Cholera

Cholera

The cause of cholera

Transmission of cholera

Symptoms and treatment of cholera

Resources

Cholera is among the most devastating of all human diseases. Although endemic in some areas of the world, cholera is usually associated with massive migrations of people, such as those occurring during war or famine. Cholera is also common in developing countries, where suboptimal sanitation practices are responsible for its spread. If not treated, cholera has a fatality rate of over 60%. Death results from dehydration, a consequence of the severe diarrhea and vomiting that characterize this disease. In the last 15 years, treatment strategies have been devised that have cut the fatality rate of cholera to 1%. Preventive measures have also reduced the incidence of cholera outbreaks. These measures, however, require swift intervention, which is not always possible during the social upheavals that can lead to cholera epidemics.

The cause of cholera

Cholera is caused by a bacteria called Vibrio cholerae, which secretes a toxin, or poison, that binds to the cells of the small intestine. One of the functions of the small intestine in humans is to regulate the amount of fluid that is absorbed by cells. Normally, small intestine cells absorb most of the fluid that is ingested; only a small amount of fluid is excreted in the feces. Under abnormal conditions, such as in response to a pathogen, cells do not absorb fluid, and as a result a lot of fluid enters the small intestine and is excreted in the feces. These frequent, watery stools are called diarrhea. Diarrhea can actually be helpful, as the rapid movement of fluid flushes the gastrointestinal tract of harmful bacteria and other pathogens, but if diarrhea is severe or long lasting, such as occurs in cholera, too much fluid is lost and the body becomes dehydrated. If fluids are not replaced, death can result.

Along with causing fluid loss, the binding of cholera toxin to small intestine cells also results in loss of electrolytes. Electrolytes are chemicals that the body needs to function properly, such as potassium chloride, sodium chloride (salt), and bicarbonate. Electrolytes are crucial in the control of blood pressure, excretion of metabolic wastes, and maintenance of blood sugar levels. If the amount of electrolytes in the body deviates even slightly, these crucial body functions are imperiled. Cholera toxin prompts the small intestine cells to secrete large amounts of electrolytes into the small intestine. These electrolytes are then excreted in the watery diarrhea.

The cholera toxin consists of two subunits, the A subunit and the B subunit. The B subunit is a ring, and the A subunit is suspended within it. By itself, the B subunit is nontoxic; the A subunit is the poisonous part of the toxin. The B subunit binds to the small intestine cell and creates a channel within the cell membrane through which the A subunit enters. Once inside the small intestine cell, the A subunit disrupts the cascade of reactions that regulates the cell’s fluid and electrolyte balance. Fluid and electrolytes leave the cell and enter the small intestine. The resultant diarrhea may cause a fluid loss that exceeds 1 qt (1 l) per hour.

V. cholerae lives in aquatic environments and especially favors salty or brackish waters. V. cholerae frequently colonize shellfish; in fact, cholera cases in the United States are almost always traced to eating raw or undercooked shellfish. Interestingly, V. cholerae can also cause skin and other soft tissue infections. Cases of such infection with these bacteria have been found in persons who have sustained injuries in marine environments; apparently, V. cholerae in water can penetrate broken skin and cause infection.

Transmission of cholera

Cholera is endemic in several areas of the world, including parts of India and Bangladesh. From these areas, cholera has been disseminated throughout the world during several pandemics, or worldwide outbreaks. In the United States, a cholera pandemic that lasted from 1832 until 1849 killed 150, 000 people; in 1866, another cholera pandemic killed 50, 000 U.S. citizens. The most recent pandemic, which began in the 1960s and lasted until the early 1980s, involved Africa, Western Europe, the Philippines, and Southeast Asia. Smaller outbreaks, such as the Rwanda epidemic of 1994, are characteristic of wartime and famine conditions, in which large numbers of people concentrate in one place where sanitary conditions are poor to nonexistent.

Because of the nature of V. cholerae infection, past epidemics can lead to future epidemics. People recovering from cholera continue to shed the organism in their feces for weeks to months after the initial infection. These people are called convalescent carriers. Another kind of carrier, called a chronic carrier, continues to shed the bacteria for years after recovery. In both carrier types, no symptoms are present. With the ease of worldwide transportation, carriers can travel throughout the world, spreading V. cholerae wherever they go. If a carrier visits an area with less-than-ideal sanitary conditions or does not wash his or her hands after using the bathroom, the deadly V. cholerae bacteria can be easily transmitted.

Symptoms and treatment of cholera

Cholera is characterized by sudden onset. Within several hours or days after ingesting V. cholerae, severe diarrhea and vomiting occur. Fluid losses can be up to 4-5 gal (15-20 l) per day. As a consequence of this severe fluid loss, the eyes and cheeks can appear sunken, and the skin loses its pliancy.

Treatment of cholera involves the rapid replacement of fluid and electrolytes. Most persons can be treated using special rehydration formulations that utilize rice or grain as a base and are supplemented with appropriately balanced electrolytes. If a patient is too severely ill to drink even small, frequent sips, infusions of electrolytes and fluids may be necessary. Once fluid and electrolyte balance is restored, rapid reversal of symptoms occurs. Treatment with antibiotics, typically tetracycline, neutralizes the V. cholerae and decreases the number of bacteria passed into the stool.

Prevention

In the United States, sewage treatment and water purification plants are ubiquitous, and consequently,

KEY TERMS

Electrolyte —Compounds that ionize in a solution; electrolytes dissolved in the blood play an important role in maintaining the proper functioning of the body.

Endemic —Maintains a presence in a particular area or among a particular group of people.

Pandemic —Occurring on a worldwide scale or spread over an entire country or nation.

Toxin —A poisonous substance.

the incidence of cholera is low. Almost all cases of cholera in the U.S. are caused by improperly cooked shellfish. Experts recommend that all shellfish be boiled for 10 minutes; steaming does not kill V. cholerae. Raw shellfish should be avoided.

Another way to prevent cholera is to identify and treat cholera carriers in areas where cholera is endemic. Treating carriers would eliminate a major route of cholera transmission.

Currently, several cholera vaccines are being developed, but only one, Dukoral^®, is likely to be effective for long periods. Although not yet approved for use in the U.S., Dukoral is an oral vaccine, which is more easily delivered to populations in the developing world than injectable vaccines. Once an outbreak has occurred, Orochol-E^® is usually a more effective vaccine to prevent further spread of the disease. Neither vaccine is effective against a relatively new type of cholera bacteria that emerged in India and Bangladesh in the early 1990s known as Vibrio cholerae O139. Cholera outbreaks caused by V. cholerae O139 have since been reported in eleven other countries in Southeast Asia. For travelers to cholera-endemic areas, preventive measures rather than vaccines are often advised. Suggestions include drinking only boiled or chlorine- or iodine-treated water; avoiding ice; avoiding fruits and vegetables unless cooked thoroughly or peeled; and eating only thoroughly cooked seafood.

Resources

BOOKS

Coleman, William, and Alcamo, I. Edward. Cholera (Deadly Diseases and Epidemics). New York: Chelsea House, 2003.

Hayhurst, Chris. Cholera. New York: Rosen Publishing Group, 2001.

Hempel, Sandra. The Strange Case of the Broad Street Pump: John Snow and the Mystery of Cholera. Berkeley: University of California Press, 2006.

OTHER

“Cholera.” Centers for Disease Control and Prevention. <http://www.cdc.gov/ncidod/dbmd/diseaseinfo/cholera_g.htm> (accessed November 25, 2006).