Anthrax – a natural disease and a biological weapon

Introduction The bacterium that causes anthrax, Bacillus anthracis, has been with us throughout recorded history. The organism is unique because it is the only aerobic spore forming bacterium that appears to be an obligate parasite. It has been suggested that the organism has a natural cycle of replication within the soil1 but there is no evidence that this occurs consistently, if at all, because the nutrient supply is limited and the environment is hostile. However when an animal dies of anthrax and the carcass is consumed by carnivores the release of large quantities of nutrient-rich body fluids containing huge concentrations of anthrax organisms might encourage the organism to replicate in the soil,1 especially in warm conditions. Eventually, when the nutrient supply declines, the bacteria will produce the resistant spores that are the major reason for the persistence of the disease in the wild. Animals that have died of anthrax can cause considerable contamination of the local area, becoming a source of infection particularly for browsing herbivores2. Vultures and other scavengers, which are relatively resistant to anthrax infection can, in their droppings, disperse the agent over vast distances. Although anthrax is enzootic in many regions of the world, it is in Africa that the problem is particularly acute. The huge herds of herbivores graze every square metre of grassland and it is inevitable that some of them will consume an infectious dose of the anthrax spores lying dormant close to the surface of the soil. Although anthrax is not a contagious disease the natural cycle of infection, death, disruption of the carcass by scavengers and the dispersal of the spores can produce an enormous increase in the number of animals infected. The consequent decline in the local animal population can be to the point of annihilation in some instances. The role of the scavengers in this cycle of infection is critical because without their activity the carcass of the fallen animal will remain undisturbed, without the anthrax bacteria becoming exposed to oxygen and forming spores. The non-sporulating bacteria will then decay within a short time, thus ending the infectious cycle.