Giardia infection, also called giardiasis, is caused by a tiny parasite. It infects part of the digestive system called the small intestine. Symptoms include stomach cramps, bloating, upset stomach and loose stools. The parasite that causes giardia lives all over the world, mainly in areas poor sanitation and that don't have safe water.

Giardia infection, also called giardiasis, is one of the most common causes of disease carried by water in the United States. The parasites can live in streams and lakes, public water supplies, swimming pools, whirlpool spas and wells. Giardia infection can spread through water, food and person-to-person contact.

Giardia infections most often clear up within weeks. But you may have stomach problems long after the parasites are gone. Several medicines work against giardia parasites, but they don't work for everyone. It's better to prevent infection.

Some people with giardia infection don't have symptoms. But they still carry the parasite and can spread it to others through their stool. For those who do get sick, symptoms most often appear 1 to 3 weeks after getting the parasite. Symptoms may include:

• Loose stools that are often watery and sometimes smell bad. Sometimes stools may be soft and greasy.
• Tiredness.
• Stomach cramps and bloating.
• Gas.
• Upset stomach.
• Weight loss.

Symptoms of giardia infection may last 2 to 6 weeks. But for some people they last longer or come back.

Call your healthcare professional if you have loose stools, stomach cramping, bloating and upset lasting more than a week. Also call if you lose too much fluid, called dehydration.

Tell your healthcare professional if you're at risk of giardia infection. You may be at risk if you have a child in child care, you've been to an area where the infection is common, or you've swallowed water from a lake or stream.

Giardia parasites can live in the small intestine. Before the tiny parasites pass in stool, they form hard shells called cysts. The cysts let the parasites live outside the intestines for months. Once inside a person or animal, the cysts break down, letting the parasites out.

Infection happens when you swallow the parasite cysts. This can be from swallowing water that isn't safe, by eating infected food or through person-to-person contact.

Swallowing tainted water

The most common way to get infected with giardia is by swallowing tainted water. Giardia parasites are in lakes, ponds, rivers and streams around the world. They also may be in public water supplies, wells, cisterns, swimming pools, water parks and spas.

Giardia can infect ground and surface water through runoff from farms, wastewater or animal feces. Children in diapers and people with loose stools may taint pools and spas.

Eating tainted food

Giardia parasites can spread through food. This can happen if food handlers with giardia infection don't wash their hands well. Giardia parasites also can spread when raw produce gets watered or washed with tainted water. Because cooking food kills giardia, food is a less common source of infection than water.

Person-to-person contact

You can get infected with giardia if you get stool on your hands. Parents who change children's diapers are at higher risk. So are child care workers and children in child care centers, where outbreaks happen often. The giardia parasite also can spread through anal sex.

The giardia parasite is common. Anyone can pick up giardia parasites, but some people are at higher risk, including:

• Children. Giardia infection is far more common in children than it is in adults. Children are more likely to be in contact with stool, mainly if they wear diapers, are toilet training or spend time in a child care center. People who live or work with small children also are at higher risk of giardia infection.
• People who don't have safe drinking water. There are many giardia infections in places with poor sanitation or where water isn't safe to drink. You're at risk if you travel to places where giardia infection is common, mainly if you aren't careful about what you eat and drink. The risk is greatest in rural or wilderness areas.
• People who have anal sex. Those who don't use a condom are at higher risk of giardia infection and sexually transmitted infections.

Giardia infection almost never leads to death in developed countries. But it can cause symptoms that last and serious complications, mainly in infants and children. The most common complications include:

• Dehydration. Often a result of loose stools, dehydration happens when the body doesn't have enough water to do its work. Infants and people who are pregnant are at high risk of dehydration.
• Failure to thrive. Continuing loose stools from giardia infection can lead to poor nutrition. This can harm children's physical and mental growth.
• Lactose intolerance. Many people with giardia infection get lactose intolerance. That means they can't digest the sugar in milk. The condition may go on long after the infection clears.

No medicine or vaccine can prevent giardia infection. But the following can go a long way toward cutting the chances that you'll become infected or spread the infection to others.

• Wash your hands. This is the simplest and best way to prevent most infections. Wash your hands with soap and water for at least 20 seconds after using the toilet or changing diapers and before eating or preparing food. When you don't have soap and water, you can use alcohol-based sanitizers. But sanitizers don't work as well to destroy the cyst form of giardia that can live outside the body.
• Purify wilderness water. Don't drink water from shallow wells, lakes, rivers, springs, ponds and streams unless you filter it or boil it.
• Wash produce. Wash raw fruits and vegetables with safe water. Peel the fruit before eating it. Don't eat raw fruits or vegetables if traveling in countries where they may have contact with water that isn't safe.
• Don't swallow water. Try not to swallow water when swimming in pools, lakes or streams. Keep your mouth closed in the shower when in parts of the world where the water is not likely to be safe.
• Use bottled water. When traveling to parts of the world where the water supply is not likely to be safe, drink and brush your teeth with bottled water that you open yourself. Don't use ice.
• Practice safer sex. If you have anal sex, use a condom every time. Don't have oral-anal sex unless you're fully protected.

To help diagnose giardia infection, also called giardiasis, your healthcare professional is likely to test a sample of your stool. Your healthcare team may ask you for several stool samples collected over a few days. The samples then go to a lab to look for parasites.

Children and adults who have giardia infection without symptoms most often don't need treatment unless they're likely to spread the parasites. Many people who do have symptoms often get better on their own in a few weeks.

When symptoms are bad or the infection doesn't go away, the following medicines can treat giardia infection:

• Metronidazole. Metronidazole is often prescribed to be taken three times a day for five days. Side effects may include upset stomach and a taste of metal in the mouth. Don't drink alcohol while taking this medicine.
• Tinidazole. Tinidazole works as well as metronidazole and has many of the same side effects. But you can take it in a single dose.
• Nitazoxanide. Because it comes in a liquid form, nitazoxanide may be easier for children to swallow. Side effects may include upset stomach, gas, yellow eyes and brightly colored yellow urine.

If you have giardia while pregnant, the medicines may be harmful to the unborn baby. If your symptoms are mild, your healthcare professional might ask you to wait until after the first trimester or longer for treatment. If you need treatment, talk with your healthcare professional about what you can take.

You might first contact your main healthcare professional, who may send you to a doctor who specializes in digestive system conditions, called a gastroenterologist.

What you can do

Before your appointment, you may want to write a list of answers to the following questions:

• When did your symptoms begin?
• How bad are your symptoms?
• Does anything make them better or worse?
• Do you work or live with small children?
• What medicines, vitamins and supplements do you take and at what dosages?

What to expect from your doctor

During the physical exam, your healthcare professional may gently press on parts of your belly while you're lying down to check for tender areas. Your healthcare professional also may look for signs of dehydration and tell you how to bring in a sample of your stool.