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The Epidemiology of Anthrax

Anthrax occurs worldwide.  The organism exists in the soil as a spore.   Animals, domestic or wild, become infected when they ingest spores while grazing on contaminated land or eating contaminated feed.  Only when the organism in the carcass is exposed to air does sporulation occur.  Environmental conditions such as drought, which may promote trauma in the oral cavity on grazing, are thought to increase the chances of acquiring anthrax, as Pasteur originally reported1.

Anthrax in humans is associated with agricultural, horticultural, or industrial exposure to infected animals or contaminated animal products. In the less-developed countries, primarily in Africa, Asia, and the Middle East, disease occurs from contact with infected domesticated animals or contaminated animal products. This includes handling contaminated carcasses, hides, wool, hair, and bones; and ingesting contaminated meat. Cases associated with industrial exposure, rarely seen today, occur in workers processing contaminated hair, wool, hides, and bones. Direct contact with contaminated material leads to cutaneous disease, while ingestion of infected meat gives rise to oropharyngeal or gastrointestinal forms of anthrax. Inhalation of a sufficient quantity of spores, usually seen only during generation of aerosols in an enclosed space associated with processing contaminated wool or hair, gives rise to inhalational anthrax.

Unreliable reporting makes it difficult to estimate with accuracy the true incidence of human anthrax.  It was estimated in 1958 that worldwide between 20,000 and 100,000 cases occurred annually2.  In more recent years, anthrax in animals has been reported from 82 countries, and human cases continue to be reported from Africa, Asia, Europe, and the Americas3.  In the United States, the annual incidence of human anthrax has steadily declined—from about 127 cases in the early years of this century to about one per year for the past 10 years.  The vast majority of cases have been cutaneous.  Under natural conditions, inhalational anthrax is exceedingly rare, with only 18 cases having been reported in the United States in the 20th century4.  In the early years of this century, cases of inhalational anthrax were reported in rural villagers in Russia who worked with contaminated sheep wool inside their homes5.  Five cases of inhalational anthrax occurred in woolen mill workers in New Hampshire in the 1950s6.  During times of economic hardship and disruption of veterinary and human public health practices, such as occurs during war, there have been large epidemics of anthrax.  The largest reported epidemic of human anthrax occurred in Zimbabwe from 1978 through 1980, with an estimated 10,000 cases.  Essentially all were cutaneous, with very rare cases of gastrointestinal disease and eight cases of inhalational anthrax, although no autopsy confirmation was reported7.

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1.  Wilson GS, Miles AA. Topley and Wilson’s Principles of Bacteriology and Immunity. Vol 2. Baltimore, Md: Williams & Wilkins; 1955: 1940

2.  Glassman HN. World incidence of anthrax in man. Public Health Rep. 1958;73:22–24.

3.  Fujikura T. Current occurrence of anthrax in man and animals. Salisbury Med Bull Suppl. 1990;68:1.

4.  Brachman PS. Inhalation anthrax. Ann N Y Acad Sci. 1980;353:83–93.

5.  Elkina AV. The epidemiology of a pulmonary form of anthrax. Zh Mikrobiol Epidemiol Immunobiol. 1971;48:112–116.

6.  Plotkin SA, Brachman PS, Utell M, Bumford FH, Atchison MM. An epidemic of inhalation anthrax, the first in the twentieth century, I: Clinical features. Am J Med. 1960;29:992–1001.

7.  Davies JC. A major epidemic of anthrax in Zimbabwe. Cent Afr J Med (Zimbabwe).

(The preceding material was derived from the Virtual Naval Hospital web site www.vnh.org)

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