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The report, entitled ‘Review of latest available evidence on risks to human health through potential transmission of avian influenza (H5N1) through water and sewage’ [PDF], raises serious questions about the safety of the public using water which may have been contaminated by asymptomatic migratory birds, as well as a possible risks to sewage workers from infected human and other excreta.

The possibility that the H5N1 virus may be able to be spread to humans drinking or swimming in contaminated water has long been the stuff of legend among amateur flu trackers. Their argument being that asymptomatic migratory waterfowl excrete the virus; that the virus can live in untreated water for some time; and that therefore humans might be at risk, has now been officially recognised. The threat remains theoretical because there is little proven evidence to support the hypothesis and it should be made clear that that properly treated water poses little or no risk to humans.

The report makes it clear that the fact that waterfowl excrete influenza viruses into water does not necessarily mean that it is a route of transmission between birds, nor proves the extent of the risk posed to humans. Some other viruses are also excreted into water without being transmitted. However, there is some evidence which suggests that this may not be true for H5N1.

The report however calls for more research and suggests that where treatment is impossible, it may become necessary to restrict access to reservoirs of water which might be shared by migratory waterfowl and humans – or to put it another way, stop people drinking or swimming in water where H5N1 has been found.

The report also suggests that there is evidence that excreta from infected humans, just like bird excreta, may theoretically provide an avenue for human to human spread of the H5N1 virus, although again the data is limited. If further research confirms this, it may have major implications for the disposal and treatment of human sewage and require improvements to safety procedures for sewage workers.

Drinking water remains safe – it’s important not to over dramatize the implications of the report – but you may want to think twice about swimming in open water frequented by migratory birds.