10/11/2023: 

We have been monitoring the presence of West Nile Virus (WNV) in the Netherlands for some years now. It is a compulsory notification virus which mainly circulates between mosquitoes and birds, but can also infect humans and horses. Only mosquitoes can actually pass on the virus; mammals cannot infect each other. This virus can result in neurological clinical signs.

As of 2021, the Equine animal group is included in the basic health monitor conducted by GD as a statutory task. Climate change is facilitating the migration of diseases from elsewhere in the world to our region. The European Animal Health Regulation requires member states to monitor such emerging diseases, of which West Nile Virus is an example.

Symptoms of West Nile Virus

Both humans and horses are the ‘end host’ for the WNV, becoming infected through a bite by an infected mosquito. It is generally asymptomatic, though some cases resembling influenza have been seen, and 1 percent of human cases may lead to meningitis in humans. The elderly are particularly vulnerable. The disease progression is generally more serious in horses, resulting in around 10 percent of neurological cases. A Hungarian study has shown ataxia to be the main symptom of West Nile Virus in horses. General weakness, behavioural changes, paralysis and muscle vibrations are also relatively common. Muscle vibrations in the muzzle, shoulder and flanks are typical.

Alertness required

In Germany and a number of Southern European countries, a few dozen cases of affected horses are seen annually. In 2022, around 1,000 human WNV infections were reported in Italy, Greece, Hungary and Germany, among other countries. There were also notifications of 87 WNV outbreaks in horses (in Italy, Germany and Croatia) and 284 outbreaks in birds. Despite the reasonably extensive basic monitoring in the Netherlands, no infected horses have ever been discovered. However, there have been cases of infected mosquitoes, birds and people in 2020. An infected grey heron was detected in 2022. Alertness is required during the mosquito season, from mid-May to late November. WNV needs a sufficient number of mosquitoes, the right species of birds and sufficiently warm temperatures in order to cause an outbreak. In the event of neurological clinical signs in horses, veterinarians can submit serological samples to GD free of charge, in order to exclude WNV, thanks to a project financed by the Ministries of Agriculture and Public Health (LNV and VWS). This is not only important for the horse, but timely detection also helps prevent people in the vicinity being bitten by infected mosquitoes, as far as possible. Any infections must be reported to the Netherlands Food and Consumer Product Safety Authority (NVWA), though farms need not be closed down as the virus cannot be transferred from one horse to another.

Animal health surveillance