What is anthrax?
The bacterium that causes anthrax is a large, encapsulated, gram-positive rod which produces exotoxins and spores. The capsule and exotoxins are important virulence factors, and both are necessary for disease to occur. Spores are ellipsoidal or oval and are located within the bacilli. The endospores have no reproductive significance, as only one spore is formed by each bacillus and a germinated spore yields a single bacillus. Spores form in soil or dead tissue and with no measurable metabolism may remain dormant for years. They are resistant to drying, heat, and many disinfectants.
Anthrax is primarily a disease of herbivorous animals that has spread to humans through association with domesticated animals and their products. Herbivorous animals grazing in pastures with soil contaminated with endospores become infected when the spores gain entry through abrasions around the mouth and germinate in the surrounding tissues. Omnivores and carnivores can become infected by ingesting contaminated meat. Human infection is often a result of a close association with herbivores, particularly goats, sheep, or cattle (including their products of hair, wool, and hides). Anthrax has been an uncommon infection in the United States, and only 233 human cases were reported from 1955 to 1987.
The most common clinical illness in humans is skin infection acquired when spores penetrate through cuts or abrasions. After an incubation period of three to five days, a papule develops and evolves into a vesicle, which ruptures to leave an ulcer that dries to form a characteristic black eschar. Inhaled spores reach the alveoli of the lung, where they are engulfed by macrophages and germinate into bacilli. Bacilli are carried to lymph nodes, where release and multiplication are followed by bloodstream invasion and spread to other parts of the body, including the brain, causing meningitis. The illness begins with flulike symptoms a few days after the inhalation of anthrax spores and may be associated with substernal (under the breastbone) discomfort. Cough, fever, chills, and respiratory distress with dyspnea (abnormal or uncomfortable breathing) and stridor (noisy breathing) ensue. The least common type of infection is that of the gastrointestinal tract.
The laboratory criteria for the diagnosis of anthrax are cultural growth of Bacillus anthracis from a clinical specimen collected from an affected tissue or site or other supportive laboratory data, such as polymerase chain reaction (PCR), immunohistochemical staining, or serology that shows evidence of anthrax infection.
Antibiotic treatment of cutaneous anthrax does not change the course of the evolving skin lesion, but it reduces edema (swelling) and systemic symptoms such as fever, headache, and malaise which may be part of the illness. Antibiotic therapy is also initiated to prevent complications from spreading infection and, most important, bloodstream invasion. The mortality rate of cutaneous infection with appropriate antibiotic treatment is less than 1 percent. Virtually all patients with inhalation anthrax will die if untreated. Antibiotic therapy, especially if administered during the early part of this biphasic illness, combined with other intensive supportive measures, may save up to 50 percent of patients, although long-term effects may be noted. The paucity of gastrointestinal anthrax cases has resulted in limited mortality data, but mortality has been estimated to be as high as 50 percent, with disease limited to the mouth and throat having a better prognosis. If recovery ensues, the disease subsides within two weeks. Antibiotic therapy is indicated for all cases.
Initial antibiotic therapy is with either ciprofloxacin or doxycline, plus one or two additional antibiotics for more severe cases until the patient is stable. Prolonged antibiotic therapy (one hundred days) is indicated for more serious illness. Antibiotics may also be used to prevent illness in the case of possible exposure, and an anthrax vaccine is available for persons at high risk of exposure.
A disease killing cattle in 1491 b.c.e., likely to have been anthrax, is recounted in the Book of Genesis. In Exodus 9, the Lord instructs Moses to take “handfuls of ashes of the furnace” and “sprinkle it toward the heaven in the sight of the Pharaoh.” Moses performed the deed, and “it became a boil breaking forth with blains upon man and upon beast.” This may represent the first use of anthrax as a biological agent. Greek peasants tending goats suffered from anthrax. Anthrax in Greek means “coal,” which refers to the coal-black center of the skin lesion.
Anthrax was the first pathogenic bacillus to be seen microscopically when it was described in infected animal tissue by Aloys-Antoine Pollender in 1849. Studies by Robert Koch in 1876 resulted in the four postulates which form the basis for the study of infectious disease causation. In 1881, Louis Pasteur demonstrated the protective efficacy of a vaccine for sheep made with his attenuated vaccine strain.
Anthrax spores can be easily packaged to act as aerosolized agents of war, and the genome may be bioengineered to alter the virulence of anthrax or the ability of current vaccines to protect against it. In 1979, anthrax spores were accidentally released from a Soviet biowarfare facility in the city of Sverdlovsk, resulting in at least seventy-seven human cases, with sixty-six deaths. In the United States, a bioterrorism attack with anthrax spores placed in letters and sent through the mail caused twenty-two infections and five deaths in 2001.
In the future, more rapid molecular based diagnostic methods promise to identify cases earlier so that appropriate antibiotic therapy can be initiated, ensuring a maximum opportunity for recovery. A need also exists for safer and more effective vaccines that would offer protection even against bioengineered strains.
Bartlett, John G., Thomas V. Inglesby, Jr., and Luciana Borio. “Management of Anthrax.” Clinical Infectious Diseases 35 (October 1, 2002): 851–858.
Centers for Disease Control and Prevention. “Bioterrorism-Related Anthrax.” Emerging Infectious Diseases 8 (October, 2002): 1013–1183.
Dixon, Terry C., et al. “Anthrax.” New England Journal of Medicine 341 (September 9, 1999): 815–826.
Kyrincou, Demetrios N., Alys Adamski, and Nanci Khardori. “Anthrax: From Antiquity and Obscurity to a Front-runner in Bioterrorism.” Infectious Disease Clinics of North America 20 (2006): 227–251.