West Nile virus

West Nile virus (WNV) is one of the equine arboviral encephalitides and as such, can cause encephalitis (a serious nervous disease) in horses, which can be fatal. Wageningen Bioveterinary Research conducts research on this disease.

The West Nile virus is caused by a flavivirus, which can infect many bird and mammal species, among which humans and horses.

Wat is West Nile Virus?

West Nile Virus (WNV) is a virus transmitted by insects (arbovirus) belonging to the genus Flavirus in the family Flaviviridae. Two groups (lineages) of the virus are distinguished on the basis of their genetic relationship. WNV causes the disease West Nile Fever (WNF).

Infection with the West Nile Virus

West Nile Virus is spread primarily by the mosquitoes of the Culex family. These mosquito species occur across the whole of Europe, but infections are mainly reported in warmer Southern Europe. WNV propagates primarily in birds, which in this way form a source of virus transmission. Many other vertebrates, among them mammals including horses and humans, can be infected. Approximately 10 to 20 per cent of these infections lead on to symptoms, and only a very small proportion (1 per cent) progress to becoming very serious. Infected mammals are 'dead-end hosts', i.e. too little of the virus is produced for further transmission.

Transmission of the disease

The disease originally occurred only in Africa. Transmission has expanded, particularly over recent decades, to regions that were previously free of the disease, and it has become endemic in regions around the Mediterranean Sea, India and Central and South Africa. Most worthy of note is the introduction into The Bronx, part of New York City in the United States.

Infections were first reported in Germany in 2018. WNV poses a serious threat to the health of humans and animals. Wageningen Bioveterinary Research (WBVR) is watching the situation closely and is in consultation with Dutch institutes to prepare for the possible arrival of the virus.

Any suspicion of this notifiable disease must be notified to the national government and the European commission.

Rapid spread in the US

WNV was first encountered in the United States in 1999. Its spread across the entire continent over subsequent years forms a recent example of extremely rapid dissemination following initial introduction to a new region. It turned out over the years that WNV had definitively established itself on the continent. The virus probably entered New York via infected birds.

WNV first manifested in high mortality in the crow population. Humans and horses were subsequently infected, causing a great deal of sickness and even mortality in both humans and horses. Infected humans and horses 'produce' insufficient WNV to be taken up by mosquitoes and transmitted onwards.

Spread in Europe from 1999

WNV occurs in Africa, Europe, the Middle East, West and Central Asia and Oceania. There were sporadic outbreaks in Southern Europe up to approximately the turn of the century. Infections were increasingly recorded in Southern European countries after 2000, and the region where these infections were found constantly expanded to the north.

Preparations in Europe

Since the turn of the century, there has been a slight increase in incidental infections in Europe and on the outskirts of Europe. This trend has continued in recent years, and in 2018 a noticeably large number of infections were reported. Spreading into Germany is also cause for concern. This indicates that WNV could also reach the Netherlands. A vaccine for horses has been registered, and virologic and serologic tests are in operation to indicate WNV infections.


There are no clear clinical symptoms. They are similar to those of flu. Increased bird mortality is also reason for further research into the presence of West Nile Virus. Laboratory diagnosis can be conducted by means of serologic testing, but cross-reactions with other flaviviruses could be a problem.

Consequences for humans

The disease causes no or (mild) flu-like symptoms of a temporary nature in most cases. The most frequent clinical symptoms are high fever, headache, stiffness in the neck and various neurological complaints. In the case of elderly people and those with impaired resistance, the disease may be associated with serious symptoms, such as paralysis and permanent nervous disorders. This can lead to death on occasion. To date, more than 1,000 people have died in the US as a result of WNV infection.

Infection of horses

West Nile Virus infection of horses in the US resulted in more than 15,000 fatalities up to 2004. Since 2004, a vaccination for horses has been available, with the result that the disease has been suppressed.


  • A vaccine is available for horses. Uptake of voluntary vaccination is extremely patchy and often happens only at the point when the virus is in circulation.
  • No vaccine is available for humans; development has reached an advanced stage, but a vaccine is not yet available/licensed.