Giardia Lamblia and Cryptosporidium parvum are protozoan parasites, infecting the digestive tract of man and other warm blooded animals.
It has been demonstrated that semi-aquatic mammals can serve as a zoonotic host, transmitting the disease to humans who consume contaminated water.
Domestic mammals (particularly ruminants) can serve as an infective host of Cryptosporidium, and, as in Giardia, the animal can contaminate a drinking water supply.
Research has also shown that both Giardia and Cryptosporidium are highly resistant to chlorine, a commonly used water disinfectant.
Due to the small size of Giardia cysts and Cryptosporidium oocysts, both are difficult to remove through filtration.

Cryptosporidium

It is currently believed that approximately 7% of the diarrheal cases in the U.S. are caused by Cryptosporidium, but this figure may be misleading, as causes of diarrhea are seldom tested for by physicians.

Cryptosporidium can be fatal to immuno-surpressed patients, and the current infectious dose (ID50) in healthy people is approximately 150 oocysts, whilst the ID50 for immunocompromised people is believed to be approximately 35 to 50 oocysts, although some documents suggest that the simple ingestion of as little as two oocysts can cause infection.

Cryptosporidium parvum was first recognised as a cause of human illness in 1976.
From 1976 to 1982 the disease was reported rarely in the U.S., primarily among the immunocompromised.

In 1982 the number of reported cases began to increase dramatically along with the number of HIV-infected persons; outbreaks among immunocompetent populations also were reported.

Recent municipal waterborne outbreaks of cryptosporidiosis in Texas (1984), Georgia (1987) and Oregon (1992), and a massive outbreak in Wisconsin in 1993 that affected more than 400,000 persons have raised the awareness about the waterborne transmission of cryptosporidiosis.
Since 1993 several smaller cryptosporidiosis outbreaks were reported in the U.S., two were related to drinking water, six were linked to recreational water and one was foodborne.

Cryptosporidiosis is caused by the ingestion of the environmentally tough oocysts of the protozoan parasite C. parvum, an intracellular organism that can replicate in the gut epithelial cells of most mammals.
Its oocyst is extremely resistant to chlorine, which is commonly used to treat municipal water.

"C. parvum oocysts are present in most surface water supplies; better technological tools and epidemiologic assessments are needed to determine public health risks from these oocysts.
Until the risks are fully known, efforts should be made to inform the public about cryptosporidiosis."
National Centre for Infectious Diseases
Atlanta, Georgia, U.S.A.

Giardia Lamblia

Giardia lamblia (intestinalis) is a single celled animal, i.e. a protozoa, that moves with the aid of five flagella. In Europe it is sometimes referred to as Lamblia intestinalis.

Although Giardia was discovered in the 19th century, it was not until 1981 that the World Health Organisation classified Giardia as a pathogen (capable of causing disease).

Giardiasis is the most frequent cause of non-bacterial diarrhoea in North America.

Human giardiasis may involve diarrhoea within 1 week of ingestion of the cyst, which is the environmental survival form and infective stage of the organism.

Normally illness lasts for 1 to 2 weeks, but there are cases of chronic infection lasting months to years and chronic cases, both those with defined immune deficiencies and those without, are difficult to treat.

The disease mechanism is unknown, with some investigators reporting that the organism produces a toxin while others are unable to confirm its existence.

The organism has been demonstrated inside host cells in the duodenum, but most investigators think this is such an infrequent occurrence that it is not responsible for disease symptoms.

Mechanical obstruction of the absorptive surface of the intestine has been proposed as a possible pathogenic mechanism, as has a synergistic relationship with some of the intestinal flora.

Several strains of G. lamblia have been isolated and described through the analysis of their protein and DNA; type of strain, however, is not consistently associated with disease severity, with different individuals showing various degrees of symptoms when infected with the same strain, and symptoms of an individual may vary during the course of the disease

Ingestion of one or more cysts may cause disease, as contrasted to most bacterial illnesses where hundreds to thousands of organisms must be consumed to produce illness.

Giardiasis is most commonly associated with the consumption of contaminated water, five outbreaks have been traced to food contamination by infected or infested food handlers, and the possibility of infections from contaminated vegetables that are eaten raw cannot be excluded. Cool moist conditions favour the survival of the organisms.

Contamination of a water supply by Giardia can occur in two ways; by the activity of animals in the watershed area of the water supply or by the introduction of sewage into the water supply.
Wild and domestic animals have been shown to be a major contributor in contaminating water supplies.

Giardiasis is more prevalent in children than in adults, possible because many individuals seem to have a lasting immunity after infection. This organism is implicated in 25% of the cases of gastrointestinal disease and the overall incidence of infection in the U.S. is estimated at 2% of the population.

About 40% of those who are diagnosed with giardiasis demonstrate disaccharide intolerance during detectable infection and up to 6 months after the infection can no longer be detected.

Lactose (i.e. milk, sugar) intolerance is most frequently observed, and some individuals (less than 4%) remain symptomatic more than 2 weeks; chronic infections lead to a malabsorption syndrome and severe weight loss.

Chronic cases of giardiasis in immunodeficient and normal individuals are frequently refractile to drug treatment, and in some immune deficient individuals giardiasis may contribute to a shortening of the life span.