A grey heron in the Noord-Holland-Noord region has tested positive for the West Nile virus. The virus is rare in the Netherlands. The virus is transmitted by mosquitoes and since the mosquito season, during which bites are common, is almost over, no further spread of the virus is expected at this stage. 

Additional screening of birds and mosquitoes

The grey heron was caught in a duck decoy in mid-September; the Erasmus Medical Center had been carrying out a study into bird flu in the area. An investigation is currently underway in the area to determine whether any other birds have been infected with the West Nile virus. The Netherlands Food and Consumer Product Safety Authority (NVWA) is also conducting a study into overwintering mosquitoes in the area. 

West Nile virus

Birds are the primary hosts of the West Nile virus, and the virus is transmitted by mosquitoes that feed on the blood of infected birds. These mosquitoes spread the virus to other birds and sometimes to humans and mammals, such as horses. Recent decades have seen the virus spread across large parts of the world, including to Southeastern and Central Europe and Germany. In the Netherlands, the West Nile virus was first detected in 2020 in a common whitethroat, and subsequently infection with the virus was detected in mosquitoes as well as in 8 people. 

Infection in humans

Most of the time, humans will not fall ill as a result of the West Nile virus, with some 80% of people not developing any symptoms at all and 20% experiencing mild symptoms, such as fever and flu-like symptoms. A small percentage of individuals (1%) will become seriously ill with encephalitis or meningitis. 

Early detection of West Nile virus

RIVM, the NVWA and other parties are working closely to monitor the spread of the West Nile virus. In order to detect the virus at an early stage, RIVM and the NVWA are conducting research into subjects including mosquitoes, and are monitoring any relevant changes. Erasmus MC is leading a national research consortium that aims to assess the risk of outbreaks of the West Nile virus. RIVM and the NVWA will inform the general public and ensure that citizens are aware what steps they can take when necessary. This will help us prevent mosquitoes from transmitting the virus and people from getting sick as much as possible.