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Cholera is a severe diarrheal illness that spreads through contaminated water. Left untreated, it can cause death in a matter of hours.

Modern sewage and water treatment facilities have virtually eliminated cholera in industrialized nations; the last major cholera outbreak in the United States occurred in 1911. But the illness is still present in much of the world, particularly India and sub-Saharan Africa. The risk of epidemics is highest when poverty, war or natural disasters force people to live in crowded conditions without adequate sanitation.

The great irony is that unlike many infectious diseases, cholera is easily treated. Death results from severe dehydration, which can be prevented with a simple and inexpensive rehydration solution.

Most people exposed to cholera don't become ill and never know they've been infected. Yet because they shed the bacteria in their stool for seven to 14 days, they still have the potential to infect others. The great majority of people who become sick experience mild or moderate diarrhea that's often hard to distinguish from diarrhea caused by other problems.

Only about one in 10 infected people develops the typical signs and symptoms of cholera, which include:

• Severe, watery diarrhea. The incubation time for cholera is brief — usually one to five days after infection. Diarrhea comes on suddenly. It's often voluminous, flecked with mucus and dead cells, and has a pale, milky appearance that resembles water in which rice has been rinsed (rice water stool). What makes cholera diarrhea so deadly is the loss of large amounts of fluids in a short time — as much as a quart an hour.
• Nausea and vomiting. Occurring in both the early and later stages of the disease, vomiting may persist for hours at a time.
• Muscle cramps. These result from the rapid loss of salts such as sodium, chloride and potassium.
• Dehydration. This can develop just hours after the onset of symptoms — far more quickly than in other diarrheal diseases. Depending on how much body weight has been lost, dehydration can range from mild to severe; a loss of 10 percent or more of total body weight indicates severe dehydration. Signs and symptoms include irritability, lethargy, sunken eyes, a dry mouth, extreme thirst, dry, shriveled skin that's slow to bounce back when pinched into a fold, little or no urine output, low blood pressure, and an irregular heartbeat (arrhythmia).
• Shock. Hypovolemic shock is one of the most serious complications of dehydration. It occurs when low blood volume causes a drop in blood pressure and a corresponding reduction in the amount of oxygen reaching your tissues. If untreated, severe hypovolemic shock can cause death in a matter of minutes.

Signs and symptoms in children
In general, children with cholera have the same signs and symptoms adults do, but they may also experience:

• Extreme drowsiness or even coma
• Fever
• Convulsions

Mid-19th-century London was hardly the stuff of Masterpiece Theater. The air was filthy, the streets streamed with waste and animal droppings, and underneath it all was a foul system of cesspits and decaying sewers. This made the city a prime breeding ground for infectious disease, including cholera.

Between 1831 and 1854 in England alone, tens of thousands of people died of the disease. Prevailing wisdom had it that London's bad air gave rise to cholera, but John Snow, a British anesthesiologist and a leading figure in the history of public health, proved otherwise.

In 1854, at the height of what he called "the most terrible outbreak of cholera that ever occurred in the kingdom," Snow traced the epidemic to a single contaminated water pump. Today, contaminated water supplies continue to be the main source of 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. Environmental changes, notably global warming, and mutations in the bacterium itself raise questions about the future of cholera and its eventual eradication.

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 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. The rampant algae growth is further fueled by the urea found in sewage and in agricultural runoff.

In the 18th and 19th centuries, cholera bacteria also followed human travelers from port to port, inadvertently carried in kegs of contaminated water and in the feces of those who were infected. The spread of the disease was fueled not only by unsanitary conditions in most parts of the world but also by the reluctance of cities like London, flush with profits from international trade, to quarantine ships that came into their ports.

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. Some scientists are concerned that global climate change and a rise in ocean temperatures will encourage the growth of both algae and copepods and increase the likelihood that cholera bacteria will survive and spread.

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 the 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 bacteria. Because more than a million bacteria — approximately the amount in a glass of 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. V. cholerae can lie dormant in water for long periods of time, and contaminated public wells are frequent sources of large-scale outbreaks. Epidemics are most likely to occur in communities without adequate sanitation and in areas hard hit by natural disasters or war. People living in crowded refugee camps are especially at risk. Water stored at home also can become contaminated. Bathing or washing dishes in water containing cholera bacteria can spread the disease.
• Seafood. Eating raw or undercooked seafood, especially shellfish, that originate 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 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 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 are a perfect medium for the growth of V. cholerae.

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, 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 past and present
Cholera is an ancient disease. The first epidemics seem to have originated in India, and references to dehydrating diarrhea appear in old Sanskrit texts. But in the early 19th century, the disease began to appear in other parts of the world, its path following the trade routes that opened with colonial expansion. Just 100 years later, six major epidemics had swept the globe.

The seventh pandemic began in Indonesia in 1961 and has since affected more than 100 countries. In 1998 and 1999, more than 400,000 people in 14 African nations contracted the disease. Hardest hit was sub-Saharan Africa, where cholera remains endemic. In the same year, cholera appeared in South America for the first time in nearly a century. The initial outbreak quickly assumed epidemic proportions, eventually spreading to Central America and Mexico. By 1995, cholera had sickened over a million people in the Western Hemisphere.

These worldwide epidemics not only were deadly, they were caused by a single strain of cholera bacteria, V. cholerae O1. But in 1993, a new strain, V. cholerae O139, appeared in Bangladesh and India. Just as virulent as O1, O139 has undergone genetic changes that make it a threat even for people immune to the older strain.

Everyone is susceptible to cholera, with the exception of nursing infants who derive immunity from their mothers' milk. Still, certain factors can make you more vulnerable to the disease or more likely to experience severe signs and symptoms. These risk factors include:

• Malnutrition. Malnutrition and cholera are interconnected. People who are malnourished are more likely to develop cholera infection, and cholera is more likely to flourish in places where malnutrition is common, such as refugee camps, impoverished countries, and areas devastated by famine, war or natural disasters.
• Reduced or nonexistent stomach acid (hydrochlorhydria or achlorhydria). Cholera bacteria can't survive in an acidic environment, and ordinary stomach acid often serves as a first-line defense against infection. But people with low levels of stomach acid lack this protection and so are more likely to develop cholera and to have severe signs and symptoms of the disease. Children and older adults, especially, tend to have lower than normal stomach acid levels. So do people who have had gastric surgery, who have untreated Helicobacter pylori infection, or who are taking antacids, H2 blockers or proton pump inhibitors for ulcer disease. Antacids help neutralize stomach acid, and H2 blockers and proton pump inhibitors reduce the amount of acid your body produces.
• Household exposure. You're more likely to develop cholera if you live with someone who has the disease. Up to half of household contacts of infected people also become sick.
• Compromised immunity. If your immune system is compromised for any reason, you're more susceptible to cholera infection.
• Type O blood. For reasons that aren't entirely clear, people with type O blood are twice as likely to develop cholera as are people with other blood types.
• Raw or undercooked shellfish. Although large-scale cholera outbreaks no longer occur in industrialized nations, eating raw shellfish — particularly oysters — from waters known to harbor the bacteria or shellfish transported by travelers from countries where cholera is endemic greatly increases your risk.

The risk of cholera is slight in industrialized nations, and even in endemic areas you're not likely to become infected if you follow food safety recommendations. Still, sporadic cases of cholera occur throughout the world. If you have diarrhea, especially severe diarrhea, and think you may have been exposed to cholera, seek treatment right away. Severe dehydration is a medical emergency and requires immediate care.

Although the signs and symptoms of severe cholera may be unmistakable in endemic areas, the only way to confirm a diagnosis is to identify the bacteria in a stool sample. But because cholera needs immediate treatment and because all cases of watery diarrhea are treated in the same way, doctors are likely to begin rehydration without a definitive diagnosis.

They're also likely to carefully monitor vital signs such as blood pressure and pulse as well as blood sugar levels, electrolytes, and the amount of oxygen and carbon dioxide in the blood — all of which can be affected by the severely dehydrating effects of cholera.

Cholera can quickly become fatal. In the most severe cases, the rapid loss of large amounts of fluids and electrolytes can lead to death within two to three hours. In less extreme situations, people who don't receive treatment may die of dehydration and shock 18 to 48 hours after symptoms first appear.

Although shock and severe dehydration are the most devastating complications of cholera, other problems can occur, such as:

• Low blood sugar (hypoglycemia). A common complication in children, this occurs when blood levels of glucose, the body's main energy source, fall abnormally low. Glucose is absorbed directly into your bloodstream after eating and enters your cells through the action of the hormone insulin. When blood sugar reaches normal levels, your body stores extra sugar in your liver in the form of glycogen. As blood sugar levels drop between meals, your liver breaks down glycogen and releases glucose into your bloodstream. In emergencies, your liver and kidneys also can manufacture glucose (gluconeogenesis). But severe cholera interferes with all of these processes. Most people become too ill to eat, so they don't get glucose from food, their glycogen stores are quickly depleted, and the body becomes unable to carry out normal glucose production. This can lead to unusually low blood sugar levels, which in some cases can cause seizures, unconsciousness and even death.
• Low potassium levels (hypokalemia). People with cholera lose large quantities of minerals, including potassium, in their stools. Very low potassium levels interfere with heart and nerve function and are life threatening. Hypokalemia is especially serious in people whose potassium stores have already been depleted by malnutrition.
• Kidney (renal) failure. When the kidneys lose their filtering ability, excess amounts of fluids and wastes build up in your body — a potentially life-threatening condition. In people with cholera, kidney failure often accompanies shock.

The World Health Organization (WHO) has established guidelines for treating cholera that can be used in the most severe cases and circumstances. The goal is to replace fluids and electrolytes lost through diarrhea using a simple rehydration solution that contains specific proportions of water, salts and sugar. The solution, called World Health Organization Oral Rehydration Solution (WHO-ORS), is available as a powder that can be reconstituted in boiled or bottled water.

During an epidemic, most people can be helped by oral rehydration alone, but severely dehydrated people need intravenous fluids, which are more expensive and difficult to administer. No matter which method is used, immediate treatment is critical because death from cholera can occur within hours. Without rehydration, approximately half of people with cholera die; with treatment, the number of fatalities drops to less than one percent.

Rehydration usually occurs in two stages. The initial phase treats existing dehydration; in the maintenance phase, fluids are continually replenished until diarrhea stops. The amount of solution needed to maintain hydration varies greatly, depending on the severity of dehydration and the degree of diarrhea. But most people require large amounts of fluids, especially at the start of treatment. People who have trouble drinking ORS, either because of frequent vomiting or the sheer volume of fluids, may need infusion through their veins (intravenous treatment). In addition to rehydration, people who are very sick may benefit from antibiotics, which can cut the length of the illness in half.

All recent cholera outbreaks in the United States have been traced to raw or undercooked shellfish. The best advice for prevention is to avoid raw oysters and shrimp and to make certain that sushi or other uncooked seafood comes from waters uncontaminated by cholera bacteria.

If you're traveling to cholera-endemic areas, your risk of contracting the disease is extremely low if you follow these precautions:

• Wash your hands. Frequent hand washing is the best way to control infection. Wash your hands thoroughly with hot, soapy water, especially before eating or preparing food, after using the toilet, and when you return from public places. Carry an alcohol-based hand rub for times when water isn't available.
• Avoid untreated water. Contaminated drinking water is the most common source of cholera infection. For that reason, drink only bottled carbonated water or water you've boiled or disinfected yourself. Hot beverages such as coffee and tea as well as bottled or canned soft drinks, and wine and beer are generally safe. Carefully wipe the outside of all bottles and cans before you open them, and ask for drinks without ice. Use bottled water to brush your teeth.
• Get it while it's hot. Choose food that's been thoroughly cooked and is served piping hot. Cholera bacteria can survive on room temperature food for up to five days and aren't destroyed by freezing. It's best to avoid street food, but if you do buy from local vendors, make sure your meal is cooked in your presence and served hot.
• No sushi for you. Avoid raw fish and seafood of all kinds.
• Ignore your mother's advice. These days, many experts recommend eating up to nine servings of fresh fruits and vegetables a day. It's sound advice, but when you're traveling, make sure that all fruits and vegetables are cooked or have thick skins that you peel yourself. Avoid lettuce in particular because it may have been rinsed in contaminated water.
• Be careful with dairy foods. Avoid ice cream, which is often contaminated, and unpasteurized milk.

Update on the cholera vaccine
Because travelers have a low risk of contracting cholera and because the traditional injected vaccine offers minimal protection, no cholera vaccine is currently available in the United States. A few countries offer two new oral vaccines that may provide longer and better immunity than the older version did. If you'd like more information about these vaccines, contact your doctor or local office of public health. Keep in mind that no country requires immunization against cholera as a condition for entry.

Community-wide prevention
Any community affected by cholera is in urgent need of clean, safe food and drinking water and an effective and sanitary method of waste disposal. Providing these things where cholera is a threat is a challenge, but progress is being made.

Mobile water purification plants, for instance — simple systems in which contaminated water is chlorinated, filtered and then trucked to refugee camps — may have helped prevent diarrheal disease following the December 2004 Indian Ocean earthquake and tsunami. Also key is providing sanitary facilities that respect local beliefs about privacy and cleanliness.

Although always important, these measures are particularly critical with regard to cholera because mass vaccinations and routine treatment of a population with antibiotics won't stop the spread of the disease. Nor will travel restrictions between countries or between affected areas.