Bacillus anthracis is a large, gram-positive, spore-forming bacillus that is found worldwide. The spores are very resistant to heat and drying and may survive for decades in certain soil conditions. Domestic and wild animals become infected when they ingest spores while grazing on contaminated land or eat contaminated feed. Humans become infected by ingesting contaminated meat or through agricultural or industrial exposure to include contaminated carcasses, hides, wool, hair, and bones. With the use of vaccine in at-risk workers, as well as a vaccine for animals, there has only been approximately 1 case of anthrax reported per year for the last 10 years in the U.S. Most human cases today occur in Africa and Asia where use of the vaccine is not as widespread.

To produce disease, the anthrax spore must be ingested, inhaled, or enter the body through a break in the skin. Macrophages ingest the spores at the site of entry, whereupon, the spores germinate into bacteria that rapidly replicate and release toxins. Inhaled spores are carried to tracheobronchial lymph nodes, where they are ingested and germinate. Toxins elaborated by multiplying B. anthracis cause edema, hemorrhage, and local tissue necrosis. Bacteremia and septicemia result and other organs are usually seeded, including the meninges. In inhalation anthrax, death results from a combination of respiratory failure with pulmonary edema, overwhelming bacteremia, and often, meningitis.

Diagnosis of Infection

There are three forms of anthrax: cutaneous anthrax (which accounts for 95 percent of cases of anthrax occurring naturally in the world); gastrointestinal; and inhalation anthrax. Inhalation anthrax develops following an incubation period of 1-6 days. The initial symptoms are nonspecific and include malaise, fatigue, myalgia and fever, as well as a nonproductive cough and mild chest pain. These symptoms usually persist for 2-3 days, and may even be followed by a short period of improvement. The terminal symptoms appear suddenly and include increasing respiratory distress with difficulty breathing, stridor, cyanosis, increased chest pain, and diaphoresis. The most critical aspect of making the diagnosis of inhalation anthrax is having a high index of suspicion, since the early symptoms are entirely nonspecific. The clinical picture of respiratory distress is helpful, especially in association with CXR evidence of a widened mediastinum, a result of the edema and hemorrhage occurring in the tracheobronchial lymph nodes. Sputum stains and cultures are not helpful in diagnosis since this is a mediastinal disease and not a pneumonia. Blood cultures are positive late in the course of the illness.