Anthrax

Anthrax (Bacillus anthracis) gets its name from the Greek word for coal (anthrakis) due to the black skin lesions the disease causes. Anthrax has appeared throughout history; Virgil describes it among domesticated wild animals in his Georgics (29 b.c.e.). It was also the first disease of microbial origin identified by Robert Koch (1876). The first new form of anthrax, inhalation anthrax, occurred among wool sorters in England and may have been the first occupational respiratory disease. In 1979, anthrax spores were accidentally released from a military research facility in Sverdlovsk, Russia, resulting in many deaths. The 2001 postal anthrax attack in the United States showed that anthrax could be used as a bioterrorism weapon.

B. anthracis is a large, aerobic, gram-positive, spore-forming, nonmotile Bacillus species. Spores grow readily on all ordinary lab media at 37 degrees C and are shaped like a “jointed bamboo rod” or a “curled hair.” B. anthracis forms a prominent capsule in tissue in vivo and in vitro in the presence of bicarbonate and carbon dioxide. Germination occurs when the bacillus is exposed to an environment rich in amino acids, nucleosides, and glucose. However, spore formation (sporulation) will occur when the infected body is opened and exposed to air. These spores can survive for decades in ambient conditions.

Anthrax is a zoonotic disease that predominantly occurs in domesticated and wild animals. Contact with infected animals or animal products can result in human infection. There are three main forms of anthrax infection: inhalation, cutaneous, and gastrointestinal. Other forms of anthrax infection include meningococcal and oropharyngeal. Natural infection occurs predominantly by the cutaneous route.

Inhalation anthrax occurs when spores are breathed in. It is the most lethal form, with mortality greater than 80 percent. The spores must be 1–2 μ in length for them to stay in the lungs. Spores that are greater than 5 μ in size pose a minimal threat to the lungs because they are either trapped in the nasopharynx or cleared by the mucociliary escalator system. Cutaneous anthrax infection occurs when spores enter the body through an abrasion on the skin. This may occur when handling animal hides, wool, leather, or even hair from infected animals. With proper therapy, death is rare for this form.

Gastrointestinal anthrax follows consumption of grossly contaminated and undercooked food. This is characterized by acute inflammation of the intestinal tract. Oropharyngeal anthrax is a milder form of gastrointestinal anthrax and has a favorable prognosis. B. anthracis can also cause meningitis. The most common portal of entry is the skin. However, inhalation and gastrointestinal anthrax can also lead to meningitis. These meningococcal cases are almost always fatal.

B. anthracis is found worldwide. It is a normal part of the soil flora and can periodically undergo bursts of multiplication that are not well understood. Conditions for multiplication are favorable when the soil has a pH above 6.0, is rich in organic matter, and a dramatic change occurs in the soil environment such as a drought or abundance of rainfall. It is very common for herbivores to be infected while grazing. Epizootic anthrax continues to occur in highly endemic areas such as Iran, Iraq, Turkey, Pakistan, and sub-Saharan Africa. Anthrax reportedly killed over 1 million sheep in Iran in 1945. It is thought that anthrax may be spread from animal to animal by biting flies, and to different environments by vultures and nonbiting flies. Animal vaccination programs have [Page 145]helped in some areas. Nevertheless, epizootics still occur. For example, in 2000 over 30 farms in North Dakota were quarantined due to anthrax, and 157 animals died.

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