Anthrax is primarily a disease of domesticated and wild animals, particularly herbivorous animals such as cattle, sheep, horses, mules, and goats. Humans become infected incidentally when brought into contact with diseased animals, which includes their flesh, bones, hides, hair and excrement.

Anthrax is caused by the endospore-forming bacterium, Bacillus anthracis. These bacteria have been found naturally in soil samples from around the world, but they are not readily found or isolated. In the United States there are recognized areas of infection in South Dakota, Nebraska, Arkansas, Texas, Louisiana, Mississippi and California; small areas exist in other states. Even in endemic areas, anthrax occurs irregularly, often with many years between occurrences.

Bacillus anthracis spores and vegetative cells (ABC News)

Bacillus anthracis is classified as a Gram-positive, nonmotile, spore-forming bacterium. It is capable of growth under aerobic and anaerobic conditions. Vegetative cells are 4-8um long x 1-1.5um wide. The spores of Bacillus species, called endospores, are known to be the most hardy and durable of all resting cells produced in nature.

Pathogenesis
In humans, anthrax is fairly rare; the risk of infection is about 1/100,000. The most common form of the disease in humans is cutaneous anthrax, which is usually acquired via injured skin or mucous membranes. A minor scratch or abrasion, usually on an exposed area of the face or neck or arms, is inoculated by spores from the soil or a contaminated animal or carcass. The spores germinate, vegetative cells multiply, and a characteristic lesion develops at the site. In severe cases, where the blood stream is eventually invaded, the disease may be fatal.

Another form of the disease, inhalation anthrax (woolsorters' disease), results most commonly from inhalation of spore-containing dust where animal hair or hides are being handled. The disease begins abruptly with high fever and chest pain. It progresses rapidly to a systemic hemorrhagic pathology and is often fatal if treatment cannot stop the invasive aspect of the infection.

Intestinal anthrax is analogous to cutaneous anthrax but occurs on the intestinal mucosa. As in cutaneous anthrax, the organisms probably invade the mucosa through a preexisting lesion. The bacteria spread from the mucosal lesion to the lymphatic system. Intestinal anthrax results from the ingestion of poorly cooked meat from infected animals. Intestinal anthrax, although extremely rare in developed countries, has an extremely high mortality rate.

Meningitis due to B. anthracis is a very rare complication that may result from a primary infection elsewhere.

The infectious dose of anthrax organisms is highly variable from animal species to species, and it varies widely dependent on the route of inoculation. See Bacillus, Chapter 15 for symptoms and clinical manifestations of the disease.

The bacteria are able to invade their animal hosts because they are resistant to the hosts' phagocytic defenses. During and after invasion, the bacteria produce a toxin which has a lethal mode of activity. Death from anthrax in humans or animals frequently occurs suddenly and unexpectedly. The level of the lethal toxin in the circulation increases rapidly quite late in the disease, and it closely parallels the concentration of organisms in the blood.

Antibiotics should be given to unvaccinated individuals exposed to inhalation anthrax. Penicillin, tetracyclines and fluoroquinolones (Cipro) are effective if administered before the onset of lymphatic spread or septicemia, estimated to be about 24 hours. Antibiotic treatment is also known to lessen the severity of disease in individuals who acquire anthrax through the skin. Inhalation anthrax was formerly thought to be nearly 100 percent fatal despite antibiotic treatment, particularly if treatment is started after symptoms appear. A recent Army study resulted in successful treatment of monkeys with antibiotic therapy after being exposed to anthrax spores. The antibiotic therapy was begun one day after exposure.

Immunization
Currently, the anthrax vaccine is produced under contract to the Department of Defense, and only small quantities are made available as needed to civilians who are exposed to anthrax hazards in their work environment, such as veterinarians, lab workers and others. An attempt to immunize 2.5 million members of the military ended three years ago, but that policy is being reevaluated. If the manufacturer receives approval from the FDA, vaccine production will resume.

The anthrax vaccine is a preparation of the protective antigen (a fraction of the toxin) recovered from the culture filtrate of an avirulent, nonencapsulated strain of Bacillus anthracis. Anthrax immunization consists of three subcutaneous injections given two weeks apart followed by three additional subcutaneous injections given at 6, 12, and 18 months. Annual booster injections of the vaccine are required to maintain a protective level of immunity.

The inhalation of anthrax spores can lead to infection and disease. The possibility of creating aerosols containing anthrax spores has made B. anthracis a chosen weapon of bioterrorism. Iraq, Russia, Korea and as many as ten nations have the capability to load spores of B. anthracis into weapons. Domestic terrorists may develop means to distribute spores via mass attacks or small-scale attacks at a local level.

As an agent of biological warfare it is expected that a cloud of anthrax spores would be released at a strategic location to be inhaled by the individuals under attack. Spores of B. anthracis can be produced and stored in a dry form and remain viable for decades in storage or after release.

There is no evidence of person-to-person transmission of anthrax. Quarantine of affected individuals is not recommended. Anthrax spores may survive in the soil, water and on surfaces for many years. Spores can only be destroyed by steam sterilization or burning. The U.S. Navy Manual on Operational Medicine and Fleet Support, entitled Biological Warfare Defense Information Sheet states "Disinfection of contaminated articles may be accomplished using a 0.05% hypochlorite solution (1Tbsp. bleach per gallon of water). Spore destruction requires steam sterilization." It has also been reported that boiling (100 degrees C°) for 30 minutes kills endospores of B. anthracis.

An infection of local animal populations such as sheep and cattle could follow a biological attack with spores. Infected animals could then transmit the disease to humans through the cutaneous, intestinal or inhalation route by spores from a contaminated animal, carcass or hide.

A segment of the U.S. military population has been vaccinated against anthrax. Anthrax vaccine consists of a series of six doses with yearly boosters. The first vaccine of the series must be given at least four weeks before exposure to the disease. This vaccine protects against anthrax that is acquired through the skin and it is believed that it would also be effective against inhaled spores in a biowarfare situation.