Report No DWI0756

Feb 1996

There are rare occasions when the water delivered to consumers contains invertebrates and this has raised questions about the health risks which the presence of these animals represent. In these situations the Department of Health and the Drinking Water Inspectorate advise that whilst the presence of animals is aesthetically unpleasant there is no evidence of any risk to public health.

To provide a more secure basis for this advice, the Drinking Water Inspectorate contracted WRc to review the significance to health of animals in water distribution.

In reviewing the literature WRc sought answers to the following questions;

• are any of the invertebrates found in tap water pathogenic to humans?
• to what extent do aquatic invertebrates ingest and concentrate micro-organisms?
• how long do micro-organisms survive within aquatic invertebrates?
• do invertebrates excrete micro-organisms in a viable form?
• can ingestion of invertebrates potentially provide an infective dose of pathogens?

The review established that animal infestations of water distribution systems are associated with low flow rates and the presence of established biofilms. Three categories of animals are involved; those whose whole life cycle is completed in water (e.g. water fleas) and which may establish permanent populations, those whose life cycle is partially aquatic (e.g. insect larvae) which may survive for long periods, and terrestrial animals (e.g. earthworms) which are only transient inhabitants.

It was found that very few systematic investigations of the occurrence and concentration of animals in water distribution systems had been made and that there were even fewer studies of the microbiological aspects of these infestations. In the UK and other temperate climates it is unlikely that any of the animals would create a health problem. However, their mode of feeding is such that they will ingest, harbour and concentrate micro-organisms and there was some evidence that animals could contain opportunist pathogens, such as some Aeromonas species. Little information is available on the survival, or conditions which would encourage proliferation, of micro-organisms within the gut of the animals. There was general agreement that the animals afforded the micro-organisms with protection from the effects of water treatment processes, including disinfection.

Although the overall conclusion was that the presence of animals in water distribution poses little or no health risk to the population in general, it is possible that persons with impaired immune systems are at risk. Three suggestions have been made for studies which should provide the Department with more secure information on the microbiological implications of animal infestations.

This study was funded by the Department of the Environment and supervised by the Drinking Water Inspectorate.

Copies of this report may be available as an Acrobat pdf download under the 'Find Completed Research' heading on the DWI website.