Dust-Bathing Behaviors of African Herbivores and the Potential Risk of Inhalational Anthrax

Abstract

: Anthrax in herbivorous wildlife and livestock is generally assumed to be transmitted via ingestion or inhalation of Bacillus anthracis spores. Although recent studies have highlighted the importance of the ingestion route for anthrax transmission, little is known about the inhalational route in natural systems. Dust bathing could aerosolize soilborne pathogens such as B. anthracis, exposing dust-bathing individuals to inhalational infections. We investigated the potential role of dust bathing in the transmission of inhalational anthrax to herbivorous wildlife in Etosha National Park, Namibia, an area with endemic seasonal anthrax outbreaks. We 1) cultured soils from dust-bathing sites for the presence and concentration of B. anthracis spores, 2) monitored anthrax carcass sites, the locations with the highest B. anthracis concentrations, for evidence of dust bathing, including a site where a zebra died of anthrax on a large dust bath, and 3) characterized the ecology and seasonality of dust bathing in plains zebra ( Equus quagga), blue wildebeest ( Connochaetes taurinus), and African savanna elephant ( Loxodonta africana) using a combination of motion-sensing camera traps and direct observations. Only two out of 83 dust-bath soils were positive for B. anthracis, both with low spore concentrations (≤20 colony-forming units per gram). We also detected no evidence of dust baths occurring at anthrax carcass sites, perhaps due to carcass-induced changes in soil composition that may deter dust bathing. Finally, despite observing some seasonal variation in dust bathing, preliminary evidence suggests that the seasonality of dust bathing and anthrax mortalities are not correlated. Thus, although dust bathing creates a dramatic cloud of aerosolized soil around an individual, our microbiologic, ecologic, and behavioral results in concert demonstrate that dust bathing is highly unlikely to transmit inhalational anthrax infections.

Review: The risk of contracting anthrax from spore-contaminated soil – A military medical perspective

“…Apart from the relatively low infectivity for humans, there are a number of factors that may explain why only very few cases of soil associated anthrax in humans were registered [ 28 , 40 , 49 , 61 , 156 , 157 , 209 , 345–349 ]: Small size and number of active anthrax soil foci, and animals that died of anthrax (depending on the season and weather conditions) in an endemic combat area, and thus a low probability of the release of spores and exposure in association with combat situations “Dilution” and distribution of spores in anthrax soil foci by agricultural activities and decrease of spore concentration by antagonistic soil microbial activities “Dilution” of spores as a result of the impact of bombs, grenades or mines when large amounts of soil from different layers are blasted into the air and mix with spores Thermal inactivation of spores by explosions Inactivation of free deposited spores by UV light of sun Attachment of spores to the soil matrix (e.g. clay, bentonite, humus particles) and formation of large complexes with soil and dust particles preventing inhalation in deeper air ways Number of spores on or in contaminated environmental media too low to cause infection …”

“…This is one of the reasons, beside of the relatively high infectious doses, why human infection resulting from contaminated soil or the inhalation of dust is rather unlikely [ 157 ]. In studies on dust bathing herbivores in Etosha National Park, the highest concentrations of anthrax spores were found around and under an anthrax carcass where the soil was massively contaminated with blood, intestinal contents, and tissue fluids [ 40 ]. In the presence of blood proteins (e.g.…”

Coalescence modeling of intrainfection Bacillus anthracis populations allows estimation of infection parameters in wild populations

“…Plains zebra (Equus quagga) are the most common host for B. anthracis in Etosha. Most of these infections likely occur after ingesting spores while grazing at anthrax carcass sites (8), and not from drinking contaminated water (8) nor from inhalation of spores (12). Anthrax mortalities in zebra peak during the rainy season, where enhanced production of forage occurs at nutrient-rich carcass sites (13).…”

Spores and soil from six sides: interdisciplinarity and the environmental biology of anthrax (Bacillus anthracis)

“…Transmission takes place through several pathways, the primary one for ungulates being ingestion of B. anthracis spores during feeding at carcass sites (Hugh‐Jones & De Vos, ; Hugh‐Jones & Blackburn, ; Turner et al , ). Other potential pathways include ingesting emesis and faeces deposited by necrophagous flies on vegetation after these flies have fed on hosts that have died of anthrax (Blackburn et al , ), inhaling anthrax spores that have become airborne [in nature occurring from dust‐bathing hosts, although recent evidence casts doubt on this (Barandongo, Mfune & Turner, )], waterborne transmission from waterholes and temporary ponds (Turner et al , ), cutaneous routes, which account for the majority of human clinical cases globally (Shadomy & Smith, ), and gastrointestinal infections from eating infected meat and blood directly (Bales et al , ; Hugh‐Jones & De Vos, ; Beatty et al , ; Hugh‐Jones & Blackburn, ). In some regions, anthrax outbreaks are a consistent and predictable feature of ecosystems and occur regularly on a seasonal cycle; in other settings, epizootics are infrequent or rare events, and can be responsible for mass die‐offs among wildlife and livestock (Lindeque & Turnbull, ; Hugh‐Jones & De Vos, ; Hoffmann et al , ).…”