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Email Link   Giardia and Cryptosporidium in Drinking Water

Giardia and Cryptosporidium are microscopic parasites that can be found in water. If ingested by warm-blooded animals, including humans they can cause intestinal illnesses with mild symptoms that often go unnoticed. However in some instances, symptoms can be more severe and are especially serious for immuno-compromised individuals.

Further information on both are provided below. If you or a member of your family appears to have symptoms of infection from either pathogen or other illness, it is strongly recommended to seek professional medical advice.


Symptoms:Several studies have shown that there appears to be a certain proportion of the population (up to 20%) that carry Giardia, but of these, two thirds show no symptoms of infection.
  • Mild Infection - indigestion, flatulence, nausea
  • Severe Infection - vomiting, diarrhoea, abdominal cramps and weight loss
Sources of Infection:
  • Person to person (faecal-oral transmission)
  • Untreated drinking water supplies, especially ones infiltrated with human waste.
  • Children day care centres are also well known to be regular sources of infection due to the lack of adequate hygienic practices by young children. It has been shown that children are up to three times more susceptible to infection than adults.
  • Current research is trying to determine whether cross-infections can occur between humans and other animals. Some recent work has indicated quite strong evidence for infections between humans and dogs.
Description:This is the most common enteric protozoan pathogen of humans, domestic animals and wildlife.
Once a person has ingested Giardia, the Giardia become flagellated trophozoites which are the active feeding stage colonizing the upper part of the small intestine. Generally, only trophozoites are present in faeces when an infected person has diarrhoea. When trophozoites pass slowly toward the large intestine, they develop into cysts.



Fever, loss of appetite, nausea, crampy abdominal pain and watery diarrhoea

Sources of Infection:

Similar to Giardia, however Cryptosporidium strains that infect humans appear to have a much wider host range with other animals. Young dairy calves for example, are sources of large numbers of cysts of the same species that infect humans.

Description:While recognised in animals for over a century, Cryptosporidium wasn’t identified as a threat to humans until the AIDS epidemic in the 1980’s. In 1993 a water-borne outbreak infecting more than 403,000 people in Milwaukee, Canada, has shown that it is a potential major hazard to the general population. It is now regarded as the most important water-borne human pathogen in developed countries.
In comparison to Giardia, Cryptosporidium has a more complex life-cycle. It has intracellular development in the gut wall as well as asexual and sexual reproduction.


After ingestion, the organisms germinate, reproduce, and cause illness. Then, the organisms form cysts which are then passed in the faeces. These cysts are very resistant to harsh environmental conditions, meaning they can survive for long periods in drinking water. Both organisms are quite resistant to chlorine, so the best water treatment processes are ones that can filter out these organisms prior to chlorination.

Tweed’s Bray Park Water Treatment Plant has Membrane Ultrafiltration with a nominal pore size of 0.04 micrometers. This removes protozoa such as Giardia and Cryptosporidium, but also much smaller organisms such as bacteria.

Testing the Water

The methods for detecting Giardia and Cryptosporidium in water samples are still not reliable and expensive to perform. Also, routine testing cannot determine between species that are infective to humans or different hosts. For these reasons, no guideline values are given for either organism in the Australian Drinking Water Guidelines. Tweed Lab

Instead, potential threats to drinking water supplies are determined using indicators such as E. coli which are shed in huge numbers from faeces in all individuals (whether infected or not). The presumption is that if no E. coli are present in a water sample, then no recent faecal contamination has occurred. See Tweed Laboratory Centre

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