Written by Amanda Delgado | Published on July 16, 2012
Medically Reviewed by George Krucik, MD


Giardiasis is an infection that affects your small intestine, caused by microscopic parasites called giardia. Giardiasis is spread through contact with infected people and by eating contaminated food or drinking contaminated water. It is the most common cause of parasitic infections in the United States, according to the Wexner Medical Center at Ohio State University. Giardiasis also commonly occurs in developing countries that are overcrowded and lack sanitary conditions and water quality control.

What Are the Causes of Giardiasis?

Giardia are present in the stool of people and animals, as well as in contaminated food, water, and soil. They are capable of surviving outside of a host for long periods of time. Accidentally ingesting these parasites can lead to an infection.

Drinking water that contains giardia is the most common way that people become infected. Contaminated water can be found in swimming pools, spas, and bodies of water, such as lakes. Sources of contamination include animal feces, diapers, and agricultural runoff.

Contracting giardiasis from food is less common, since heat kills the parasites. It can be spread by food handlers with poor hygiene or when produce is rinsed in contaminated water.

Giardiasis can also be spread through person-to-person contact. You can become infected after changing your child’s diaper or from picking up the parasites while working in a child care center. Children are at high risk for giardiasis since they are likely to encounter feces when wearing diapers or potty training. Unprotected anal sex is another way that this infection is passed from one person to another.

What Are the Symptoms of Giardiasis?

Some people can carry giardia parasites without experiencing any symptoms. Otherwise, symptoms of giardiasis generally show up one or two weeks after exposure. Common symptoms include:

  • fatigue
  • nausea
  • diarrhea or greasy stools
  • loss of appetite
  • vomiting
  • bloating and abdominal cramps
  • weight loss
  • excessive gas
  • headaches
  • abdominal pain

How Is Giardiasis Diagnosed?

Your doctor will ask you to submit one or more stool samples for testing. The samples will be checked for giardia parasites. You might also be asked to submit more samples during treatment. Your doctor might also perform an enteroscopy, which involves running a flexible tube down your throat and into your small intestine in order to examine your digestive tract and perhaps take a tissue sample.

What Are the Treatments for Giardiasis?

In most cases, giardiasis clears up on its own. Your doctor might prescribe medication if your infection is severe or prolonged. Medications that are commonly used to treat giardiasis include the antibiotics metronidazole, tinidazole, nitazoxanide, and paromomycin.

Metronidazole is an antibiotic that can cause nausea and leave a metallic taste in your mouth.

Tinidazole is often given in a single dose and is just as effective as metronidazole.

Nitazoxanide is typically given to children since it is available in liquid form.

Paromomycin has a lower chance of causing birth defects than these other antibiotics, although pregnant women are advised to wait until after delivery before taking any medication for giardiasis.

What Is the Prognosis for Giardiasis?

Giardiasis infections usually last about six weeks, but problems such as lactose intolerance can persist after the infection clears up.

What Complications Are Associated With Giardiasis?

Giardiasis can lead to complications such as weight loss and dehydration from diarrhea. The infection can also cause lactose intolerance in some people. Children under five are at risk for malnutrition, which can interfere with their physical and mental development.

How Can I Prevent Giardiasis?

You can’t prevent giardiasis, but you can lower your risk of getting it by thoroughly washing your hands, especially if you work in places where germs are easily spread, such as day care centers.

Ponds, streams, rivers, and other bodies of water are all potential sources of giardia. Don’t swallow water if you go swimming in one of these. Avoid drinking surface water unless it’s been boiled, treated with iodine, or filtered. Bring bottled water with you when you go hiking or camping.

When traveling in a region where giardiasis occurs, do not drink tap water. Keep in mind that tap water can also be present in ice and other beverages. Avoid eating local produce that hasn’t been cooked.

Be cautious about sexual practices, such as anal sex, that are associated with the spread of this infection. Use a condom to reduce the chance of contracting giardiasis.

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