Cryptosporidium: the health issues

Cryptosporidium and cryptosporidiosis

Consumer Information Produced by the Drinking Water Inspectorate, current August 2004.

What is it?

Cryptosporidium is a parasite that infects man and a wide range of domestic and wild animals. It causes cryptosporidiosis, which in healthy adults, is an unpleasant diarrhoea lasting for up to two weeks. At present there is no cure for cryptosporidiosis and the condition can be serious for, and fatal to, the immuno-compromised e.g. those receiving chemotherapy or AIDS patients.

Cryptosporidiosis is relatively uncommon. It can be contracted through person to person contact, from contaminated food, poorly operated swimming pools or contaminated drinking water. The environmentally resistant form of the parasite, the oocyst, is excreted in the faeces of infected animals and humans. Pollution of watercourses can occur where there is poor control over the disposal of faecal slurries from infected farm animals. There is evidence that sewage effluents discharged to rivers used for drinking water abstraction can play a role in recycling oocysts excreted by infected humans. There have been a number of drinking water-related outbreaks of cryptosporidiosis, some of which have attracted considerable media attention. These outbreaks have involved from a few dozen to thousands of cases of cryptosporidiosis.

Cryptosporidium poses a challenge to water treatment processes because of its small size and resistance to disinfection processes. However, the Inspectorate's investigations of outbreaks of cryptosporidiosis have shown that they are invariably related to inadequate provision or poor operation of water treatment. There is good evidence that careful operation of the coagulation/flocculation and filtration processes provides consumers with a very high level of protection against exposure to the parasite.

The Regulatory Position

In June 1999 the Government introduced regulations that required water companies to carry out risk assessments to establish whether there is a significant risk from Cryptosporidium in water supplied from each of their treatment works. Where there is a risk, water companies must use a process for treating the water to ensure that the average number of oocysts is less than 1 per 10 litres of water. Water companies must use a regulatory method for sampling and analysis to check that they are complying with the standard.

The concentration of 1 oocyst in 10 litres is a treatment standard and not a health-related standard. It is set to ensure that water companies optimise their treatment processes and pay careful attention to operation and maintenance. It is not feasible to set a health related standard because of the wide variation in susceptibility of different sectors of the population. There is also a very poor understanding of virulence factors that are known to influence the ability of the parasite to cause infection. Nevertheless, there is evidence that where outbreaks of cryptosporidiosis have occurred, the concentrations of oocysts were in excess of 1 oocyst in 10 litres. Furthermore, most outbreaks have been associated with problems in the operation of treatment processes. The UK approach to risk reduction may be summarised as the requirement to install and operate effectively, physical barriers to optimise removal of Cryptosporidium from water supplies.

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