Staff at the pharmaceutical company GlaxoSmithKline in Rixensart, Walloon Brabant, dumped 45 litres of liquid contaminated by live polio virus into the river Laan, the federal health ministry has revealed. The incident took place on 2 September but has only now been confirmed.

A ministry statement said the incident was the result of “human error”. The Laan rises in the municipality of the same name and flows through Rixensart before crossing the regional border into Flemish Brabant, passing through part of Overijse before joining the Dijle at Sint-Agatha-Rode. The contaminated water will have first passed through the purification plant at Rosières.

“The Scientific Institute for Public Health and the Superior Council for Health carried out an analysis of the risk and concluded there is no public health danger for the population,” the statement says. “The risk of people exposed to contaminated water developing poliomyelitis is extremely limited, given the high level of dilution and the high incidence of vaccination among the population.”

Poliomyelitis is an acute infectious disease that produces no symptoms in 90% of cases. In the other 10%, the central nervous system can be affected, leading to muscle weakness and paralysis. Once a common disease, polio has now been reduced to fewer than 1,000 cases a year worldwide thanks to vaccination programmes.

Vaccination against polio is compulsory in Belgium. Anyone who thinks they may have been exposed to the contaminated water, such as swimmers or anglers, should contact their doctor.

The health ministry took samples of water and mud from the water purification station, the Laan and the Dijle on 6 September. All samples were negative for polio virus.