Giardiasis, or beaver fever, is a parasitic infection of the digestive system. Symptoms include severe abdominal discomfort and diarrhea, but some people have no symptoms.

The parasite that causes it is a microscopic, single-celled organism called Giardia lamblia.

According to the Centers for Disease Control and Prevention (CDC), giardiasis is the most common intestinal parasitic disease in the United States.

In 2012, people reported 15,223 cases of the disease in the U.S.

Giardiasis occurs all over the world, but it is more common in areas with poor sanitation. In developing countries, it can affect 20 to 30 percent of the population at any one time. In some places, it affects 100 percent of the population.

Abdominal discomfort and a feverShare on Pinterest
Abdominal discomfort and a fever are common symptoms of giardiasis.

Not everyone with giardiasis has symptoms, but they can still spread the disease.

Symptoms can appear between 1 and 3 weeks after infection.

They may include:

Stools usually eventually become greasy but do not contain blood.

Diarrhea can lead to dehydration. If water loss is severe, it can become serious, especially for infants. It is important to consume plenty of fluids, preferably water.

Symptoms normally improve within 2–6 weeks, but this may take longer. Medications can help speed up recovery.

There are two types of Giardia.

The active form is present in the gut of an infect person or animal.

The inactive cyst form can remain dormant in the environment for several months.

Parasites can leave their host’s body when a person or animal has a bowel movement.

Outside, they form a hard, protective shell and become a Giardia cyst. A cyst can survive for several months in the environment.

A gust of wind can pick up the cysts from infected manure on farmland and blow it in any direction.

The infection usually spreads when people drink water that has come into contact with infected feces, especially in countries with poor sanitation.

Transmission can occur in the following ways:

  • touching the mouth after touching surfaces where Giardia parasites are present
  • through water supplies, for example, by consuming improperly treated water or ice, swimming, or eating food washed in contaminated water or grown in a place where giardia are present
  • through poor handwashing after using the bathroom or when handling food
  • by touching the anus of a person who has the disease
  • when taking care of someone who has the infection

An infant with giardiasis wearing diapers in a swimming pool can contaminate the water. Anybody who swims there is then at risk of becoming infected.

In the U.S., giardiasis is more common during the summer, possibly because people spend more time doing outdoor activities and traveling in the wilderness.

People may contract it while travelling but not notice the symptoms until they are home again.

There is a small risk of catching giardiasis from a pet, but animals usually have a different kind that does not affect humans.

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Giardiasis happens when a parasite causes intestinal problems, especially when traveling.

A doctor will take a stool sample and send it to a lab to check for Giardia cysts.

Sometimes, they will ask for several samples, because a person does not expel cysts every time they use the bathroom.

If the results do not show giardia, but the person has symptoms that seem to suggest giardia, the doctor may carry out further tests.

This may include an endoscopy and possibly a biopsy from the small intestine.

Metronidazole (Flagyl) is a drug that is effective for treating giardiasis. A doctor will prescribe the drug and advise about dosage.

Side effects are rare, but they may include:

  • convulsions
  • confusion
  • hallucinations
  • rash
  • nausea
  • dark or cloudy urine
  • vomiting
  • drowsiness

Metronidazole may interfere with alcohol dehydrogenase, an enzyme which breaks down alcohol. For this reason, a person should avoid alcohol during treatment.

Other possible medications

Other options include:

  • Tinidazole (Fasigyn)
  • Nitazoxanide (Alinia)

Without treatment, giardia can become chronic, but this is rare. It can last for years, with repeated bouts of diarrhea, vitamin deficiencies, and tiredness.

A person who recovers without treatment may continue to have and to shed the parasite, although this does not always happen. It depends on the individuals’ defense immunity.

In children, giardiasis may lead to malnutrition and conditions connected with this, such as problems with growth and cognitive development, according to a review published in 2013.

Other complications include:

  • long-term, non-specific gastrointestinal problems
  • chronic fatigue

Studies have also suggested that it may lead to a higher risk of:

  • eye problems
  • muscular complications
  • allergy symptoms, such as urticaria

Some reports have linked the development of cancer to Giardia infection, but more research is needed to confirm this.

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Boil or purify any drinking water that comes from a well or another untreated source.

Giardiasis is widespread in sub-Saharan Africa, South and Southeast Asia, Central America, western parts of South America, Russia, Turkey, Romania, and Bulgaria.

Statistics suggest that some 200 million people in Africa, Asia, and Latin America have Giardia with symptoms, and many others do not know they have it, because they do not have any symptoms.

For these reasons, it is important for travelers to be aware of the signs of giardiasis and to take certain precautions:

Hand-washing: Wash hands after using the bathroom or changing a diaper, and before eating or handling food.

Drinking water: Beware of water in rivers and lakes. When camping, people should take their own water supply or some means of boiling or purifying it.

Food: Avoid eating raw foods and foods washed in water that could be contaminated.

Brushing teeth: Use purified water.

Giardiasis is a common parasitic disease that affects the intestines. While most common in countries with poor sanitation, it also affects people in the U.S.

A doctor can provide medication to treat the infection.

People should take care to use treated, boiled, or purified water for drinking and washing food, especially when camping or visiting countries where Giardia is common.

Good handwashing practices can reduce the risk.


I am going on a 2-month backpacking trip to South America soon. Some of my friends have told me I should take some Flagyl with me, just in case. Should I ask my doctor to give me some?


Doctors do not typically give Flagyl for Giardia infection because most people do not have symptoms. The benefit of treating giardiasis without symptoms is unclear, and most people who develop symptoms recover quickly.

Using the above precautions for food and water can prevent infection with this parasite.

Daniel Murrell, MD Answers represent the opinions of our medical experts. All content is strictly informational and should not be considered medical advice.

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