Review
The ecology of Bacillus anthracis

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Abstract

The global distribution of anthrax is largely determined by soils with high calcium levels and a pH above 6.1, which foster spore survival. It is speculated that the spore exosporium probably plays a key part by restricting dispersal and thereby increasing the probability of a grazing animal acquiring a lethal dose. ‘Anthrax Seasons’ are characterized by hot-dry weather which stresses animals and reduces their innate resistance to infection allowing low doses of spores to be infective. Necrophagic flies act as case-multipliers and haemophagic flies as space-multipliers; the latter are aided by climatic factors which play a key part in whether epidemics occur. Host death is a function of species sensitivity to the toxins. The major function of scavengers is to open the carcass, spill fluids, and thereby aid bacilli dispersal and initiate sporulation. In the context of landscape ecology viable spore distribution is a function of mean annual temperature, annual precipitation, elevation, mean NDVI, annual NDVI amplitude, soil moisture content, and soil pH.

Introduction

The term ecology or oekologie was coined by the German biologist Ernst Haeckel in 1866, which he defined as “the comprehensive science of the relationship of the organism to the environment” or the study of the relationships between living organisms and their environments (Haeckel, 1866). This differs from the definition of epidemiology, a discipline which is ultimately concerned with improved disease control. So we will be examining not the livestock problem but how this pathogen interacts with the soil environment, with its various hosts, primarily wildlife, and mechanical vectors with disease as an incidental, though it is important for its long time persistence, multiplication, and spread.

Section snippets

Soils

Anthrax spores survive best in black steppe soils rich in organic matter and calcium. The persistence of anthrax was commented upon by Higgins in 1916 in that “a suitable soil must be slightly alkaline.” Citing the work of Minett and Dhanda (1941) and of Whitworth (1924), Van Ness and Stein, 1956, Van Ness, 1971, with the results of their own studies of the geographic distribution of outbreaks in the US, put forward an hypothesis that “anthrax occurs in livestock that live upon a soil with a pH

Index case infection

For multiplication B. anthracis is an obligate pathogen lurking in the soil. While the oral minimum infective oral dose is largely unknown for wildlife species it was noted (De Vos, 1990, De Vos and Scheepers, 1996) that while 100–250 spores parenterally administered consistently killed kudu in the Kruger National Park, the oral LD50 with same strain was approximately 15 million spores. In healthy unstressed sheep, horses and cattle the lethal oral dose is of the order of 1.5–5 × 108 spores;

Insects

Anthrax has long been associated with insects, primarily necrophilic and haemophagic flies. B. anthracis has also been recovered from Musca domestica and from ticks. It is unlikely that the latter reflect any meaningful epidemiologic risk. It is probable that vector ecological and feeding preferences, land cover, and host species feeding-habits and densities explain why browsing kudu are the major affected species in one area, e.g., Kruger National Park, and grazing antelope in another, e.g.,

Mortality

It has long been noted that “In certain outbreaks a single species of animal may show a more marked susceptibility than others which are apparently similarly exposed.” (Higgins, 1916). An inverse relationship exists between resistance to infection and susceptibility to the toxin complex as reflected in the level of the terminal bacteraemia (Lincoln et al., 1967). Cattle are very prone to natural infection but die with high bacteraemias indicative of a toxin resistance, thus high protective

Carcass

To quote Sterne (1959) on sporulation, a high oxygen tension is not necessary as a reduction in partial pressure does not materially affect sporulation but a high partial pressure of CO2 diminishes sporulation, which is why sporulation only occurs after the carcass has be opened. Thus the major function of scavengers is to open the carcass to spill bloody fluids and allow sporulation. If there was blood extravasation after death, spores will form in this spilt blood before it acidifies but it

Landscape ecology

To understand the macro-ecology of B. anthracis, and given that most research currently suggests that germination and multiplication occurs in the host, while spore survival occurs in the soil (vide supra), it is necessary to identify the geographic area where bacilli spores can thrive for long periods of time. Landscape ecology provides a useful perspective of scale for such analyses. Haines-Young et al. (1994) provide an overview of landscape ecology and the role that geographic information

Conclusion and discussion

Although in this paper the interactions between the various factors have to be presented in a relatively simple way, the reality is that they are complex, not unidirectional, and exist in a multidimensional space. For example rain impacts spore survival and movements, insect numbers, grazing and browse availability and quality, animal nutrition, health, and fertility, which goes to animal density and the probability of haematophagous insect vectors finding the next animal to feed upon.

There are

Acknowledgements

We are very thankful for the help of many colleagues but especially Dan Dragon, Phil Hanna, and Peter Turnbull for their shared insights, experience, and critical comments in assembling this review; also Denise Westphal, whose research librarian skills were invaluable and never ceased to amaze.

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