Cholera is an acute diarrheal illness caused by a bacterium, Vibrio cholerae. There are several environmental strains of Vibrio cholerae, which are found mainly in brackish waters and marine environments, but only two strains are responsible for cholera epidemics in humans, serogroups O1 and O139.
The first described cholera pandemic was in Europe from 1817 to 1823. However, the disease was known in Asia prior to that, with the first possible descriptions dating back as far as 2,000 years ago in India and China. Since that first pandemic there have been a total of seven pandemics. The cholera outbreaks that occurred in London, England in 1849 and 1854 are important in the history of the disease. John Snow, a physician, recognized that cholera was spread via water contaminated with human waste when he identified the source of the London outbreak as the Broad Street water pump. This discovery stimulated the future development of adequate water and sewage systems, which led to the control of many infectious diseases.
The seventh pandemic started in Sulawesi, an island in Indonesia, in 1961 and then spread rapidly through Asia and the Middle East. In 1970, for the first time in over one hundred years, cholera was found in West Africa. In 1991, cholera appeared in Peru and quickly spread throughout the remainder of South and Central America. As was the case with Africa, cholera had not been seen in the western hemisphere for over one hundred
Cholera is acquired by ingestion of V. cholerae in water, seafood, or other foods that have been contaminated by human excrement. The incubation period can range from a few hours to five days, depending on the inoculum size and the underlying health of the person. Cholera can cause a spectrum of disease, from no clinical symptoms to a mild diarrheal illness or a severe fulminant illness resulting in death. The diarrhea is caused by an enterotoxin produced by the V. cholerae that stimulates the small intestine to secrete large volumes of fluid and electrolytes. Some factors that predispose to severe disease include having blood group O, low gastric acid levels, and malnutrition. The very young and the very old are at particular risk for severe disease. Persons living in endemic areas appear to develop some natural immunity to the infection.
In symptomatic infections, there is an abrupt onset of copious diarrhea, often accompanied by abdominal cramps and vomiting. The diarrhea is typically watery and clear with mucous flecks— often described as "rice water stools." It is unusual for fever to develop. Uncomplicated cholera is a self-limited disease that resolves in three to six days. In more severe cases, fluid losses from diarrhea can amount to over 20 liters a day and can lead to profound dehydration that produces weakness, muscle cramping, loss of skin turgor, and sunken eyes and cheeks. If the fluid losses are not rapidly corrected, death results. The fatality rate can be over 50 percent in cases of severe cholera; however, with prompt and adequate rehydration the death rate may be as low as 1 to 2 percent.
The infection is diagnosed by identification of V. cholerae bacteria in stool. The organism can be grown in the laboratory on special alkaline culture media. It appears microscopically as curved, gramnegative rods. A clinical diagnosis can be made in severe cases if a patient presents with profuse, watery diarrhea in an endemic region. There are few other illness that cause such copious diarrhea.
The mainstay of treatment is fluid replacement, either intravenously or orally. In very severe cases, intravenous fluid replacement should be used. When fluids are administered by mouth, it is important to use an oral rehydration solution that contains the correct mix of sugars and electrolytes.
Prevention of cholera depends upon good sanitation and hygiene, including treatment of water supplies, adequate sewage control, and strict hygiene in food preparation. Good food preparation involves hand washing before contact with food, thorough cooking of food, eating food while it is still hot, and not allowing cooked food come into contact with raw foods or with water or ice.
There are several vaccines currently available to prevent cholera. The original cholera vaccine was a parenteral-killed preparation that provided about 50 to 60 percent protection and was only effective for a period of three to six months. This vaccine is no longer recommended for use. The World Heath Organization currently advocates the use of a killed whole cell V. cholerae O1 vaccine (WC/rBS), which is combined with one of the toxin subunits and is given in two doses one week apart. This newer vaccine has been shown to confer 85 to 90 percent protection for six months. The vaccine can be used to prevent a cholera outbreak in a population felt to be at high risk of an out-break, such as the inhabitants of refugee camps. It can also be offered to travelers going to high-risk regions. Another recently developed effective vaccine is the oral, single dose, live attenuated V. cholerae strain, devoid of the A toxin subunit (Mutachol), that provides from 62 to 100 percent protection for about six months. The level of protection varies for different cholera biotypes.
A concern about future cholera outbreaks is the possible emergence of new biotypes. Until 1992, the only strain of cholera identified as causing epidemics in humans was V. cholerae O1. That year a new serotype, O139, emerged in India. Neither previous exposure to O1 cholera, nor vaccination with current vaccines, confers protection against O139. Because V. cholerae exists naturally in brackish waters, and because of the possibility of new biotypes emerging, it is unlikely that cholera will ever be eradicated as a human pathogen. Good hygiene and sanitation are the best strategies we have for control of this disease.
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