There are 3 main types of anthrax – cutaneous, gastrointestinal and respiratory. All three types of infection can occur in animals and humans. Spores are an important factor in transmitting infection, and animals usually become infected through grazing in areas where large numbers of spores are present in the surface of the soil (link to anthrax and cattle). Therefore natural infection in humans is not likely to occur unless they are in contact with infected animals or animal products.
Infection in animals is usually gastrointestinal, and the most likely route of infection in grazing animals is through ingestion of spores during dry periods following flooding. Spores are brought to the surface during periods of heavy rainfall and remain there and become concentrated during dry spells. Ingestion alone does not necessarily result in infection – the spores require a lesion of some sort to gain entry into the tissues. Gastrointetinal lesions may occur when grazing on dry, spiky, gritty grass that grows close to the soil – infection occurs where spores have also been deposited on the soil.
Grazing animals may also become infected through inhalation of spore-laden dust (pulmonary anthrax), although infection by this route is much less common than through ingestion. Animals that feed on the carcasses of dead animals can also become infected during outbreaks in grazing animals.
Humans become infected through contact with infected animals or animal products such as carcasses, hides, wool, hair and bone meal. Therefore, in areas where infection in livestock is uncommon, human infection is also rare.
The World Health Organization (WHO) reports higher incidence of infection in certain areas of Canada such as the MacKenzie Bison Range, North West Territory and Wood Buffalo National Park in northern Alberta, with sporadic outbreaks occurring in southern Alberta and Saskatchewan. In the U.S., sporadic cases occur in South Dakota, Nebraska and Oklahoma, with more persistent outbreaks in western Texas. In other areas of the world outbreaks occur more consistently – Central and South America, Mexico, South Africa, Middle East, Soviet Union, southern India, and south-east Asian countries (Vietnam, Cambodia, western China, Thailand).
The most common form of natural human infection is cutaneous anthrax, accounting for at least 95% of cases world-wide. Cutaneous anthrax is readily treated with penicillin and a number of other antibiotics. Without treatment, 10-20% of cutaneous infections may be life-threatening. Contact with the vegetative form of the bacteria in the fluids and tissues of sick or dying animals, or with spores in dead carcasses, meats, hides, hair, wool or bone does not guarantee infection. Infection requires a skin lesion (cut, scrape, etc.) in order to gain entry to the tissues. In 2-3 days (may occur as early as 9 hours or as long after as 7 days) a pimple-like red elevated area appears, followed 1-2 days later by a ring of blister-like, watery fluid-filled vesicles with swelling in the surrounding area. By 5-7 days, an ulcer forms (eschar) (see photo). By approximately 10 days, the eschar begins to heal and may take up to 6 weeks to resolve. Treatment at this stage does not speed healing. Without treatment a small number of cases may develop systemic infection.
Gastrointestinal and pulmonary anthrax have much higher mortality rates than cutaneous anthrax, often because they are more likely to go unrecognized and untreated. Treatment in the early stages of either infection is very effective; however, the disease progresses rapidly, and in the latter stages of infection treatment is often ineffective.
Gastrointestinal infection may occur following ingestion of raw or improperly cooked meat from sick or dead animals and symptoms are similar to other food-borne illnesses –
nausea, vomiting, fever, abdominal pain. Cases may be mild or severe – in severe cases the mortality rate is approximately 50% even with treatment.
Pulmonary anthrax is even more likely to be misdiagnosed as the initial stage of infection involves flu-like symptoms – mild fever, fatigue and malaise lasting one to several days. Without treatment at this stage, infection progresses rapidly to difficulty breathing, disorientation, toxemia and death. Naturally acquired pulmonary anthrax in humans is extremely rare.