etosha National Parks

Abstract

Exposure and immunity to generalist pathogens differ among host species and vary across spatial scales. Anthrax, caused by a multi-host bacterial pathogen, Bacillus anthracis, is enzootic in Kruger National Park (KNP), South Africa and Etosha National Park (ENP), Namibia. These parks share many of the same potential host species, yet the main anthrax host in one (greater kudu (Tragelaphus strepsiceros) in KNP and plains zebra (Equus quagga) in ENP) is only a minor host in the other. We investigated species and spatial patterns in anthrax mortalities, B. anthracis exposure, and the ability to neutralize the anthrax lethal toxin to determine if observed host mortality differences between locations could be attributed to population-level variation in pathogen exposure and/or immune response. Using serum collected from zebra and kudu in high and low incidence areas of each park (18- 20 samples/species/area), we estimated pathogen exposure from anti-protective antigen (PA) antibody response using enzyme-linked immunosorbent assay (ELISA) and lethal toxin neutralization with a toxin neutralization assay (TNA). Serological evidence of pathogen exposure followed mortality patterns within each system (kudus: 95% positive in KNP versus 40% in ENP; zebras: 83% positive in ENP versus 63% in KNP). Animals in the high-incidence area of KNP had higher anti-PA responses than those in the low-incidence area, but there were no significant differences in exposure by area within ENP. Toxin neutralizing ability was higher for host populations with lower exposure prevalence, i.e., higher in ENP kudus and KNP zebras than their conspecifics in the other park. These results indicate that host species differ in their exposure to and adaptive immunity against B. anthracis in the two parks. These patterns may be due to environmental differences such as vegetation, rainfall patterns, landscape or forage availability between these systems and their interplay with host behavior (foraging or other risky behaviors), resulting in differences in exposure frequency and dose, and hence immune response.

Environmental drivers of biseasonal anthrax outbreak dynamics in two multihost savanna systems

“…This study was conducted in central ENP and northern KNP (Figure 2), where the highest incidence of anthrax mortalities is detected within the two parks (Ebedes, 1976; Ochai et al, 2022; Steenkamp et al, 2018). ENP and KNP are located at similar latitudinal ranges and have many species in common.…”

Comparing microbiological and molecular diagnostic tools for the surveillance of anthrax

“…There have been reports of a close relationship of pathogenic and non-pathogenic Bacillus spp. resulting in serological cross-reactivity (Marston et al, 2016, Zimmermann et al, 2017) which was also reported in the anthrax endemic region in northern Kruger National Park (KNP) (Steenkamp et al, 2018, Ochai et al, 2022). Anti-PA and LT-neutralising antibodies were also detected at higher rates than expected in animals from a low-incidence area of KNP (southern region) (Ochai et al, 2022).…”

“…The KNP (19,485 km 2 ; Figure 1) is situated in the northeastern part of South Africa, bordering Mozambique and Zimbabwe. The northern half of KNP (Figure 1) is considered the anthrax endemic region, where most of the anthrax mortalities have been reported (Ochai et al, 2022, Vos, 1990); this region is classified as semi-arid and is highly wooded with some grassland savannah (Huntley, 1982). KNP has variable elevations, with Pafuri (found in the northernmost part of KNP; 22.4206° S, 31.2296° E) having lower elevation floodplains and mountains towards the northwestern part of the park.…”

“…Anti-PA and LT-neutralising antibodies were also detected at higher rates than expected in animals from a low-incidence area of KNP (southern region) (Ochai et al, 2022). Thus we hypothesised that the animals could be reacting to “anthrax-like” microbes in the environment, which could possess genes with similar homologies to B. anthracis (Ochai et al, 2022). These data coupled with the discovery of anthrax cases caused by B. cereus biovar anthracis (Bcbva) strains in west and central Africa (Hoffmann et al, 2017), prompted us to re-assess the robustness of diagnostic tools currently employed in the surveillance for anthrax in southern Africa.…”

“…We utilised blood smears from the collection, covering the years 2012-2020. This period encompassed known anthrax outbreaks from 2012 to 2015 (Hassim, 2017a, Ochai et al, 2022). From these outbreaks, B. anthracis bacilli were initially identified using the microscopic evaluation of blood smears from wildlife carcasses in KNP with follow-up collection of samples (bone, hair and tissue) from these positive carcass sites in a previous study (Hassim, 2017b).…”