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Basics
Description
- Anthrax is a highly infectious disease of animals, especially ruminants (hooved animals such as cows, goats, and sheep) that is caused by the bacteria Bacillus anthracis. Cutaneous (95% of US cases), inhalational, and GI forms can be transmitted to humans by contact with the animals or their products (typically hair or hides).
- Synonym(s) for cutaneous anthrax: Charbon; Malignant pustule; Siberian ulcer; Malignant edema; Splenic fever; Milzbrand
- Synonym(s) for inhalational anthrax: Ragpicker disease; Woolsorter disease
Epidemiology
- Total of 235 anthrax cases (224 cutaneous and 11 inhalational) occurred in the US between 1955 and 1994, resulting in 20 fatalities.
- Cutaneous: 95% of cases in the US; cases of cutaneous anthrax without occupational risk should raise concern for bioterrorism:
- ~5–20% of untreated cases result in death; case fatality rate is <1% with antibiotic therapy.
- GI: Very rare in the US (a few documented cases in the last 50 years):
- Mortality rate is estimated to be 25–60%.
- Inhalational anthrax is rare in the US; if no occupational exposures to animal hides or products it should be considered a bioterrorist event in the US until proven otherwise:
- Death results in 99% of untreated cases and in 45–80% of patients with severe symptoms who are treated in a state-of-the-art facility.
- From October to November 2001, the first 10 cases of intentional inhalational anthrax were caused by a terrorist release of B. anthracis in the US.
- A new route of anthrax exposure termed “Injectional Anthrax” has occurred over the past several years where B. anthracis was a contaminant in the drug injected; >50 recognized cases occurred in the United Kingdom between December 2009 and December 2010, with a 33% case fatality rate.
- Anthrax is most common in agricultural regions, where it occurs in animals. These regions include the Middle East, Asia, Southern and Eastern Europe, Africa, South and Central America, and the Caribbean.
Risk Factors
- Contact with infected animals or their products
- Drug users
- Bioterrorist event
General Prevention
- Anthrax vaccine protects against all forms of anthrax and is as safe as other vaccines, according to the FDA, CDC, and the National Academy of Sciences.
- A 2009 review by the Cochrane Infectious Disease Group concluded that the anthrax vaccine is effective in reducing the risk of contracting anthrax and has a low rate of adverse effects (1)[A].
- A longitudinal study published in 2012 of United Kingdom military personnel who were offered anthrax vaccination showed no overall adverse health effect from the vaccination in follow-up data ranging from 3–6 years (2)[B].
- Anthrax vaccine should be effective against all known strains of B. anthracis as well as against any strains that might be bioengineered by terrorists or others.
- Vaccine schedule and route: Vaccine is given IM in 5 doses at
0, 4 weeks, and 6, 12, and 18 months plus annual boosters. IM vs. SC injection greatly reduces the incidence of injection-site adverse events (3)[A]:
- Anthrax vaccine adsorbed (trade name BioThrax) is FDA approved for ages 18–65 and is pregnancy Category D.
- If behind schedule, do not start the series over; begin where left off (delays do not reduce the resulting protection).
- Individuals are not considered protected until they have completed the full vaccination series.
- The most common (>10%) local (injection-site) adverse reactions observed in clinical studies were tenderness, pain, erythema, and arm motion limitation. The most common (>5%) systemic adverse reactions were muscle aches, fatigue, and headache.
- The Advisory Committee on Immunization Practices recommends vaccination for the following groups:
- Persons who work directly with the organism in the laboratory
- Persons who work with imported animal hides or furs in areas where standards are insufficient to prevent exposure to anthrax spores
- Persons who handle potentially infected animal products in high-incidence areas
- Military personnel deployed to areas with high risk for exposure to organisms (when used as a biologic warfare weapon)
- Pregnant women should be vaccinated for anthrax only if absolutely necessary.
- Patients with a likely inhalational exposure history, but no symptoms, are candidates for postexposure prophylaxis with either ciprofloxacin 500 mg PO b.i.d. or doxycycline 100 mg PO b.i.d. for 60 days. Levofloxacin is also FDA approved for patients age ≥18. CDC guidelines state patients should also receive 3 doses of anthrax vaccine (0, 2 weeks, 4 weeks), but since BioThrax is not licensed for postexposure prophylaxis or for a 3-dose series, this would need to be conducted under an Investigational New Drug application. Prophylactic medications are not indicated for prevention of cutaneous anthrax.
Pathophysiology
- B. anthracis is a spore-forming, gram-positive bacterium found in the soil worldwide. The word anthracis is derived from a Greek word meaning “coal,” which is used to describe the cutaneous form of the disease that leads to a characteristic black lesion.
- B. anthracis has 3 known virulence factors: An antiphagocytic capsule and 2 protein toxins (known as edema factor and lethal factor):
- The capsule provides resistance to phagocytosis.
- Lethal factor and edema factor are named for the effects they induce when injected into experimental animals.
- A protein called protective antigen binds to the host cell’s surface; when cleaved by a protease on the cell surface, it creates a site to which the lethal factor and edema factor can bind; protective antigen is required for the action of the 2 protein toxins.
- B. anthracis spores introduced into the host are ingested at the exposed site by macrophages and then germinate into vegetative forms that produce the virulence factors.
Etiology
- Cutaneous: Occurs when B. anthracis enters the skin through a cut or abrasion during the handling of animal products (e.g., meat, wool, or hides infected with B. anthracis)
- GI: Ingestion of bacillus-contaminated meat
- Inhalational: Inhalation of aerosolized B. anthracis spores
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