Giardia and Drinking Water from Private Wells

What is giardiasis?

Giardiasis (GEE-are-DYE-uh-sis) is a diarrheal illness caused by a microscopic parasite, Giardia intestinalis (also known as Giardia lamblia or Giardia duodenalis). Once an animal or person is infected with Giardia, the parasite lives in the intestine and is passed in feces. Because the parasite is protected by an outer shell, it can survive outside the body and in the environment for long periods of time (i.e., months).

During the past 30 years, Giardia infection has become recognized as a common cause of waterborne disease in humans in the United States. Giardia can be found worldwide and within every region of the United States.

For more information about giardiasis and treatment, please visit CDC's Giardia website.

Where and how does Giardia get into drinking water?

Millions of Giardia parasites can be released in a bowel movement of an infected human or animal. Human or animal waste can enter the water through different ways, including sewage overflows, sewage systems that are not working properly, polluted storm water runoff, and agricultural runoff. Wells may be more vulnerable to such contamination after flooding, particularly if the wells are shallow, have been dug or bored, or have been submerged by floodwater for long periods of time.

How can I find out whether there is Giardia in my drinking water?

If you suspect a problem and your drinking water comes from a private well, you may contact your state certification officer for a list of laboratories in your area that will perform tests on drinking water for a fee.

How can I remove Giardia from my drinking water?

To kill or inactivate Giardia, bring your water to a rolling boil for one minute (at elevations above 6,500 feet, boil for three minutes) Water should then be allowed to cool, stored in a clean sanitized container with a tight cover, and refrigerated.

An alternative to boiling water is using a point-of-use filter. Not all home water filters remove Giardia. Filters that are designed to remove the parasite should have one of the following labels:

• Reverse osmosis,
• Absolute pore size of 1 micron or smaller,
• Tested and certified by NSF Standard 53 for cyst removal, or
• Tested and certified by NSF Standard 53 for cyst reduction.

To learn more, visit CDC’s A Guide to Water Filters page.

As you consider ways to disinfect your well, it is important to note that Giardia is moderately chlorine resistant. Contact your local health department for recommended procedures. Remember to have your well water tested regularly, at least once a year, after disinfection to make sure the problem does not recur.