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Chapter: Medical Microbiology: An Introduction to Infectious Diseases: Corynebacterium,Listeria,and Bacillus

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Anthrax

Human anthrax is typically an ulcerative sore on an exposed part of the body. Constitutional symptoms are minimal, and the ulcer usually resolves without complications. If anthrax spores are inhaled, a fulminant pneumonia may lead to respiratory failure and death.

ANTHRAX

 

Human anthrax is typically an ulcerative sore on an exposed part of the body. Constitutional symptoms are minimal, and the ulcer usually resolves without complications. If anthrax spores are inhaled, a fulminant pneumonia may lead to respiratory failure and death.

 

The isolation of B. anthracis, the proof of its relationship to anthrax infection, and the demonstration of immunity to the disease are among the most important events in the his- tory of science and medicine. Robert Koch rose to fame in 1877 by growing the organism n artificial culture using pure culture techniques. He defined the stringent criteria needed to prove that the organism caused anthrax (Koch’s postulates), then met them experimen-tally. Louis Pasteur made a convincing field demonstration at Pouilly-le-Fort to show that vaccination of sheep, goats, and cows with an attenuated strain of B. anthracis prevented anthrax. He was cheered and carried on the shoulders of the grateful farmers of the dis-trict, an experience now, unhappily, largely restricted to winning football coaches.

 

EPIDEMIOLOGY

Anthrax is primarily a disease of herbivores such as horses, sheep, and cattle who acquire it from spores of B. anthracis contaminating their pastures. Humans become infected through contact with these animals or their products in a way that allows the spores to be inoculated through the skin, ingested, or inhaled. In the 1920s, more than 100 cases oc-curred annually in the United States among farmers, veterinarians, and meat handlers, but the control of animal anthrax in developed countries has made human cases rare. A few endemic foci persist in North America and have been the source of naturally acquired dis-ease. Another source is animal products such as wool, hides, or bone meal fertilizer that have been imported from a country where animal anthrax is endemic.

The real threat associated with anthrax comes from its continuing appeal to those bent on using it as an agent of biological warfare or terrorism. The long life, stability, and low mass of the dried spores make the prospect of someone producing a “cloud of death” leading to massive pulmonary anthrax a chilling reality. A 1979 episode resulting in more than 50 anthrax deaths in the former Soviet Union is now attributed to an accidental ex-plosion at a biological warfare research facility that involved more than 20 pounds of anthrax spores. United Nations inspection teams in the Middle East recently uncovered facilities for the production of massive amounts of spores together with plans to create and spread infectious aerosols using missile warheads. The inhalation anthrax among postal workers following the September 11, 2001 terrorist attacks appears to have been due to the mailing of envelopes containing “weapons-grade” anthrax spores stolen from a biologic warfare research facility. Such spores had been treated to enhance their aerosolization and dissemination.

 

PATHOGENESIS

When spores of B. anthracis reach the rich environment of human tissues, they germinate and multiply in the vegetative state. The antiphagocytic properties of the capsule aid in survival, eventually allowing production of large enough amounts of the exotoxins to cause disease. The tripartite nature of the anthrax exotoxin complex must play an important role but the timing and relative importance of the components are not known. The adenylate cy-clase activity is believed to correlate with the striking edema seen at infected sites.

 

IMMUNITY

 

The specific mechanisms of immunity against B. anthracis are not known. Experimental evidence favors antibody directed against the toxin complex, but the relative role of the components of the toxin is not clear. The capsular glutamic acid is immunogenic but anti-body against it is not protective.

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