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Cryptosporidiosis and Drinking Water

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At least six outbreaks of cryptosporidiosis have been associated with contaminated drinking water, including a large, well-publicized outbreak in Milwaukee, Wisconsin in 1993. This information sheet answers the most common questions VDH receives about drinking water and cryptosporidiosis.

How do Cryptosporidium oocysts get into drinking water?
Cryptosporidium gets into surface water sources such as rivers and lakes in the stool (feces) of infected animals or people. Municipal water from treatment plants that get their water from these surface water sources can contain Cryptosporidium oocysts (the egg-like form of the parasite).

Does the treatment process remove the oocysts?
Filtration treatment will usually remove Cryptosporidium oocysts. Chlorination by itself is not effective. All Virginia localities that use surface water sources provide filtration treatment. The better the equipment and the more experienced the operators, the better the removal of the Cryptosporidium oocysts.

In addition, in an effort to reduce health risks associated with Cryptosporidium, the Environmental Protection Agency (EPA) has promulgated the Long Term 2 Enhanced Surface Water Treatment Rule (LT2). LT2 requires that all water systems that obtain their water from surface water sources must monitor the raw (source) water for Cryptosporidium oocysts or indicator organisms. Monitoring results will indicate whether systems will be required to provide additional treatment to achieve effective oocyst reduction.

What does it mean if Cryptosporidium oocysts are found in drinking water?
Authorities believe that the presence of a few oocysts in drinking water does not pose a threat to people with healthy immune systems. It takes an unusual combination of events to lead to a situation where the drinking water would be considered unhealthy. A change in the source water (such as an increase in the number of organisms or increase in turbidity) and a failure of the treatment system would have to occur at the same time. This is what happened in Milwaukee in 1993.

How will officials decide that water is not safe to drink?
Officials evaluate the treatment technique by looking at several indicators of water quality, including such things as changes in the source water, turbidity (cloudiness of the water), particle counts, presence of other organisms, water plant performance, and maintenance records. The evaluation of the treatment process is used to determine if the treatment process has failed.

What will officials recommend if water isn't safe?
Bringing water to a rolling boil for one minute will kill all organisms, including Cryptosporidium.

What laws regulate Cryptosporidium in drinking water?
There are no federal or state regulatory standards for Cryptosporidium in drinking water. However, the EPA's LT2 rule sets standards to ensure compliance with Cryptosporidium removal requirements.

Why aren't laboratory tests used to determine whether there is a problem with Cryptosporidium in water?
There are several things to be aware of regarding Cryptosporidium test results:

  • The laboratory test most commonly used to detect Cryptosporidium in water cannot tell the difference between viable (able to cause illness) and nonviable oocysts.
  • In most cases, the results of the tests on drinking water will not be available until several weeks after the sample was taken and so will not be an accurate measure of present conditions.
  • The number or concentration of oocysts is not necessarily a predictor of when illness will occur. Other factors, such as clumping of oocysts and water temperature may play a role.

How can I obtain more information on Cryptosporidium in drinking water?

Additional information is available at:

A report of the 1993 Milwaukee Cryptosporidium outbreak was published in the New England Journal of Medicine (N Engl J Med1994:331:161-7). The report is available at: (

Last Updated: 08-03-2011

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