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Giardia infection is an intestinal infection marked by stomach cramps, bloating, nausea and bouts of watery diarrhea. Giardia infection is caused by the parasite Giardia intestinalis (also called Giardia lamblia). The parasite is found worldwide but is especially prevalent in countries with poor sanitation and unsafe water, where it's responsible for most cases of childhood diarrhea.

Yet the giardia parasite isn't just a problem in developing nations. Giardia infection (giardiasis) is one of the most common waterborne diseases in the United States. Though the parasites are often associated with backcountry streams and lakes, they also turn up in municipal water supplies, swimming pools, whirlpool spas and wells. Giardia infection, which is highly contagious, also can be transmitted through food and person-to-person contact.

Most cases of giardia infection clear up in two to six weeks without lasting effects. But some people have recurrent episodes for years or have intestinal problems long after the parasites are gone. Children who have a severe giardia infection may fail to grow normally or have learning difficulties later in life.

Although several drugs are generally effective against giardia parasites, not everyone responds to these medications. Drug resistance is also a growing problem, especially in people who need longer therapy or who are treated more than once. Prevention is your best defense against giardiasis.

Some people with giardia infection — particularly children — never develop symptoms but still carry the parasite and can spread it to others. For those who do get sick, signs and symptoms usually appear a week to 10 days after exposure and may include:

• Watery, sometimes malodorous diarrhea that may alternate with soft, greasy stools
• Abdominal cramps and bloating
• Nausea
• Fatigue
• Weight loss — as much as 10 to 15 pounds in adults

Signs and symptoms of giardia infection usually improve in two to six weeks, but in some people they become chronic, lasting for months or years.

Giardia parasites have two forms: an active form that lives in the intestine of an infected animal or human and an inactive cyst form that can survive for months in the environment. When you ingest the cysts, their hard shells break down in your stomach, releasing the parasites. The parasites then attach to the wall of your small intestine, eventually reproducing themselves millions of times and damaging the intestinal wall. This interferes with your ability to absorb food, leading to diarrhea and weight loss.

In time, the parasites detach from the small intestine, transform back into cysts and are shed by the millions in your feces. They can survive in soil or water for long periods until they're ingested by another host. You're likely to pick up the parasites in one of three ways:

• In water. This is the main source of giardia infection. The parasites are found in lakes, ponds, rivers and streams worldwide, as well as in municipal water supplies, wells, cisterns, swimming pools, water parks and spas. Giardia parasites have even turned up in touch tanks in aquariums and museums. Ground and surface water can become contaminated from agricultural runoff and wastewater discharge. Because giardiasis affects beavers, muskrats, small rodents and a variety of birds, reptiles and fish, water can also become contaminated from animal feces. Children in diapers and people with diarrhea may accidentally contaminate pools and spas. You can become infected if you drink from streams or lakes that contain the parasites, drink contaminated tap water at home or abroad, or accidentally swallow water from a tainted pool or spa.
• In food. Giardia parasites can be transmitted through food — either because food handlers with giardiasis don't wash their hands or because raw produce is irrigated or washed with contaminated water. Food is a less common source of infection than water is, especially in industrialized countries, but outbreaks have been traced to contaminated ice and infected food service workers.
• By direct contact. You can contract giardiasis if your hands become contaminated with fecal matter — parents changing a child's diapers are especially at risk. So are child care workers and children in child care centers, where outbreaks are increasingly common. The giardia parasite also can spread through anal sex.

The giardia parasite is the most common intestinal parasite worldwide, infecting an estimated 200 million people each year. Although anyone can pick up giardia parasites, some people are especially at risk:

• Children. Giardiasis is far more common in children than it is in adults. Children are more likely to come in contact with feces, especially if they wear diapers, are toilet training or spend time in a child care center. Nearly half of all children in child care may be infected, though many don't have symptoms. Children also tend to put their hands or other objects in their mouths, may swallow lake or pool water, and can be careless about hand washing.
• Child care workers and parents. Adults who work with young children — especially in schools and child care centers — and parents of infants and toddlers are at risk of infection. Giardiasis usually travels through families; once the infection takes hold, it's difficult to prevent it from spreading.
• People without access to safe drinking water. Giardiasis is rampant wherever sanitation is inadequate or water isn't safe to drink. Outbreaks are common in the former Soviet Union, Mexico, Southeast Asia, South and Central America, and Africa.
• Travelers. You're at risk if you travel to places where giardiasis is common, especially if you aren't careful about what you eat and drink. The risk increases with the amount of time you spend in a region and is greatest in rural or wilderness areas.
• Hikers and campers. In the United States, giardia parasites primarily occur in mountainous areas where lakes and streams have become contaminated. Hikers and campers should avoid drinking untreated water, even if it appears clean.
• People who drink from shallow wells. According to the Environmental Protection Agency, all surface water in the United States contains giardia parasites, which means you're at risk if you drink untreated water from shallow wells.
• People who have anal sex. Having anal sex without using a condom puts you at risk of giardia infection, as well as for serious sexually transmitted diseases.

Diarrhea that goes away within a week or less probably isn't giardiasis. Call your doctor if you have loose stools, abdominal bloating and nausea lasting two weeks or more, or you become dehydrated. It's easy to overlook giardiasis because the signs and symptoms resemble those of other gastrointestinal diseases. Be sure to tell your doctor if you're at risk of infection — that is, you have a child in child care, you've recently traveled to an endemic area, or you've swallowed water from a lake or stream.

To help diagnose giardiasis, your doctor is likely to test a sample of your stool. For accuracy, you may be asked to submit several samples collected over a period of days. The samples are then examined in a laboratory for signs of the parasites. Stool tests may also be used to monitor the effectiveness of any treatment you receive.

Giardia infection is almost never fatal in industrialized countries, but it can cause lingering symptoms and serious complications, especially in infants and children under 5. The most common complications include:

• Dehydration. Often a result of severe diarrhea, dehydration occurs when the body doesn't have enough water to carry out its normal functions. Infants and children are especially vulnerable.
• Failure to thrive. Chronic diarrhea from giardia infection can lead to malnutrition and harm children's physical and mental development.
• Lactose intolerance. Many people with giardia infection develop lactose intolerance — the inability to digest milk sugar. The problem may persist long after the infection has cleared, leading to malabsorption and weight loss in some.
• Skin rash. In rare cases, giardia infection may cause a skin rash or hives.

Children and adults who have giardia infection but no symptoms usually don't need treatment unless they're likely to spread the parasites. Many people who do have problems often get better on their own in a few weeks. When signs and symptoms are severe or the infection persists, doctors usually treat giardiasis with antibiotics such as metronidazole or tinidazole. The two drugs are equally effective, but tinidazole requires a shorter course of treatment. Both can cause side effects such as a metallic taste, nausea and vomiting, and you must avoid drinking alcohol while taking them. Pregnant women shouldn't use these drugs during the first trimester because they can lead to birth defects. Some doctors prefer not to treat women at all during pregnancy, or they may opt to use a less toxic — though less effective — drug.

No drug can prevent giardia infection. And exposure to giardia parasites in food or municipal drinking water is unavoidable. Still, common-sense precautions can go a long way toward reducing the chances that you'll become infected or spread the infection to others.

For your own safety:

• Wash your hands. This is the simplest and best way to prevent most kinds of infection. Wash your hands after using the toilet or changing diapers and before eating or preparing food. Scrub your hands briskly with soap and warm water for at least 15 seconds. For children, this is about as long as it takes to say the ABCs or sing the "Happy birthday" song. Rinse thoroughly, allowing the water to run down your hands, and then dry your hands with a disposable paper towel. When soap and water aren't available, alcohol-based sanitizers containing at least 62 percent alcohol are an excellent alternative.
• Purify wilderness water. Avoid drinking untreated water from shallow wells, lakes, rivers, springs, ponds and streams unless you boil or filter it first.
• Leave no trace. If you're camping without access to a toilet, bury your waste and your pet's at least 6 inches deep and 200 feet from a water source. People with portable toilets should use sewage dump points.
• Keep your mouth closed. Try not to swallow water when swimming in pools, lakes or streams.
• Be wary of tap water. Each year in the United States, about 250 people in every 10,000 contract giardiasis from public drinking water. Water can become contaminated if standard treatments aren't effective or natural events such as flooding overwhelm treatment systems. If an outbreak of giardia infection occurs in your area, buy bottled water or boil or filter tap water before you use it.
• Use bottled water. When traveling to parts of the world where the water supply is likely to be unsafe, drink and brush your teeth with bottled water that you open yourself. Don't use ice, and avoid raw fruits and vegetables, even those you peel yourself.
• Practice safer sex. If you engage in anal sex, use a condom every time. Avoid oral-anal sex unless you're fully protected.

Giardiasis is extremely contagious. If you're infected, these measures can help keep others safe:

• Wash your hands. Scrub your hands with soap and warm water for at least 15 seconds after using the toilet and before handling food. If possible, have a friend or family member prepare meals until you're free of infection. Keep in mind that you can spread the parasites for several weeks after you stop having symptoms. Away from home, use an alcohol-based hand sanitizer containing at least 62 percent alcohol.
• Stay out of the water. Avoid all types of recreational water — pools, hot tubs, whirlpool spas, lakes and streams — for at least two weeks after your diarrhea has stopped. You can continue to spread the infection even if you no longer have symptoms.
• Be sex-safe. Avoid having anal or oral-anal sex until you're free from infection.

To purify water
If you're not sure water is safe to drink, you can purify it using one of these methods:

• Heat or boil water to at least 158 F (70 C). Ten minutes should be enough to kill any parasites.
• Use a water filter. The Centers for Disease Control and Prevention recommends a filter having a pore size of 1 micron or smaller or one that has been rated by the National Science Foundation (NSF) as effective for cyst removal.
• Don't rely on chlorine or iodine water purification tablets — they're not effective against giardia parasites.