West Nile virus mosquito

West Nile fever

West Nile fever is caused by the West Nile virus (WNV). It is transmitted by mosquito species that are present in large numbers in warmer climate areas, but can also be present in areas with mild temperatures. Mosquitoes, birds, mammals and humans can be infected. Wageningen Bioveterinary Research (WBVR) conducts research into this disease.

What is West Nile virus?

West Nile fever is a disease caused by the West Nile virus (Familie: Flaviviridae, genus: Flavivirus). The virus is transmitted by insects, what is called an arbovirus. Culex mosquito species spread West Nile virus from bird to bird. Occasionally, mosquitoes can also transmit the virus to other animals such as horses and to humans (zoonosis), who can become very sick.

Where does West Nile virus occur?

West Nile virus originally only occurred in Africa. Sporadically, it was also detected elsewhere. Since the turn of the century, this has been happening with increasing frequency. The disease has manifested itself in the United States. In Europe there are species of culex mosquitoes that can spread the virus too. Especially in the warmer south, infections are regularly reported. In the Netherlands, West Nile virus was found for the first time in wild birds and mosquitoes in 2020. Antibodies against West Nile virus were also detected in some people.

What are symptoms of West Nile fever?

Increased mortality in birds is a reason for further research into West Nile fever. The susceptibility to the virus in bird species is very different and crows are especially sensitive.

West Nile virus infections in horses cause clinical signs in approximately 20% of infected horses. A small percentage progress from serious to very serious, including neurological symptoms. Less than 1% of infections in horses lead to death. In the USA, the pathogenic potential of West Nile virus appeared to have increased after its introduction to this continent. Up to 2004, 5 years after the introduction, more than 15,000 dead horses have been reported.

Clinical signs in humans resemble flu symptoms. The majority of infections progress without or (mild), temporary flu-like symptoms. The most common clinical signs are high fever, headache, neck stiffness and various neurological complaints. In older people and people with reduced resistance, the disease can be accompanied by serious symptoms such as paralysis and permanent nervous disorders and can sometimes even lead to death. In the USA 100-200 people die every year as a result of West Nile virus infections.

Notifiable animal disease

Suspicions of West Nile fever in animals must be reported to NVWA (Netherlands Food and Consumer Product Safety Authority). It is a “Category E” disease (Animal Health Law).

Mortality in birds is usually the first indication of the presence of WNV and prompts further research on animals, insects and humans.

Spread of West Nile virus

West Nile fever occurs in Central and South Africa, areas around the Mediterranean, India and North America. West Nile virus infections are regularly reported in warmer southern Europe. The replication in and spread by mosquitoes also depends on the ambient temperature. Affected areas are mainly limited by the expansed area of ​​mosquitoes, the biological vector of West Nile virus.

Mosquito spread the virus

Mosquito species can spread West Nile virus from bird to bird or from bird to mammal such as horses or humans. West Nile virus is spread by mosquitoes by uptake of infected blood and releasing virus with each subsequent blood meal. Culex mosquito species, such as the mosquito Culex pipiens, and Aedes mosquito species, including the tiger mosquito Aedes albopictus, are biological vectors. Mosquitoes once infected continue to spread the virus for a lifetime.

Birds are the virus source

West Nile virus multiplies in both vertebrates and these mosquito species. Birds produce enough virus for uptake by mosquitoes. Direct contact between birds does not lead to virus spread. Birds are the virus source for mosquitoes to spread to other animals and humans. The uptake by mosquitoes from mammals is negligibly small, because too little virus is produced. Infected horses and humans therefore produce too little virus for further spread by mosquitoes (dead-end host). West Nile virus can be passed on through blood donation and breast milk.


West Nile virus was originally only found in Africa. Sporadically, it has also been detected in countries around the Mediterranean Sea. From the turn of the century, epidemics were increasingly reported outside the endemic area, including in the Camargue wetland, Provence in southern France. In 1999, West Nile virus was first detected in the United States (New York). After the first introduction, probably via infected birds, WNF first manifested itself in the United States by many deaths in the crow population. Subsequently, people and horses were also found to be infected. The continental spread in the following years is a recent example of the very rapid spread. West Nile virus turned out to be permanently present on this continent.

In Europe, it is now also found much further north, such as in Germany for the first time in 2018 and in the Netherlands in 2020.

Diagnostics West Nile virus

West Nile fever caused by both subtypes (lineages) of the virus are notifiable. Wageningen Bioveterinary Research (WBVR) is the national reference laboratory for West Nile fever in animals and has ISO-certified tests to diagnose suspicion of West Nile fever in animals.

Serological test

Antibodies against West Nile virus after infection or after vaccination can be detected with ELISAs and serum neutralization test (SNT). These tests detects antibodies against both subtypes. The cELISA detects all types of antibodies against West Nile virus, while the IgM ELISA is specific for equine IgM antibodies. IgM antibodies are detected for short period after infection. The IgM ELISA is therefore used to detect recent infections, because the virus itself is not or hardly detectable in horses. Antibodies against other flaviviruses, such as usutuvirus can also react in the ELISA for West Nile fever (false positive test result). The SNT for West Nile fever does distinguish neutralizing antibodies against different flaviviruses and is therefore very specific for this disease. Clotted blood (serum blood) is used in these serological tests.

PCR test

Both subtypes (lineages) of West Nile fever are detected in birds by the PCR test for West Nile virus. The subtype can be determined by unravelling the genetic code of the virus (‘sequencing’). The unknown West Nile virus is preferably first cultured in the laboratory. Infections in mammals cannot be detected or can only be detected in a very short period of time. Whole blood (EDTA blood) or brain tissue is used in these virological tests. Blood or brain samples must be transported on ice (not frozen).

Vaccine West Nile virus

A vaccine for horses has been available since 2004. This has largely reduced the disease of horses in the US. The motivation in the US for voluntary vaccination of horses is very variable and only increases when infected horses are detected. In the Netherlands, the vaccine for WNF is also available for horses, but is hardly vaccinated or not at all. Still, vaccination is the most effective method to prevent the consequences of West Nile virus infections.

There is no vaccine against WNV available for humans.

Prevention and control West Nile virus

To prevent is better than to cure. Preventing the introduction of West Nile virus via infected animals is impossible, because (wild) birds are the virus source for new incursions. Although mosquitoes do not spread over large distances, introduction via infected mosquitoes from neighbouring countries is not excluded.

Removal of infected birds, if possible and acceptable, reduces virus uptake and thus the virus spread by mosquitoes to susceptible animal species and humans. Mosquitoes can also be controlled to reduce the spread of disease. After a West Nile virus infection has been detected, intensified monitoring is carried out to get better insight of the situation. Depending on this, it may be advisable to protect all sensitive animal species to avoid infection by mosquitoes. This can be done, for example, by horse blankets and stabling of horses during sunrise and sunset. People can also reduce the risk of mosquito bites by the well-known measures. All these contribute to reducing the impact on animal and public health.


Wageningen Bioveterinary Research, together with the Animal Health Service (GD), monitors the spread of the virus in the animal sector. RIVM monitors the situation of West Nile virus in people.

As part of the One Health PACT, Wageningen University & Research (WUR) is conducting research on West Nile fever together with RIVM and GD. The Laboratory for Virology and Entomology of WUR is investigating the role of Dutch mosquitoes (Culex pipiens) in the spread of the disease. This research is performed in the biosafety laboratory level 3 laboratory. In addition, field research is being conducted on West Nile virus in mosquito populations. Sampling of mosquito populations occurs with special mosquito traps. The Culex pipiens mosquito is most commonly found in the mosquito traps.

Read the article in Wageningen World (March 2021)