West Nile virus mosquito

West Nile virus (WNV)

West Nile virus (WNV) 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. In the Netherlands, the first infected bird was found in September and the first human in October 2020.

What is West Nile virus?

West Nile virus (WNV) is a virus transmitted by insects; this is called an arbovirus. It causes the disease West Nile fever (WNF). Occasionally, mosquitoes can also transmit the virus to other animals such as horses and to humans, who can become very sick.

How is the situation in the Netherlands?

For years it has been thought that the West Nile virus can emerge in the Netherlands, as Dutch mosquitoes can transmit the virus and Dutch birds are also susceptible to infection. In September 2020, a warbler with the virus was found in the Netherlands.

What Dutch research is being done?

Together with partners, Wageningen University & Research studies the virus in the One Health PACT. Wageningen Bioveterinary Research (WBVR) is watching the situation closely to prepare for possible outbreaks. Tests are at hand that can detect WNV infections. In addition, the Laboratory of Virology and the Laboratory of Entomology in Wageningen are jointly investigating the susceptibility of Dutch mosquitoes to the West Nile virus.

What can you do to reduce WNV?

The West Nile virus is mainly spread by mosquitoes of the Culex family. Larvae of this mosquito grow in water-filled reservoirs such as rain barrels, watering cans and clogged gutters. In the light of current developments, keeping these mosquitoes under control will become an important strategy in the future.

You can prevent nuisance from mosquitoes by tackling these breeding grounds, or by ensuring that your home is "mosquito-proof", for example with window screens. The West Nile virus has been circulating in America since 1999. There the motto is 'Dress, DEET, Drain': dress in such a way that as little bare skin as possible can be reached by mosquitoes (especially at dusk), use anti-mosquito repellants containing DEET and do not leave (rain) water out unnecessarily; throw it away.


Infection with WNV through mosquitos

The Culex mosquito species occur across the whole of Europe, but especially in warmer Southern Europe, where infections are regularly reported. WNV propagates in these mosquito species as well as in many animal species. Birds in particular form a source of virus transmission by mosquitoes. Many other vertebrates, among them mammals including horses and humans, can be infected. Infected mammals are 'dead-end hosts', i.e. too little of the virus is produced for further transmission. 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.

Increased bird mortality cause for concern

Increased mortality among birds is reason for further research and may indicate the presence of the West Nile virus. Laboratory diagnosis is possible by serological testing, but cross-reactions with other flaviviruses can form a problem. Recent infections in birds can also be detected with a PCR test specific for West Nile virus.

West nile virus in 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. In the United States 100-200 people die every year as a result of a WNV infection.

There is no vaccine available for humans. Development has reached an advanced stage, but a vaccine is not yet available/licensed.

West Nile virus in horses

West Nile virus infection of horses in the United States resulted in more than 15,000 fatalities up to 2004. Since 2004, a vaccin for horses is available, with the result that the disease has been suppressed. Uptake of voluntary vaccination is extremely patchy and often happens only at the point when the virus is in circulation.

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 late last century into The Bronx, part of New York City in the United States. Infections were first reported in Germany in 2018 and in the Netherlands in 2020.

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.

Since the turn of the century there has thus been a slight increase in incidental infections in and on the outskirts of Europe. This trend has continued in recent years and a striking number of infections were reported in 2018. In addition, the infected area also appeared to be expanding northwards with infections in Germany near Berlin. In September 2020, WNV was found in the Netherlands for the first time.

Virus type

The virus belongs to the genus Flavirus in the family Flaviviridae. Two groups (lineages) of the virus are distinguished on the basis of their genetic relationship.