Health ministry inspectors are trying to trace the source of an apparent leak of polio virus into the waste water system on an industrial estate near Utrecht, where a large pharmaceuticals company makes vaccines, the Volkskrant said on Wednesday.

The presence of the virus was confirmed in early August and officials are now trying to establish the source of the contamination and find out if a member of staff has been infected, the paper said.

Polio has been virtually eradicated in the west, thanks to a strict strategy of vaccinations, although people who have been vaccinated can still carry and pass on the virus without symptoms. It is spread via saliva and faeces, hence the regular checks on sewage.

The virus can survive outside the body but is deactivated during the water purification process and its presence in waste water is not a health threat, a spokesman for the inspectorate told the paper.

The location where the virus was found is home to the Intravacc research institute and public health council RIVM, as well as Bilthoven Biologicals.