Cholera
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Distribution_of_the_cholera.PNG
Cholera (also called Asiatic cholera) is an infectious disease of the gastrointestinal tract caused by the Vibrio cholerae bacterium. These bacteria are typically ingested by drinking water contaminated by improper sanitation or by eating improperly cooked fish, especially shellfish. Symptoms include diarrhea, abdominal cramps, nausea, vomiting, and dehydration. Death is generally due to the dehydration caused by the illness. When left untreated cholera generally has a high mortality rate. Treatment is typically an aggressive rehydration regimen usually delivered intravenously, which continues until the diarrhea ceases. With treatment, mortality rates plummet. Cholera was first described in a scientific manner by the physician Garcia de Orta in the 16th century.
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Pathology
Susceptibility
Carriers of the cystic fibrosis gene are protected from the severe effects of cholera because they don't lose water as quickly. This explains the high incidence of cystic fibrosis among populations which were formerly exposed to cholera.
Recent genetic research has determined that a person's susceptibility to cholera (and other diarrheas) is affected by their blood type. Those with type O blood are the most susceptible. Those with type AB are the most resistant, virtually immune. Between these two extremes are the A and B blood types, with type A being more resistant than type B.
About one million Vibrio cholerae bacteria must be ingested to cause cholera in normally healthy adults, although increased susceptibility may be observed in those with a weakened immune system, individuals with decreased gastric acidity (as from the use of antacids), or those who are malnourished.
Transmission
Cholera is transmitted through ingestion of substances contaminated with the bacterium. The contamination usually occurs when untreated sewage is released into waterways, affected the water supply, any foods washed in the water, and shellfish living in the affected waterway — it is only rarely spread directly from person to person. The resulting diarrhea allows the bacterium to spread to other people under unsanitary conditions.
Symptoms
Symptoms include those of general GI tract upset (diarrhea, abdominal cramps, nausea, vomiting) along with those of the resulting dehydration (thirst, muscle cramps, weakness, loss of tissue turgor, sunken eyes and wrinkled skin, severe metabolic acidosis with potassium depletion, anuria, circulatory collapse and cyanosis). Death is through circulatory volume shock, and can occur within hours.
Causes of symptoms
More information at Vibrio cholerae
The root causes of these symptoms are the enterotoxins that V. cholerae produces. The main enterotoxin, known as cholera toxin, interacts with G proteins and cyclic AMP in the intestinal lining to open ion channels. As ions flow into the intestinal lumen, water follows from osmotic pressure.
History
Origin
Cholera originated in India or elsewhere in Asia, with the Ganges River likely serving as a contamination reservoir.
Discovery
The scientists with major contributions to fighting cholera were John Snow, who found the link between cholera and drinking water in 1854, and Robert Koch, who identified Vibrio cholerae as the bacillus causing the disease. The bacterium was originally isolated thirty years earlier by Italian anatomist Filippo Pacini, but his results were not widely known.
Spread
Cholera was originally endemic in India, but spread by trade routes (land and sea) to Russia, then to Western Europe, and from Europe to North America. It is now no longer considered an issue in Europe and North America, due to filtering and chlorination of the water supply.
Mortality/morbidity patterns
Cholera is often endemic in the tropics, but may cause epidemics anywhere. Populations suffer outbreaks, after which near-total immunity is achieved by most who are affected.
Current status
Endemic in India and sub-Saharan Africa, cholera also produces periodic epidemics in South America.
- 1,099,882 cases and 10,453 deaths were reported in the Western Hemisphere between January 1991 and July 1995.
- On average, one case of cholera is reported in the United States every week.
Treatment
Prevention
Although cholera can be life-threatening, it is easily prevented and treated. In the United States, because of advanced water and sanitation systems, cholera is not a major threat. The last major outbreak of cholera in the United States was in 1911. However, everyone, especially travelers, should be aware of how the disease is transmitted and what can be done to prevent it.
Simple sanitation is usually sufficient to stop an epidemic. There are several points along the transmission path at which the spread may be halted:
- Sickbed: Proper disposal and treatment of waste produced by cholera victims.
- Sewage: Treatment of general sewage before it enters the waterways.
- Sources: Warnings about cholera contamination posted around contaminated water sources.
- Sterilization: Boiling, filtering, and chlorination of water before use.
Filtration and boiling is by far the most effective means of halting transmission. In general, education and sanitation are the limiting factors in prevention of cholera epidemics.
Medicine
Cholera may be treated with antibiotics and oral rehydration (note that this requires non-contaminated water, which is often not available in outbreak areas). Other than the antibiotics used to kill the bacteria, it is sufficient to treat the symptoms of dehydration. If blood pressure is within acceptable bounds, oral rehydration is used. If blood pressure drops too far, rehydration is accomplished by means of an IV fluid-replacement line. The goal is to prevent circulatory collapse and electrolyte depletion. Tetracycline is often the antibiotic of choice. It should not be used by pregnant women or children under 6. In the latter it may result in permanent staining of the teeth. The CDC recommends avoiding use of antibiotics in non-severe cases in an effort to prevent antibiotic resistance.
Vaccines against Cholera are available, but because the vibrio bacteria keep mutating it keeps becoming resistant meaning there is a need to produce new vaccines. It is generally recommended that only those travelling from countries which are practically devoid of the disease to countries such as India where the disease is endemic take the vaccine, as the vaccine only lasts a few months.
Sources
- FDA "Bad Bug Book" entry: http://vm.cfsan.fda.gov/~MOW/chap7.html
- The Cholera toxin: http://tinyurl.com/6232m
References:
- Cholera facts from the United States FDA website: http://vm.cfsan.fda.gov/~MOW/chap7.html
External links:
- What is Cholera? (http://www.cdc.gov/ncidod/dbmd/diseaseinfo/cholera_g.htm) (CDC)
- Posen-L:consise description, extended history of epidemics, with timeline (http://www.posen-l.com/Cholera.htm)
- Todar's Online Textbook of Bacteriology: Cholera (http://textbookofbacteriology.net/cholera.html)
- Study on the relationship between blood type and cholera susceptibility (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=8035040) (Medline)cs:Cholera
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