Human anthrax has three major clinical forms depending on the route of infection: cutaneous, inhalation, and gastrointestinal. Cutaneous anthrax begins as a pruritic papule or vesicle that enlarges and erodes (1-2 days) leaving a necrotic ulcer with subsequent formation of a central black eschar; inhalation anthrax may begin as a prodrome of fever, chills, nonproductive cough, chest pain, headache, myalgias, and malaise, with more distinctive clinical hallmarks of hemorrhagic mediastinal lymphadenitis, hemorrhagic pleural effusions, bacteremia and toxemia resulting in severe dyspnea, hypoxia and septic shock; gastrointestinal anthrax may result in pharyngeal lesions with sore throat, dypshagia marked neck swelling and regional lymphadenopathy, or intestinal infection characterized by fever, severe abdominal pain, massive ascites, hematemesis, and bloody diarrhea. With any form of anthrax, hemorrhagic meningitis and spread to other organ systems can result from hematogenous and lymphatic spread of the organism from the primary site.
Bacillus anthracis is an encapsulated gram-positive, nonmotile, aerobic, spore-forming bacterial rod with a spore size of approximately 1 µm x 2 µm. The three virulence factors of B. anthracis are edema toxin, lethal toxin, and an antiphagocytic capsule. The toxins are responsible for the primary clinical manifestations of hemorrhage, edema, and necrosis.
In the United States, incidence of naturally-acquired anthrax is extremely rare (~ 1-2 cases of cutaneous disease per year). Gastrointestinal anthrax is rare, but may occur as explosive outbreaks associated with ingestion of infected animals. Worldwide, the incidence is unknown, though B. anthracis is present in most of the world. Unreliable reporting makes it difficult to estimate the true incidence of human anthrax worldwide. However, in fall 2001, 22 cases of anthrax (11 inhalation, 11 cutaneous) were identified in the United States following intentional contamination of the mail.
If untreated, anthrax in all forms can lead to septicemia, hemorrhagic meningitis, and death. The case fatality ratio for patients with appropriately treated cutaneous anthrax is usually less than 1%, but for inhalation or gastrointestinal disease it can exceed 50%. Case-fatality rates for inhalation anthrax are high, even with appropriate antibiotics and supportive care. Among the eighteen cases of inhalation anthrax in the United States during the twentieth century, the overall case fatality was greater than 85%. Following the bioterrorist attack in fall 2001, the case-fatality rate among patients with inhalation disease (all of whom received antibiotic therapy and aggressive supportive care) was 45% (5/11). The case-fatality rate of gastrointestinal anthrax is unknown but is estimated to be 25%-60%.
For humans, the source of infection in naturally-acquired disease is through contact with infected livestock, wild animals, or contaminated animal products (including carcasses, hides, hair, wool, meat, and bone meal). Person-to-person transmission is extremely unlikely and only reported with cutaneous anthrax where discharges from cutaneous lesions are potentially infectious.
Cutaneous anthrax is the most common manifestation of naturally-acquired infection with B. anthracis. Inhalation (pulmonary) anthrax occurs in persons working in certain occupations where spores may be aerosolized from contaminated animal products, such as animal hair processing or through intentional release. Occupational risk groups include those coming into contact with livestock or products from livestock, e.g., veterinarians, animal handlers, abattoir workers, and laboratorians.
For both livestock and humans, anthrax is a notifiable disease in the United States.
In the United States, the annual incidence of naturally-occurring human anthrax declined from estimated 130 cases annually in the early 1900's to fewer than 2 cases each in 2000, 2001, and 2002. The recent cases of anthrax that occurred after B. anthracis spores were distributed through the U.S. mail have further underscored the potential dangers of this organism as a bioterrorism threat. In addition to aerosolization, there is a theoretical health risk associated with B. anthracis spores being introduced into food products or water supplies.
Recently, cases of cutaneous and inhalation anthrax have occurred in workers using contaminated animal hides for drum making, and have also been reported associated with playing contaminated goatskin drums.