Report from the Waterborne Cryptosporidiosis Subcommittee of the Scientific Advisory Committee of the Irish National Disease Surveillance Centre

Introduction

At the request of the Department of Health and Children in Ireland, a subcommittee of the Scientific Advisory Committee of the National Disease Surveillance Centre was established to advise on the risk to public health from Cryptosporidium in drinking water supplies and in water used for recreational purposes; to advise on appropriate surveillance activities that should be undertaken to detect and prevent waterborne cryptosporidiosis; to draw up national guidelines for the public health response to the detection of Cryptosporidium in water supplies; and to advise on prevention strategies that would minimise the risk in the general population, and in target groups such as immunocompromised individuals.

A draft consultation document was prepared and a three-month consultation period was initiated in 2002. The document is now being reviewed taking into consideration the many submissions received, and it is hoped to have a finalised document prepared in the near future.

The members of the subcommittee represent the main professional groups that have responsibility in this area, and include microbiology, infection control, clinical infectious diseases, public health medicine, environmental health, laboratory science, water engineering, Teagasc and the Department of Environment and Local Government.

Bearing in mind the fact that the final report has not yet been completed, and that the recommendations are subject to amendment, this paper will summarise how the subcommittee addressed the terms of reference as outlined, and the recommendations.

Risk to human health from Cryptosporidium in water

There is plenty of evidence from outbreaks worldwide that drinking water is a source for outbreaks of cryptosporidiosis in humans, outbreaks having been documented in drinking water and water used for recreational purposes such as swimming. However, the actual risk to health from Cryptosporidium in water supplies is not known. It is probably related to parasite characteristics, dose size and the immunity of those exposed. There is no internationally agreed threshold level of Cryptosporidium oocysts in water above which human illness is likely to occur. This issue has been reviewed in many countries, and recent experience has shown that Cryptosporidium outbreaks can occur in the presence of low Cryptosporidium levels, and conversely, there may be no human illness when levels are substantial in the water supply. Given the lack of an evidence base for recommending a threshold level, no threshold level is recommended in these guidelines.

The history of the quality of the drinking water supply needs to be considered as well as the Cryptosporidium oocyst count. The quality of the water is influenced by the source of the supply, the amount of pollutants present in the source supply, and the treatment and monitoring process. Hunter (2000) has suggested a number of factors that should be taken into account in determining whether an oocyst count is significant or not, and the subcommittee recommend that these are used when assessing the risk to public health. These include when and where the sample was taken, the number of oocysts detected, the source and treatment of the affected water supply, the distribution area and size of population served, whether any problems with the supply, such as treatment failure or high turbidity have been identified, high oocyst counts in consecutive samples, recent changes in the source or treatment, the history of sampling and whether any waterborne outbreaks of cryptosporidiosis have been associated with the supply in the past.

In order to assess the risk using these criteria, there is a need for regular structured liaison between the Local Authorities and the Health Boards. In some regions in Ireland, this is happening already, with meetings being held regularly between the Local Authority and the Health Board. Where these are not in place already, the subcommittee recommend establishing this liaison. This is detailed in the public health response section.

Surveillance activities to detect and prevent waterborne cryptosporidiosis

At present it is hard to accurately estimate the amount of cryptosporidiosis in Ireland, as there are no national statistics representing all cases detected in the community, and cryptosporidiosis is not a notifiable disease. The information that is available, namely information from voluntary regional laboratory reporting systems, shows a wide variation throughout the country, a rate varying from 1 per 100,000 to 10 per 100,000 population. As well as no national data on the incidence of the disease, there are no nationally agreed standard operating procedures for the circumstances in which testing for cryptosporidiosis should be undertaken in laboratories, and hence it is difficult to interpret regional variations in incidence rates.

The subcommittee have made several recommendations regarding surveillance of Cryptosporidium. Most importantly, cryptosporidiosis should be a notifiable disease, and the detection of Cryptosporidium in stools should be notifiable by laboratories. There should be routine testing of all stool samples for Cryptosporidium oocysts in children less than 10 years and where clinically indicated e.g. diarrhoea in immunocompromised individuals. Consideration should be given to instituting routine testing of stools for Cryptosporidium oocysts in patients over 10 years of age if Cryptosporidium is detected with increased frequency in other stool samples in the laboratory. Consideration should also be given to the establishment of a Cryptosporidium reference laboratory in Ireland. This is important particularly in an outbreak situation where early typing of isolates can help in identifying the source, i.e. whether human or animal. Laboratory facilities for monitoring Cryptosporidium in water should also be available nationally.

Information from surveillance of gastrointestinal illness in humans should be regularly shared with Local Authority colleagues on the Local Liaison Groups. In an outbreak situation, attack rates for cryptosporidiosis should be calculated for each water supply zone.

Other issues, including the dataset that should routinely be collected in each case identified are under consideration at present.

National Guidance on Public Health response to the detection of Cryptosporidium oocysts in the water

The subcommittee recommend that where there currently is no regular liaison between the Local Authority and the Health Board on water, Local Liaison Groups (LLGs), comprising representatives from the Local Authority and Health Board/Authority, should be established now. This needs to happen prior to any incident occurring. The Director of Public Health and the Director of Water Services should initiate this process as a matter of urgency and devise a structure for liaison that is appropriate to their region. The subcommittee recommend that the function of the LLGs should be:

1. To establish local procedures for the monitoring of contamination of water, including contamination with Cryptosporidium.
2. To share information across agencies on trends in human gastro-intestinal illness and in cryptosporidiosis, and on the results of water quality sampling.
3. To provide access to maps and other information on water supply zones. The Local Authorities are using Geographic Information Systems (GIS) to map the public water supply schemes. It is planned to complete this project within the next two years and then extend the system to cover private schemes.
4. To identify and interpret locally the significance of deviations in water quality indicators such as turbidity.
5. To meet and review water anomalies, including the presence of Cryptosporidium in the water, and to advise on appropriate actions, using local knowledge.
6. To develop local incident response plans in the event of an incident and to agree joint working procedures.
7. To advice on when the medical officer (MO) should be notified.
8. To review results of risk assessments of water sources.

LLGs should use local knowledge to aid in interpretation of water quality indicator test results, including interpreting the finding of Cryptosporidium oocysts in the water to establish whether a cryptosporidial incident has occurred or not. Each Local Authority should have written protocols on what to do in the event of a cryptosporidial incident.

When a cryptosporidial incident has occurred, the Local Authority should establish an Incident Response Team (IRT). The Medical Officer, Principal Environmental Health Officer and other Health Board staff should be members of the IRT as appropriate. If there are any cases of illness, then an Outbreak Control Team should also be established as per Health Board procedures. A detailed outline of what to do in the situation is provided in the document.

Prevention strategies to minimise the risk of cryptosporidiosis

Conclusion

These draft recommendations of the subcommittee may be amended following completion of the review of submissions received. However, it is clear that there are many steps to be taken, if the potential for waterborne Cryptosporidium to harm human health is to be minimised.

Reference

Hunter PR. (2000) Advice on the response from public health and environmental health to the detection of cryptosporidial oocysts in treated drinking water. Commun Dis Public Health 3: 24-27.