Water UK

Summary

 • UK water suppliers place the highest priority on assuring the quality of water provided to their customers.
 • Cryptosporidium is a microscopic parasite that can cause severe and persistent diarrhoea.
 • The most common routes of Cryptosporidium infection are through person-to-person or animal-to-person contact. Infection through recreational waters (e.g. swimming pools) is also a known route.
 • Occasionally the organism has been transmitted through contaminated drinking water supplies. Although uncommon, this has the potential to affect large numbers of people and therefore must be strictly controlled.
 • Based on considerable research and experience, it is widely accepted that Cryptosporidium contamination of drinking water is best avoided by a risk management approach in which the sources of Cryptosporidium contamination are controlled. Failing that, treatment is established to deal with the risks from Cryptosporidium contamination of source waters, which will vary considerably between sources.
 • Although Cryptosporidium is resistant to chlorine, the multi-barrier approach to water treatment adopted by the industry includes effective barriers to the organism. However, control of contamination in the catchment remains the first barrier.
 • Because of the difficulty of routine analysis, there are no specific standards for the organism in the European Union Drinking Water Directive or UK national regulations, although there is a general requirement that no organism should be present in sufficient quantities to constitute a threat to public health.
 • Increased understanding of Cryptosporidium contamination and the considerable amounts of monitoring data now available have helped water suppliers to establish effective means of controlling Cryptosporidium in drinking water, although there is a need for constant vigilance. Water companies and health authorities are also required to establish well rehearsed procedures for dealing with any emergency that might occur.

Technical background

Where does Cryptosporidium come from?

Cryptosporidium is a protozoan parasite that causes a severe diarrhoeal disease known as cryptosporidiosis. The organism is of common worldwide occurrence and while there are several different species, only Cryptosporidium hominis and Cryptosporidium parvum are of significance for human health. The former affects humans and the latter affects both humans and animals.

The organism is spread via small oocysts 4-5µ in diameter which are shed in large numbers by infected hosts and subsequently can be ingested by a new host. These oocysts are known to be environmentally resistant and can survive for some time. Routes of transmission are most often by direct contact with infected animals or humans. Transmission can also be indirect, for example via recreational waters such as swimming pools or occasionally via food or water. While water borne transmission through the public water supply is relatively rare, it has the potential to affect large numbers of people and it is therefore vitally important to ensure that effective barriers are in place.

Not all ingestion causes illness. The number of oocysts required to initiate infection is uncertain but is thought to be of the order of 10-100 and depends on the immunological status of the individual. Therefore, while the presence of low levels of oocysts in drinking water requires attention, it does not necessarily mean that cases of cryptosporidiosis will be seen in the community.

What standards apply to Cryptosporidium in drinking water?

Techniques for sampling and analysis are complicated and time consuming, requiring the filtration of large volumes of water (100 - 1000 litres), followed by several stages of elution, isolation and concentration of the oocysts, and then identification and enumeration by immunofluorescent microscopy. Initial testing does not provide information on whether the oocysts are viable and therefore capable of causing disease. This requires further testing. Identification of the species sub-type can be helpful in tracing the source of contamination - this requires much additional work.

For these reasons there is no specific standard for Cryptosporidium in the EU Drinking Water Directive or the national regulations¹. However, there is a general requirement that drinking water should not contain any micro-organism or parasite at a concentration that would constitute a potential danger to human health. This would include Cryptosporidium.

In the UK, water companies adopt a formal risk-based approach to assessing and managing Cryptosporidium and any other substance that could constitute a potential danger to human health. Companies carry out routine monitoring of treated water. On the rare occasions where cryptosopridium is detected in the treated water, companies immediately investigate the cause and make corrections while consulting with local health protection units to determine measures needed to protect public health.

How is the quality of drinking water assured?

Based on considerable research and experience, it is widely accepted that Cryptosporidium contamination of drinking water is best avoided through a risk-based approach. This assesses potential sources of Cryptosporidium in the catchment and the risks to drinking water then puts in place appropriate barriers, starting with control in the catchment, and ensures that they are working efficiently. This is particularly important in surface water catchments with upstream stock, but the risk to certain types of shallow groundwater must not be overlooked.

Effective controls to prevent animal or human waste from discharging upstream of the abstraction point include fencing water courses to prevent livestock contaminating the raw water and ensuring septic tanks are not leaking into water courses or groundwater. However, for many larger water supplies this is not possible and therefore effective water treatment barriers need to be installed.

Cryptosporidium oocysts are resistant to normal chlorine disinfection, although ultraviolet disinfection can be very effective for oocyst inactivation. However, effective oocyst removal is normally best achieved through other well operated conventional water treatment processes, particularly clarification and filtration.

While rare, there have been outbreaks of water-borne cryptosporidiosis in both the UK and the US. Generally, these have been shown to be caused by inadequate water treatment or breaches of the integrity of distribution systems. Increased understanding of Cryptosporidium contamination and the considerable amounts of monitoring data that are now available have helped water suppliers to establish effective means of controlling Cryptosporidium in drinking water. However, there is a need for constant vigilance. Water companies and health authorities are therefore also required to establish well rehearsed procedures for dealing with any emergency that might occur.

Note

1 E.g. The Water Supply (Water Quality) Regulations 2000 (2001 Wales) incorporating the Water Supply (Water Quality) 2000 (Amendment) Regulations 2007 (England and Wales)

Phill Mills
Deputy Chief Executive, Water UK