Merck Manual

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Quick Facts



The Manual's Editorial Staff

Last full review/revision Aug 2021| Content last modified Aug 2021
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What is giardiasis?

Giardiasis is an infection of your small intestine by a microscopic parasite called Giardia.

  • You can get giardiasis from drinking unclean water

  • Sometimes you get giardiasis from other people who have the infection

  • The main symptoms are belly cramps and diarrhea

  • Doctors can tell you have the infection by testing a stool (poop) sample

  • To avoid giardiasis, boil infected water before you drink it, especially water from a stream or lake

  • Doctors treat giardiasis with anti-parasite medicine

What causes giardiasis?

Giardiasis is caused by small parasites called Giardia.

Giardia are common in lakes, rivers, and streams all over the world. They can even be in water that looks clean and safe. Sometimes, they are also in poorly treated water from a city water supply or swimming pool.

Most people get giardiasis from drinking water that has the parasite in it. Because people with giardiasis pass the parasite in their stool, they can accidentally spread the infection if they touch something that you then put in your mouth.

What are the symptoms of giardiasis?

Symptoms usually start about a week or two after you’re infected. They include:

  • Belly cramps

  • Passing gas

  • Watery, foul-smelling diarrhea, which can last for weeks

  • Feeling sick to your stomach—the feeling may come and go

  • Feeling tired and uncomfortable and not hungry

If diarrhea lasts longer than a few weeks, it can stop your body from getting the nutrition it needs. You may lose weight. Children with ongoing diarrhea may have growth problems.

How can doctors tell if I have giardiasis?

Doctors will ask about your symptoms and do:

  • Stool tests

How do doctors treat giardiasis?

Doctors treat giardiasis with:

  • Anti-parasite medicine

Doctors only treat people who have symptoms.

How can I prevent giardiasis?

You and your community can help prevent giardiasis by:

  • Having properly treated public water, including drinking water and water in swimming pools

  • Washing hands before preparing food

  • Washing hands thoroughly after using the toilet

  • Avoiding contact with stool during sex

  • Boiling stream or lake water before drinking it

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