Bacillus anthracis is a spore-forming, aerobic gram-positive rod that can cause acute infection (anthrax) in humans and animals, and has the potential to be used as a biologic weapon due to its rapid systemic progression and high mortality in those who are exposed and untreated.


  • Anthrax is primarily a zoonotic disease. Most naturally acquired anthrax infections are cutaneous (95%). Inhalational (5%) and gastrointestinal (GI) (<1%) forms are particularly rare.
    • In recent times, injection anthrax has been described in Europe and is associated with injecting drug users.
    • This form has not yet been reported in the United States nor in the pediatric population.
  • Incidence is extremely low, averaging 1 to 2 cases of cutaneous anthrax per year in the United States.
  • Prior to October 2001, only 18 cases of inhalational anthrax were reported in the United States during the 20th century.
  • No human-to-human spread of inhalational anthrax has been reported.
  • Rare cases of human-to-human transmission of cutaneous anthrax have been reported after direct contact with infected skin lesions.
  • Anthrax has been used as an agent of bioterrorism.

General Prevention

  • Antibiotics are effective against germinating B. anthracis but not against the spores. Therefore, if prophylactic antibiotics are stopped prematurely, remaining spores can cause disease when they germinate. This phenomenon of delayed-onset disease occurs with inhalational anthrax and not with cutaneous or GI exposures.
  • Where the threat of transmission of B. anthracis spores is deemed credible, decontamination of skin and potential fomites (e.g., clothing) may be considered to reduce the risk for cutaneous and GI forms of the disease.
  • Anthrax vaccine absorbed (AVA) is the only licensed human anthrax vaccine in the United States.
    • Primary vaccination consists of subcutaneous injections at 0, 2, and 4 weeks and three booster vaccinations at 6, 12, and 18 months.
    • Annual booster injections are required to maintain immunity.
    • The most common adverse event is injection-site discomfort (e.g., edema, pain, local hypersensitivity).
  • Infection control
    • Immediately notify the hospital epidemiologist, infection control department, or local health department of suspected cases.
    • No data suggest that patient-to-patient transmission of inhalational anthrax occurs.
    • Standard barrier isolation precautions are recommended for all hospitalized patients with all forms of anthrax infection.
    • High-efficiency particulate air-filter masks or other measures for airborne precautions are not indicated.
    • There is no need to immunize or provide prophylaxis to patient contacts unless they, like the patient, were exposed to the aerosol.
    • If anthrax is used as a bioweapon, spores may be detected on environmental surfaces. The risk of inhalation anthrax from secondary aerosolization of these spores is uncertain.
Pulmonary disease caused by anthrax is a hemorrhagic mediastinitis with pleural effusions and not a bronchopneumonia.


  • After inhalation, wound inoculation, or ingestion, B. anthracis spores infect macrophages, germinate, and proliferate.
    • Proliferation occurs at the site of infection and in regional lymph nodes.
    • Replicating bacteria release toxins, leading to edema, hemorrhage, and necrosis.
  • Incubation period depends on the route of transmission.
    • Inhalational anthrax: Infection requires inhalation of >8,000 spores; incubation period is 2 to 60 days.
    • Cutaneous anthrax: Spores enter a cut or abrasion in the skin; incubation period is 1 to 12 days.
    • GI anthrax: Spores are ingested in undercooked, infected meat; incubation period is 1 to 7 days; infection occurs in the upper (oropharyngeal lesions) or lower (intestinal lesions) GI tract.
    • Injection anthrax: Spores enter injection site in injection drug users.
  • Hematogenous spread of the bacteria causes infection at other sites, including the CNS, liver, spleen, and kidney.

Commonly Associated Conditions

If anthrax is intentionally released, physicians must be alert for diseases caused by other potential biologic warfare agents (e.g., plague, tularemia, Q fever, smallpox, and botulism).

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