How does West Nile virus spread?

Birds are important for maintaining the infection cycle of West Nile virus. A mosquito can become infected when it feeds on the blood of a bird that has the virus in its bloodstream. During a subsequent blood meal, the infected mosquito can then pass the virus to another bird. In this way, the virus can continue to spread among bird populations.

The same infected mosquito can also bite a horse, person or other mammal. Horses and people are regarded as so-called dead-end hosts. The amount of virus in their blood is too low to infect another mosquito sufficiently and thereby continue the transmission cycle.

A horse with West Nile fever therefore poses no risk of infection to other horses or people. You cannot contract the virus by stroking or caring for an infected horse. A mosquito also cannot pick up the virus from an infected horse and subsequently pass it to a person.

West Nile virus in birds

Birds are the natural reservoir of West Nile virus. Many infected birds show no signs of illness, but some species are more susceptible. Illness and death are seen in songbirds, corvids and birds of prey, among others.

Because the virus is spread between birds by mosquitoes, infections in birds can also be an important indication that the virus is circulating locally. Both living and dead birds, as well as mosquitoes, are therefore used in West Nile virus monitoring.

West Nile virus in horses

Horses can become infected when they are bitten by a mosquito carrying the virus. Fortunately, most infected horses, approximately 80%, show no signs of illness. Around 20% of horses develop mild signs. Only a small proportion become seriously ill.

However, the situation can become serious when the virus affects the nervous system. An infection involving neurological signs is also known as West Nile encephalomyelitis.

What are the symptoms of West Nile fever in horses?

The initial signs can be quite general. A horse may develop a fever, eat less and become sluggish or listless. This makes West Nile fever difficult to recognise based only on the initial signs.

More serious infections can cause neurological signs. These include an uncoordinated gait, muscle tremors, weakness and paralysis. Tremors around the lips or muzzle are described relatively frequently. Horses may also become hypersensitive to touch.

Neurological signs can also be caused by other conditions. If you suspect West Nile fever in your horse, contact a vet immediately. Additional laboratory testing is required to support the diagnosis.

How is West Nile fever treated in horses?

There is currently no specific antiviral treatment that can eliminate West Nile virus in a horse. Treatment is therefore supportive and depends on the severity of the signs.

The vet may, for example, provide fluids and nutrition when a horse is not eating or drinking enough and use medication to reduce pain and inflammatory reactions. Good nursing care is particularly important for horses with neurological involvement. Horses that are unsteady or can no longer stand independently may injure themselves and require intensive care.

What is the prognosis?

The prognosis depends greatly on the severity of the signs. Most infected horses do not become ill at all or develop only mild illness and recover fully. The prognosis becomes more guarded when serious neurological signs develop. Horses that can no longer stand independently or develop severe paralysis have a particularly poor outlook.

Horses that survive a neurological infection can recover fully, but this recovery may take weeks to months. Some horses retain residual neurological signs, such as weakness, coordination problems or reduced stamina.

Can you protect your horse against West Nile virus?

Because mosquitoes are responsible for transmission, prevention begins with reducing mosquito bites. Mosquitoes are particularly active around dawn and dusk. Keeping a horse indoors at these times can reduce exposure. Suitable mosquito repellents and an appropriate fly rug, for example, can also help.

It is also advisable to limit mosquito breeding sites around the stable and field as much as possible. Do not allow water to remain in buckets, containers or other objects for long periods, keep gutters clean and change the water in drinking troughs regularly. Mosquitoes develop mainly in stagnant water.

Vaccines against West Nile virus are also available for horses. Whether vaccination is advisable for your horse depends on factors including the current risk of infection, the location and, for example, travel to areas where the virus is circulating. Discuss this with your vet.

West Nile virus in people

People can also develop West Nile fever after being bitten by an infected mosquito. Approximately 80% of infected people have no symptoms at all. Around 20% develop flu-like symptoms, such as fever, headache and muscle pain. Abdominal complaints, a skin rash and enlarged lymph nodes may also occur.

Fewer than 1% of infected people develop serious neurological complications. The virus may, for example, cause inflammation of the brain or the membranes surrounding the brain.

There is currently no vaccine against West Nile virus for people. Protection therefore focuses mainly on preventing mosquito bites, for example by wearing clothing that covers the skin, using insect screens and applying a suitable mosquito repellent to exposed skin.

It is important to emphasise once again that people do not contract West Nile virus from an infected horse. Horses and people generally become infected independently of each other through infected mosquitoes.

Does West Nile virus also occur in wild animals?

In addition to birds, horses and people, other mammals can also be infected by an infected mosquito. Research is therefore also carried out into the presence of West Nile virus in wild animals. Research into wild birds is particularly important because birds form part of the natural transmission cycle. Unexplained deaths among wild birds may be an indication that requires further investigation.

Is West Nile virus notifiable?

West Nile virus is closely monitored because it can affect the health of both animals and people. The precise reporting requirements and the authority to which a suspected case or infection must be reported vary by country. In the Netherlands, for example, a suspected case in a kept animal must be reported. This requirement applies to animal keepers as well as vets and laboratories. Human infections are also notifiable there.

If you suspect West Nile fever in a horse or another kept animal, contact a vet immediately and follow the reporting rules of the country in which the animal is located.

West Nile virus: stay alert without unnecessary concern

West Nile virus is spread by mosquitoes, and birds play a central role in maintaining the infection cycle. Horses and people can be infected by the same infected mosquitoes, but cannot transmit the virus to each other. Most infections cause no signs of illness, but neurological signs in horses in particular require prompt veterinary assessment.

By limiting mosquito bites and breeding sites as much as possible and discussing with your vet whether vaccination is appropriate for your horse, you can reduce the risk. As the spread of West Nile virus in Europe is changing, it also remains important to follow current information from the health and veterinary authorities in your own country.