Anthrax is caused by Bacillus anthracis, toxin-producing, encapsulated, aerobic or facultative anaerobic organisms. Anthrax, an often fatal disease of animals, is transmitted to humans by contact with infected animals or their products. In humans, infection typically occurs through the skin. Inhalation infection is less common; oropharyngeal, meningeal, and GI infections are rare. For inhalation and GI infections, nonspecific local symptoms are typically followed in several days by severe systemic illness, shock, and often death. Empiric treatment is with ciprofloxacin or doxycycline. A vaccine is available.
(See also the Centers for Disease Control and Prevention's Emergency Preparedness and Response information regarding anthrax and the American College of Physicians' Quick Facts About Anthrax.)
Anthrax is an important domestic animal disease, occurring in goats, cattle, sheep, and horses. Anthrax also occurs in wildlife, such as hippos, elephants, and Cape buffalo. It is rare in humans and occurs mainly in countries that do not prevent industrial or agricultural exposure to infected animals or their products (eg, hides, carcasses, hair). The incidence of natural infection has decreased, particularly in the developed world.
However, the potential use of anthrax as a biological weapon has increased fear of this pathogen. Spores have been prepared in very finely powdered form (weaponized) to be used as agents of warfare and bioterrorism (see Biological Agents as Weapons); in anthrax bioattacks of 2001, spores were spread via the United States Postal Service.
Bacillus anthracis readily form spores when they dry—an environmental condition unfavorable for growth. Spores resist destruction and can remain viable in soil, wool, and animal hair for decades. Spores germinate and begin multiplying rapidly when they enter an environment rich in amino acids and glucose (eg, tissue, blood).
Human infection can be acquired by
Cutaneous infection is usually acquired by contact with infected animals or spore-contaminated animal products. Open wounds or abrasions increase susceptibility, but infection may occur when skin is intact. Skin infection may be transmitted from person to person by direct contact or fomites.
GI (including oropharyngeal) infection may occur after ingestion of inadequately cooked meat containing the vegetative forms of the organism, usually when a break in the pharyngeal or intestinal mucosa facilitates invasion. Ingested anthrax spores can cause lesions from the oral cavity to the cecum. Released toxin causes hemorrhagic necrotic ulcers and mesenteric lymphadenitis, which may lead to intestinal hemorrhage, obstruction, or perforation.
Pulmonary infection (inhalation anthrax), caused by inhaling spores, is almost always due to occupational exposure to contaminated animal products (eg, hides) and is often fatal.
GI and inhalation anthrax are not transmitted from person to person.
After entering the body, spores germinate inside macrophages, which migrate to regional lymph nodes where the bacteria multiply. In inhalation anthrax, spores are deposited in alveolar spaces, where they are ingested by macrophages, which migrate to mediastinal lymph nodes, usually causing a hemorrhagic mediastinitis. Bacteremia may occur in any form of anthrax and occurs in nearly all fatal cases; meningeal involvement is common.
The virulence of B. anthracis is due to its antiphagocytic capsule, its toxins (factors), and its rapid replication capability.
The predominant toxins are edema toxin and lethal toxin. Protective antigen binds to target cells and facilitates cellular entry of edema toxin and lethal toxin. Edema toxin causes massive local edema. Lethal toxin triggers a massive release of cytokines from macrophages, which is responsible for the sudden death common in anthrax infections.
Symptoms and Signs
Most patients present within 1 to 6 days of exposure, but for inhalation anthrax, the incubation period can be > 6 wk.
Cutaneous anthrax begins as a painless, pruritic, red-brown papule 1 to 10 days after exposure to infective spores. The papule enlarges with a surrounding zone of brawny erythema and marked edema. Vesiculation and induration are present. Central ulceration follows, with serosanguineous exudation and formation of a black eschar (the malignant pustule). Local lymphadenopathy is common, occasionally with malaise, myalgia, headache, fever, nausea, and vomiting. It may take several weeks for the wound to heal and the edema to resolve.
GI anthrax ranges from asymptomatic to fatal. Fever, nausea, vomiting, abdominal pain, and bloody diarrhea are common. Ascites may be present. Intestinal necrosis and septicemia with potentially lethal toxicity ensue.
Oropharyngeal anthrax manifests as edematous lesions with central necrotic ulcers on the tonsils, posterior pharyngeal wall, or hard palate. Soft-tissue swelling in the neck is marked, and cervical lymph nodes are enlarged. Symptoms include hoarseness, sore throat, fever, and dysphagia. Airway obstruction may occur.
Inhalation anthrax begins insidiously as a flu-like illness. Within a few days, fever worsens, and chest pain and severe respiratory distress develop, followed by cyanosis, shock, and coma. Severe hemorrhagic necrotizing lymphadenitis develops and spreads to adjacent mediastinal structures. Serosanguineous transudation, pulmonary edema, and bloody pleural effusion occur. Typical bronchopneumonia does not occur. Hemorrhagic meningoencephalitis or GI anthrax may develop.
Occupational and exposure history is important. Cultures and Gram stain of samples from clinically identified sites, including cutaneous or mucosal lesions, pleural fluid, CSF, ascites, or stool, should be done. Sputum examination and Gram stain are unlikely to identify inhalation anthrax because airspace disease is frequently absent. A PCR test and immunohistochemical methods can help. Nasal swab testing for spores in people potentially exposed to inhalation anthrax is not recommended because the predictive value is unknown.
Chest x-ray (or CT) should be done if pulmonary symptoms are present. It typically shows widening of the mediastinum (because of enlarged hemorrhagic lymph nodes) and pleural effusion. Pneumonic infiltrates are uncommon. Lumbar puncture should be done if patients have meningeal signs or a change in mental status. An enzyme-linked immunosorbent assay (ELISA) is available, but confirmation requires a 4-fold change in antibody titer from acute to convalescent specimens.
Mortality in untreated anthrax varies depending on infection type:
Cutaneous anthrax without significant edema or systemic symptoms is treated with ciprofloxacin 500 mg (10 to 15 mg/kg for children) po q 12 h or doxycycline 100 mg (2.5 mg/kg for children) po q 12 h for 7 to 10 days. Treatment is extended to 60 days if concomitant inhalation exposure was possible. Children and pregnant or breastfeeding women, who typically should not be given ciprofloxacin or doxycycline, should nonetheless be given one of these drugs; however, if prolonged treatment is needed, they may be switched to amoxicillin 500 mg (15 to 30 mg/kg for children) tid after 14 to 21 days if the organism is shown to be susceptible to penicillin. Mortality is rare with treatment, but the lesion will progress through the eschar phase.
Inhalation and other forms of anthrax, including cutaneous anthrax with significant edema or systemic symptoms, require therapy with 2 or 3 drugs: ciprofloxacin 400 mg (10 to 15 mg/kg for children) IV q 12 h or doxycycline 100 mg (2.5 mg/kg for children) IV q 12 h, plus penicillin, ampicillin, imipenem/cilastatin, meropenem, rifampin, vancomycin, clindamycin, or clarithromycin. Corticosteroids may be useful for meningitis and severe mediastinal edema but have not been evaluated adequately. Ca channel blockers and ACE inhibitors may be considered. Raxibacumab is a monoclonal antibody now available to treat effects of toxins already present and can be combined with antibacterial therapy. Raxibacumab has shown efficacy in animal models of inhalation anthrax, particularly when given early. With early diagnosis and intensive support, including mechanical ventilation, fluids, and vasopressors, mortality may be reduced to less than the rate in previously documented cases (45% in the US 2001 anthrax attacks and 90% in cases before these attacks). If treatment is delayed (usually because the diagnosis is missed), death is likely.
Drug resistance is a theoretical concern. Although normally sensitive to penicillin, B. anthracis manifests inducible β-lactamases, so single-drug therapy with a penicillin or a cephalosporin is not recommended. Biological warfare researchers may have created strains of anthrax that are resistant to multiple antibiotics, but these strains have not yet been encountered in a clinical situation.
An anthrax vaccine, composed of a cell-free culture filtrate, is available for people at high risk (eg, military personnel, veterinarians, laboratory technicians, employees of textile mills processing imported goat hair). A separate veterinary vaccine is also available. Repeated vaccination is required to ensure protection. Local reactions from vaccine can occur.
Limited data suggest that cutaneous anthrax does not result in acquired immunity, particularly if early effective antimicrobial therapy was used. Inhalation anthrax may provide some immunity in patients who survive, but data are very limited.
Postexposure measures include
Asymptomatic people (including pregnant women and children) exposed to inhaled anthrax require prophylaxis with oral ciprofloxacin 500 mg (10 to 15 mg/kg for children) q 12 h or doxycycline 100 mg (2.5 mg/kg for children) q 12 h for 60 days. If the organism has been shown to be susceptible to penicillin, amoxicillin 500 mg (25 to 30 mg/kg for children) tid is an option when ciprofloxacin and doxycycline are contraindicated.
The Centers for Disease Control and Prevention (CDC) recommends that the anthrax vaccine be administered with antibiotic prophylaxis to patients exposed to anthrax spores. Postexposure antibiotic treatment is extended to 100 days in patients who are vaccinated.
Last full review/revision April 2013 by Larry M. Bush, MD; Maria T. Perez, MD
Content last modified September 2013