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Causes
By Mayo Clinic staffContaminated water supplies are the main source of cholera infection, although raw shellfish, uncooked fruits and vegetables, and other foods also can harbor cholera bacteria.
The bacterium that causes cholera, Vibrio cholerae, has two distinct life cycles - one in the environment and one in humans.
Cholera bacteria in the environment
Cholera bacteria occur naturally in coastal waters, where they attach to tiny crustaceans called copepods. As many as 10,000 bacteria may adhere to a single crustacean. The cholera bacteria travel with their hosts, spreading worldwide as the crustaceans follow their food source - certain types of algae and plankton that grow explosively when water temperatures rise. Algae growth is further fueled by the urea found in sewage and in agricultural runoff.
Most cholera outbreaks occur in spring and fall when ocean surface temperatures and algae blooms are at their height. More algae mean more copepods, and more copepods mean more cholera bacteria.
Cholera bacteria in people
When humans ingest cholera bacteria, they may not become sick themselves, but they still excrete the bacteria in their stool and can pass cholera disease to others through the fecal-oral route. This mainly occurs when human feces contaminate food or water supplies, both of which can serve as ideal breeding grounds for the cholera bacteria. Because more than a million cholera bacteria - approximately the amount you'd find in a glass of contaminated water - are needed to cause illness, cholera usually isn't transmitted through casual person-to-person contact.
The most common sources of cholera infection include:
- Surface or well water. Cholera bacteria can lie dormant in water for long periods, and contaminated public wells are frequent sources of large-scale cholera outbreaks. Cholera epidemics are most likely to occur in communities without adequate sanitation and in areas affected by natural disasters or war. People living in crowded refugee camps are especially at risk of cholera.
- Seafood. Eating raw or undercooked seafood, especially shellfish, that originates from certain locations can expose you to cholera bacteria. Most cases of cholera occurring in the United States since the 1970s have been traced to oysters and crab from the Gulf of Mexico and to seafood transported or smuggled from countries where cholera is endemic. Shellfish are a particular problem because they filter large amounts of water, concentrating the levels of cholera bacteria.
- Raw fruits and vegetables. Raw, unpeeled fruits and vegetables are a frequent source of cholera infection in areas where cholera is endemic. In developing nations, uncomposted manure fertilizers or irrigation water containing raw sewage can contaminate produce in the field. Fruits and vegetables may also become tainted with cholera bacteria during harvesting or processing.
- Grains. In regions where cholera is widespread, grains such as rice and millet that are contaminated after cooking and allowed to remain at room temperature for several hours become a medium for the growth of cholera bacteria.
Bacteria produce strong toxin
Although V. cholerae is the source of cholera infection, the deadly effects of the disease are the result of a potent toxin, called CTX, that the bacteria produce in the small intestine. CTX binds to the intestinal walls, where it interferes with the normal flow of sodium and chloride. This causes the body to secrete enormous amounts of water, leading to diarrhea and a rapid loss of fluids and salts (electrolytes).
- Cholera. World Health Organization. http://www.who.int/mediacentre/factsheets/fs107/en/. Accessed Feb. 19, 2009.
- Cholera. Centers for Disease Control and Prevention. http://www.cdc.gov/nczved/dfbmd/disease_listing/cholera_gi.html. Accessed Feb. 19, 2009.
- Cholera. The Merck Manuals: The Merck Manual for Healthcare Professionals. http://www.merck.com/mmpe/sec14/ch173/ch173e.html. Accessed Feb. 19, 2009.
- Butterton JR. Approach to the patient with Vibrio cholerae infection. http://www.uptodate.com/home/index.html. Accessed Feb. 19, 2009.
- Menon MP, et al. Vibrio cholerae (Cholera). In: Long SS, et al., eds. Long: Principles and Practice of Pediatric Infectious Diseases. 3rd ed. Philadelphia, Pa.: Churchill Livingstone Elsevier; 2008:844.
- Seas C, et al. Vibrio cholerae. In: Mandell GL, et al. Mandell, Bennett, & Dolin: Principles and Practice of Infectious Diseases. 6th ed. Philadelphia, Pa.: Churchill Livingstone Elsevier; 2005:2536.
