Anthrax and other Bacillus species

Introduction Anthrax is a disease caused by the gram-positive, aerobic bacterium Bacillus anthracis and was recognized in antiquity. The disease figures prominently in the history of modern medicine because it was the first bacterial illness for which successful vaccines were prepared, almost simultaneously by William Smith Greenfield in London and Louis Pasteur in Paris. Anthrax is a zoonosis of herbivores which is encountered worldwide and human cases continue to be seen not infrequently in Turkey, Iran, Afghanistan, Thailand, and countries of sub-Saharan Africa. Grazing wild animals and cattle are very susceptible and human disease in animal husbandmen and herders is closely tied to exposure to infected beasts. Propagating the bacteria in the spore form is used in bioterrorism, a novel and more recent form of human exposure to anthrax, utilizing delivery systems such as the postal service. To understand anthrax one must keep in mind the natural cycle of disease in animals: spores survive prolonged periods in alkaline soils, rainwater concentrates spores in low-lying depressions and susceptible herbivores gather in these locales during dry periods and inhale aerosolized spores or swallow spores loosely attached to forage. These geographic and climatic factors are usually present prior to animal outbreaks and may culminate in humans being infected accidentally. Spores arise from bacilli exposed to ambient air when blood from dying animals reaches the soil or carcasses are torn apart by scavengers. When the bacilli are exposed to air, spores form in the central and subterminal part of the bacillus. Spores may survive for prolonged periods (~90 years) in soil rich with organic material, a pH greater than 6.1 (alkaline soils) with high concentrations of Ca⁺⁺. This characterizes a wide geographic swathe in the middle United States from Texas to North Dakota. It is also true for soils in the steppes of Asia and in sub-Saharan Africa where anthrax remains common in wildlife. Although anthrax is considered an obligate pathogen, it is likely that in some circumstances a vegetative bacillus-spore cycle occurs independent of infection in the soil alone.