7 Guidance on the epidemiological investigation of outbreaks of infection
7.1 Introduction
7.1.1 There is a need for more general recognition that water is not the only source of Cryptosporidium infection in humans. The organism can be acquired from water, food, milk, swimming pools, contact with farm and domestic animals and person to person transmission. Some epidemiological surveys following outbreaks have been deficient and there is a need for greater consistency in the quality of investigations.
7.1.2 Outbreaks of illness associated with drinking water are uncommon but have the potential to affect large numbers of people. There are several problems unique to the investigation of outbreaks of infectious disease thought to be related to contaminated drinking water. These include:
- Some water treatment works have very large outputs of water which will supply a wide geographic area.
- The water supply to an area may be a blend from more than one treatment works, only one of which may be the source of contamination.
- By the time an outbreak is recognised the contamination incident may have passed and therefore pathogens are often absent from water samples.
- The presence of low numbers of Cryptosporidium oocysts in water does not imply that water is the source of the outbreak.
- General exposure of the population to mains water may obscure the actual source of contamination.
- Gastroenteritis, the illness most often caused by waterborne outbreaks, is not uncommon in the population and the majority of cases are not due to water.
- Many pathogens associated with water can also be transmitted in other ways such as food and milk, animals or person to person contact.
7.2 Guidance Manual
7.2.1 High quality epidemiological information is vital in the investigation of possible outbreaks of waterborne infection associated with mains water consumption because microbiological evidence of water contamination by pathogenic organisms is usually difficult to obtain. Epidemiological investigations of such outbreaks are not straightforward; they are relatively uncommon and may be statistically complex, such that individual consultants in communicable disease control (CCDCs) or consultants in public health medicine (CPHMs) may appreciate help with investigations.
7.2.2 Guidance has been produced to give CCDCs/CPHMs and other members of the Outbreak Control Team (OCT) practical advice to assist in the conduct of epidemiological studies. It is aimed at the investigation of outbreaks that may be associated with the consumption of mains water. As Cryptosporidium is the most commonly reported organism involved in such outbreaks the emphasis is on investigation of outbreaks caused by this organism. However, the general principles set out in this guidance will apply to all outbreaks of potentially waterborne infection and to the investigation of all alternative hypotheses. The manual is not intended to replace local outbreak control plans, but it is hoped that CCDCs/CPHMs will find it a useful supplement and that its use will lead to the establishment of an agreed best practice in the investigation of water associated infection. The guidance manual is given in Appendix A4.
7.3 The Groups advice
7.3.1 The Group recognises that high quality epidemiological information is vital to the investigation of possible outbreaks of waterborne infection associated with mains water consumption because microbiological evidence of water contamination by pathogenic organisms is often difficult to obtain and even when it is available, such evidence is rarely conclusive. Some previous epidemiological surveys have been deficient and there is a need for greater consistency in the quality of investigations. The Group commends the use of the Guidance on the Epidemiological Investigation of Outbreaks of Infection Associated with Mains Water (Appendix A4 of this Report) to assist in the conduct of epidemiological studies for CCDCs/CPHMs and other members of OCTs.
Recommendation
7.3.2 The Group recommends the Chairman and members of the Outbreak Control Team use the Guidance on the Epidemiological Investigation of Outbreaks of Infection (Appendix A4 of this Report) in all outbreaks where waterborne infection is suspected.
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