Environmental Science & Engineering - www.esemag.com - January 1997

How giardia and cryptosporidium in drinking water can affect human health

Giardia and Cryptosporidium are microscopic parasites that can be found in water. Giardia causes an intestinal illness called giardiasis or "beaver fever". Cryptosporidium is responsible for a similar illness called cryptosporidiosis.

Q. How do these parasites cause illness?

A. Both parasites produce cysts that are very resistant to harsh environmental conditions. When ingested they germinate, reproduce, and cause illness. After feeding, the parasites form new cysts, which are then passed in the faeces. Studies with human volunteers have shown that ingestion of only a few of the cysts will cause illness.

Q. What are the symptoms?

A. Diarrhoea, abdominal cramps, gas, malaise, and weight loss are the most common symptoms caused by Giardia. Vomiting, chills, headache and fever may also occur. These symptoms usually happen within 6 to 16 days of the initial contact and can continue as long as a month.

The symptoms of cryptosporidiosis are similar; the most common are water diarrhoea, abdominal cramps, nausea, and headaches. These symptoms occur within 2 to 25 days of infection and can last from one to two weeks or as long as a month.

Giardia lamblia cysts.

Q. How can drinking water become contaminated with these parasites

A. Giardia are often found in faeces from humans, beaver, muskrat, and dogs. Cattle appear to be the primary source of Cryptosporidium, although they have also been found in humans and other animals.

Drinking water sources become contaminated when faeces containing the parasites are deposited or flushed into water. If water treatment is inadequate, drinking water may contain sufficient numbers of parasites to cause illness. Other sources include direct exposure to faeces of infected humans and animals, eating contaminated food and accidental ingestion of contaminated recreational water. The comparative importance of these various routes of exposure is unknown.

Q. Have these parasites been found in Canadian drinking water supplies?

A. Low levels of both parasites, especially Giardia, were detected in a national survey of drinking water conducted by Health Canada. Only a small fraction of the parasites appeared to be viable. Nevertheless, outbreaks linked to drinking water have been reported in several provinces. Their spread in swimming pools has also been reported

Q. How can these waterborne illnesses be prevented?

A. Municipal drinking water treatment providing filtration and disinfection can reduce the risk of giardiasis and cryptosporidiosis. Protection of the raw water supply is also beneficial.

In the outdoors, water should be boiled for at least one minute before it is used for drinking, food preparation or dental hygiene. This will destroy Giardia and Cryptosporidium, plus any other disease-causing microorganisms that might be present. Certain types of water filters can remove the parasites.

Travellers to countries where the safety of drinking water is suspect should boil or disinfect and filter water that is to be used for drinking, food preparation or dental hygiene.

Q. How are these infections treated?

A. Giardia is usually cleared without treatment from healthy people within a month. Anti-parasitic drugs are available and are particularly helpful to immunocompromised people in whom the illness could otherwise develop into a persistent state.

Cryptosporidium will usually disappear from healthy people within a month. Anti-diarrhoeal drugs and rehydration therapy may be used if diarrhoea becomes severe. There are no approved drugs to fight the illness although many are now being tested.

Cryptosporidium parvum oocysts.
Photos courtesy Dr. G.R. Finch

Q. What extra precautions can immunocompromised people take

A. Both parasites, but particularly Cryptosporidium, can pose a more serious threat to immunocompromised people such as those with AIDS or cancer, or transplant patients receiving immunosuppressive drugs. For these people, the symptoms are more severe and can be life threatening.

At present, it is unknown whether immunocompromised individuals are at greater risk to waterborne Giardia and Cryptosporidium than the general public. Nevertheless, immunocompromised individuals should discuss these risks with their physicians.

Those who wish to take extra precautions can boil their water for 1 minute to kill any parasites that may be present. This practice will also destroy any other microorganisms that might be of concern to these individuals. As bottled water is not routinely monitored for Giardia and Cryptosporidium, its suitability as an alternative to boiled tap water is unknown.

Q. What should you tell your physician?

A. If you are suffering from diarrhoea and suspect that your symptoms may be due to Giardia or Cryptosporidium, visit your physician and mention any exposure to water, food or faeces that may have been contaminated with the parasites.

Q. What is Health Canada doing to ensure the safety of our drinking water?

A. A consistent approach to improving drinking water quality is provided by Health Canada's Guidelines for Canadian Drinking Water Quality (soon to include a guideline for Giardia and Cryptosporidium) which are designed to ensure that Canadians have access to safe drinking water.


Contact Health Canada, Publications, Ottawa, ON K1A 0K9, Fax: (613) 941-5366.


Related ES&E articles:
Water industry challenge - waterborne parasites - Part I
Water industry challenge - waterborne parasites - Part II
Drinking water in the post Walkerton era
How conditions and coagulant type can affect Crypto removal
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