Giardiasis is a gastrointestinal illness affecting people and animals of all ages. The symptoms of Giardiasis include diarrhea, abdominal cramps and gas. It is caused not by a bacteria or virus, but by the protozoan parasite, Giardia lamblia. One of the most common intestinal parasites of man, Giardia lamblia is carried by a number of animal hosts. Giardia organisms form a resistant cyst that is shed by the host in its waste. Infections are acquired by ingesting these cysts in food and water or by personal contact with an infected person.
Individuals exposed to Giardiasis respond differently. After parasite ingestion, the symptoms may take one to four weeks to appear. The number of cysts consumed is not related to the persistences or severity of the infection. As few as 10 cysts have been known to cause infections. Infected individuals can be treated with prescription medication that eliminates the parasite from the intestine. A large number of infections disappear spontaneously without any treatment. In persistent cases of Giardiasis, individuals can go through periods in which stool samples altemate between positive and negative. There is also evidence that some individuals can develop resistance to Giardia infections.
Drinking water is a prime carrier of Giardia cysts for several reasons. Individuals infected with the parasite shed cysts in their waste. Therefore, surface water supplies (like streams, lakes, and ponds) can be contaminated with Giardia cysts through the introduction of sewage or animal wastes. Groundwater supplies are usually protected by the filtering action of the soil which removes the cysts. Humans, dogs, cats, cattle, deer, and other mammals can carry Giardia. Beavers are often found to be the source of contamination because they can become infected and introduce their waste directly into the water near a water supply intake. Once in the water, Giardia cysts can persist for over 60 days.
Giardia cysts are difficult to identify in a water supply. There is no routine test a water company can use to check for Giardia contamination. Water testing for Giardia is currently used to confirm suspected contamination. It involves filtering several hundred gallons of water and identifying the cysts through microscopic examination by a trained analyst. Negative results are not a guarantee of a safe water supply because of the unknown sensitivity of the test. Giardia cysts are more easily identified in stool samples taken from an exposed person or animal. One gram of feces may contain as many as 2 million cysts.