About giardia infection (giardiasis)

Giardia infection is an intestinal infection marked by abdominal cramps, bloating, nausea and watery diarrhea. Giardia infection is caused by a parasite that is found worldwide, especially in areas with poor sanitation and unsafe water.

Giardia infection (giardiasis) is one of the most common waterborne diseases in the United States. The parasites are found in backcountry streams and lakes, but also in municipal water supplies, swimming pools and spas. Giardia infection can also be transmitted through food and person-to-person contact.

Infections usually clear up within six weeks. But you may have recurrent episodes or have intestinal problems long after the parasites are gone. Several drugs are generally effective against giardia parasites, but not everyone responds to them. Prevention is your best defense.


Some people with giardia infection never develop signs or symptoms but still carry the parasite and can spread it to others through their stool. For those who do get sick, signs and symptoms usually appear one to two weeks after exposure and may include:

  • Watery, sometimes foul-smelling diarrhea that may alternate with soft, greasy stools
  • Fatigue
  • Abdominal cramps and bloating
  • Nausea
  • Weight loss — as much as 10 percent of your body weight

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 takes two forms
Giardia is caused by the parasite Giardia intestinalis (also called Giardia lamblia). The giardia parasite has two forms:

  • Active. The active form of giardia lives in the intestines of an infected animal or human.
  • Inactive cyst. An inactive cyst form can survive for months in the environment.

When you ingest the inactive 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 by the millions 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 in your feces. They can survive in soil or water for long periods until they’re ingested by another host.

How you can get giardia infection
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. Because cooking food kills giardia, food is a less common source of infection than water is, especially in industrialized countries. However, outbreaks can occur through ice and infected food service workers.
  • By direct contact. You can get 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. The giardia parasite can also spread through anal sex.

The giardia parasite is a very common intestinal parasite. 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 35 percent 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. In families, once giardiasis takes hold, it’s difficult to prevent it from spreading.
  • People without access to safe drinking water. Giardiasis is common wherever sanitation is inadequate or water isn’t safe to drink.
  • 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 can 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.


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 the presence of the parasites. Stool tests may also be used to monitor the effectiveness of 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.
  • 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 properly 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 may lead to birth defects. Some doctors prefer not to treat women at all during pregnancy, or they may opt to use the less toxic — though less effective — drug paramomycin. Metronidazole may be considered in extreme cases where signs and symptoms are severe.


No drug can prevent giardia infection. 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 60 percent alcohol are an excellent alternative.
  • 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 always effective against giardia parasites. Iodine disinfection, for example, must be carried out for at least eight hours.

  • Leave no trace. If you’re camping without access to a toilet, bury your waste and your pet’s at least 6 inches (15 centimeters) deep and 150 feet (about 46 meters) 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 get 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 problems.
  • 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.