A male patient has been diagnosed with the West Nile virus last week. The man has not been abroad recently. He may have contracted the virus in the region of Utrecht due to a mosquito bite. In this region, birds and mosquitoes have been detected with the West Nile virus in August and September. This is the first time that an infection with this virus has been diagnosed in a person who has contracted it in the Netherlands. 

The West Nile virus circulates in birds and is transmitted by mosquitoes. The mosquitoes get infected by feeding on infected birds. Afterwards, these mosquitoes spread the virus to other birds and sometimes to humans and other mammals, such as horses. People and horses can become ill from infection  with this virus, but do not play a role in the further spread of the disease. The virus is not transmitted from human to human or from human to animal.

Clinical picture

Most people do not develop disease following an infection with the West Nile virus. About 80% does not display any symptoms at all; 20% develop mild symptoms such as a fever and flu-like disease. However, about 1% of the people who are infected can develop severe neurological disease, such as brain inflammation. In that case, treatment in a hospital is necessary. A small percentage of people with such a severe disease can die. An infection with the virus has previously been detected in the Netherlands, but all these infections were contracted abroad.

Spread West Nile virus

The West Nile virus has spread over large parts of the world in recent decades. The disease has been occurring for some time in southeastern Europe and the Mediterranean. In recent years, the disease has spread to Central Europe, including the central part of Germany. Further spread to Western Europe was in line with expectations.

Infected bird

In the Netherlands, in August of this year, West Nile virus was detected for the first time in a whitethroat in a nature reserve near Utrecht. The bird was examined within the framework of the One Health PACT  research project. Within this project the occurrence and prediction of mosquito-borne viruses in the Netherlands are being investigated. As a result of this discovery, the Dutch microbiological laboratories and specialists were informed to be more vigilant of this infection. That the virus had been circulating in the Utrecht region was confirmed by  research carried out on mosquitoes and the discovery of two more positive birds.

Mosquito season

With the weather change and dropping temperatures of the last few weeks, the mosquito season is coming to an end. This minimises the risk of contracting an infection with West Nile virus. Before the next mosquito season, advice and possible further actions will be proposed.