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Cryptosporidium & Giardia

Waterborne Parasites: Giardia and Cryptosporidium

Denver Water has tested for Cryptosporidium (Crypto) and Giardia in raw and treated water since the 1980s. Since that time, Denver Water never has detected a viable indication of either in treated drinking water.

Crypto and Giardia are microscopic organisms that, when ingested, can cause diarrhea, cramps, fever and other gastro-intestinal symptoms. Crypto and Giardia usually are spread through means other than drinking water.

Below are some commonly asked questions.


  • Where do parasites come from?

    Giardia and Cryptosporidium both come from the intestinal tracts of warm-blooded animals. Many of these animals — such as elk, deer, beaver, and muskrats — live in the Rocky Mountains. Other animals that can carry these parasites live right in our own back yard, including dogs, cats, and mice. Animal waste can find its way into a stream or lake and the cysts present in the waste are carried downstream. Since the cyst is environmentally resistant, it can survive for an extended period. Anyone, human or animal, who drinks this untreated water and ingests the cysts may develop the disease. Ingesting as few as 10 cysts can give someone giardiasis.

  • How do you get giardiasis and cryptosporidiosis?

    Giardiasis is the more common illness of the two, but both these diseases are transmitted in the same ways. One way for humans to become infected with Giardia or Cryptosporidium is by drinking untreated water that contains the cysts. Hikers sometimes mistakenly presume that water in mountain streams is free from harmful bacteria and Giardia. Hikers consuming even a very small quantity of mountain water without treating it are at risk of getting giardiasis. A person doing something as seemingly harmless as swimming can become infected if exposed to water contaminated with Giardia.

    However, these diseases are more commonly spread from person to person. Children playing together contact can readily spread the disease. Young children have a habit of putting everything into their mouths. They know little about personal hygiene, and their immune systems are underdeveloped. The disease can spread quickly from an infected child through an entire family. Many infected people carry the parasite without showing any symptoms of the illness.

    Both parasites can also be foodborne. An infected food handler who does not use proper sanitation can transmit the disease. Giardia can also be passed directly from an infected animal (wild or domestic) to a person.

  • Are these parasites in municipal drinking water?

    There are several steps in the treatment of drinking water that will remove or kill the cysts. If the cysts are present in the raw water that enters the treatment plant, the application of disinfectant in the water treatment process will kill many of them. In one treatment step, polymers and other chemicals are added to the water to help remove dirt particles. The water is then filtered, and any Giardia or Cryptosporidium cysts should be removed with the dirt and chemical residue. In accordance with public health regulations, a small amount of disinfectant is still present in the water in water mains, and this keeps the water disinfected even as it enters your home.

    Cryptosporidium is more resistant to common treatment methods like heat and disinfection, but it can be controlled by exposure to ultraviolet light and through reverse osmosis filtration. In areas where its presence in raw water poses health risks, these expensive water treatment processes are sometimes employed. Fortunately, monitoring of our raw water indicates no need for such costly measures.

    The federal government requires compliance with certain standards of treatment designed to prevent these parasites from contaminating your drinking water, and Denver Water complies with those regulations. Results of our tests for these parasites in the water appear each year in our Water Quality Reports. Outbreaks involving other municipal water supplies were usually caused by water that was not properly treated or system failures.