Giardiasis (beaver fever)

Last Reviewed: March 2026

What is giardiasis?

Giardiasis is an intestinal (bowel) illness caused by a tiny parasite called Giardia. It is a common illness that causes diarrhea (loose stool/poop). The most common way people get sick with giardiasis is by swallowing contaminated water, but people can also get sick from contaminated food, surfaces, and objects.

Who gets giardiasis?

Anyone can become infected with Giardia, but those at greater risk include children and staff in daycare centers, foreign travelers, and people who drink improperly treated water.

How is this parasite spread?

Giardia can infect people as well as wild and domestic animals. It is passed in the feces (poop) of an infected person or animal and may contaminate water, food, surfaces, or objects. The beaver has gained attention as a potential source of Giardia contamination of lakes, reservoirs, and streams, but human feces (poop) can also spread the parasite to contaminate water sources. People become infected by accidently swallowing Giardia and can carry the parasite in their bodies from a few weeks to a few months. Spread of the parasite directly from one person to another may occur, especially in daycare centers, settings where handwashing practices are poor, or through oral-anal sex.

What are the symptoms of giardiasis and how soon do they appear?

People exposed to Giardia may experience mild or severe diarrhea (loose stool/poop), gas, stomach cramps, nausea (a feeling of upset in the stomach), or dehydration (loss of water in the body causing weakness of dizziness). Some people experience no symptoms at all.

The symptoms usually appear from 1 to 2 weeks after becoming infected with the parasite. People usually have symptoms for 2 to 6 weeks, but occasionally people have symptoms that last for a long time, from months to years.

How is giardiasis diagnosed?

Giardiasis is diagnosed by testing samples of your stool (poop). Sometimes, several different stool samples must be tested because the number of Giardia being passed in the stool, which varies from day to day, may be too low to detect from any single sample.

What is the treatment for giardiasis?

There are many prescription drugs that are available to treat giardiasis, however, some individuals may recover on their own in a few weeks without medication. Since diarrhea can cause dehydration, it is important to drink a lot of fluids when sick.

What can be done to prevent the spread of giardiasis?

Some important preventive measures are:

  • Do not drink untreated water from lakes, rivers, springs, ponds, streams or shallow wells.
  • Do not drink untreated water from sources with unknown water quality while traveling.
  • If you are not sure about the safety of drinking water, use commercially bottled water; or use water that has been previously boiled for 1 minute and left to cool (elevations above 6,500 feet boil for 3 minutes); or use a filter (NSF '53', NSF '58', or 'absolute pore size of 1 micron or smaller')
  • Avoid swallowing water while swimming.
  • Thoroughly wash hands with soap and water after toilet visits and handling dirtied diapers.
  • Thoroughly wash hands with soap and water after working with soil (landscaping, field work, gardening, etc.)
  • Carefully dispose of sewage wastes (poop) so as not to contaminate surface water or groundwater.
  • You can spread giardia to others after your diarrhea has ended. Wait for at least 2 weeks after symptoms end before entering recreational water (pools, hot tubs, lakes, etc. ) or engaging in sex with a partner.