Anthrax--CDC review.

Clinical Features

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 (for one to two days), leaving a necrotic ulcer with subsequent formation of a central black scab. Inhalation anthrax may begin with premonitory symptoms of fever, chills, nonproductive cough, chest pain, headache, muscle pain, and malaise. More distinctive clinical hallmarks are hemorrhagic inflammation of the lymph nodes in the chest cavity, hemorrhagic pleural effusions, bacteremia and toxemia resulting in severe respiratory difficulty (dyspnea), deficiency of oxygen reaching the tissues (hypoxia), and septic shock. Gastrointestinal anthrax may result in pharyngeal lesions, with sore throat, difficulty swallowing, marked neck swelling, and regional enlargement of the lymph nodes, or with intestinal infection characterized by fever, severe abdominal pain, massive accumulation of serous fluids in the abdominal cavity, vomiting of blood, and bloody diarrhea. As with any form of anthrax, hemorrhagic meningitis can result from spread of the organism through the blood.

Etiologic Agent

Bacillus anthracis is an encapsulated Gram-positive, nonmotile, aerobic, spore-forming bacterial rod. The three virulence factors of B. anthracis are edema toxin, lethal toxin, and a capsular antigen that works against immune system cells. The toxins are responsible for the primary clinical manifestations of hemorrhage, edema, and necrosis.

Incidence

In the United States, incidence of naturally acquired anthrax infection is extremely rare (approximately one to two 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. In the fall of 2001, however, 22 cases of anthrax (11 inhalation, 11 cutaneous) were identified in the United States following intentional contamination of the mail.

Sequelae 

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 percent, but for inhalation or gastrointestinal disease it can exceed 50 percent. Case-fatality rates for inhalation anthrax are high, even with appropriate antibiotics and supportive care. Among the 18 cases of inhalation anthrax reported in the United States during the 20th century, the overall case fatality was less than 75 percent. Following the bioterrorist attack in fall 2001, the case-fatality rate among patients with inhalation disease was 45 percent (five of 11). The case-fatality rate for gastrointestinal anthrax is unknown but is estimated to be 25 to 60 percent.

Transmission

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 has been reported only with cutaneous anthrax, where discharges from cutaneous lesions are potentially infectious.

Risk Groups

Cutaneous anthrax is the most common manifestation of naturally acquired B. anthracis infection. Inhalation (pulmonary) anthrax occurs in persons working in certain occupations in which 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).

Surveillance

For both livestock and humans, anthrax is a notifiable disease in the United States.

Trends

In the United States, the annual incidence of naturally occurring human anthrax declined from an estimated 130 cases annually in the early 1900s to two cases each year 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.

(Adapted from Centers for Disease Control and Prevention [CDC], "Anthrax: Background Factsheet from the Division of Bacterial and Mycotic Disease," December 2002. For comprehensive CDC information about bioterrorism and related issues, visit http://www. bt.cdc.gov.)