Anthrax occurs worldwide. It is usually seen in areas where animals have died from anthrax and contaminated the soil with spores. Outbreaks of anthrax are most often associated with warm weather seasons, heavy rains, and/or droughts.
Anthrax is caused by Bacillus anthracis, a gram-positive, encapsulated, spore-forming aerobic rod. The organism exists in two forms: the vegetative form, which causes disease, and the sporulated form, which is dormant. Oxygen is needed for sporulation (development from the vegetative state into spores). Spores are usually found in the soil and can lay dormant for decades.
Most mammals, including humans, are susceptible to anthrax infection to some degree. Ruminants such as cattle, sheep, and goats are most commonly infected because they are grazing animals and may graze on grass growing in infected soil. Other herbivores like horses and pigs may also be infected. Anthrax has been reported in dogs and cats but is rare in these animals.
Humans are infected by ingestion or inhalation of anthrax spores or by handling contaminated carcasses, wool, hide, or hair (Figure 1). When a person is infected through handling a contaminated object, the spores enter through preexisting skin cuts or abrasions. Direct person-to-person spread of anthrax is extremely rare. Communicability is not a concern in managing or visiting with patients with inhalational anthrax.
In animals, the most common form of transmission is ingestion of anthrax spores. The spores in the soil are ingested when environmental conditions are right. Periods of heavy rain can wash the spores into low-lying areas, where they are exposed to oxygen as they are brought to the surface, and animals grazing in these areas ingest the spores. Drought conditions, in which pastures have little plant growth, can also be a contributing factor. Animals graze closer to the ground than they normally would and may ingest spores. Biting flies can, but rarely do, transmit B. anthracis from one animal to another. Outbreaks in pigs have been associated with feed containing meat and/or bone meal from infected carcasses. Wild animals or other scavengers become infected by feeding on infected dead animals.
All virulent strains of B. anthracis form a nontoxic capsule that functions to protect the organism against the bactericidal components of plasma and phagocytes and against destruction by gastric juices.
An exotoxin plays a major role in the pathogenesis of anthrax. One component of the anthrax toxin has a lethal mode of action that is not understood at this time. Death is apparently due to oxygen depletion, secondary shock, and respiratory and cardiac failure. Death from anthrax in humans or animals frequently occurs suddenly and unexpectedly.
ANTHRAX IN ANIMALS
CATTLE, SHEEP, AND GOATS
Anthrax in ruminants may cause a peracute (very sudden) disease, in which the animals are found dead, having shown no clinical signs before death. The carcasses are bloated, with little or no rigor mortis and bloody discharges that do not clot coming from body openings. Acute cases of anthrax are associated with sudden illness, characterized by high fever, localized edema (in more longstanding cases), decreased or absent rumination, decreased milk production with blood-tinged or yellow-colored milk, excitement followed by depression, difficult breathing, convulsions, bleeding that does not clot coming from body openings, and death. Anthrax can cause abortion. The incubation period is 1 to 3 days following exposure, with death usually occuring in 2 to 3 days.