category_news
First suspicion of locally acquired case of West Nile virus infection in a horse in the Netherlands
A six-year-old horse in the province of Zuid-Holland (Netherlands) has tested positive for antibodies against West Nile virus (WNV). The analyses was done on a non-official sample, therefore this case is qualified as a suspicion. The infection was detected through Dutch surveillance and confirmed by Royal GD and Wageningen Bioveterinary Research. As the case could not be confirmed by means of an official sample yet, further research for final determination is still ongoing.
The horse infected with West Nile virus (WNV) was being treated by a local veterinary practice. Due to neurological symptoms (muscle tremors, reduced awareness and weakness in the limbs), the veterinarian collected a blood sample on 18 September for testing at Royal GD (GD). The sample tested positive for WNV antibodies. The same sample was sent to Wageningen Bioveterinary Research (WBVR, part of Wageningen University & Research) for confirmation. WBVR confirmed the result and also detected IgM antibodies, indicating a recent infection. The European Reference Laboratory for WNV in horses has since confirmed the infection using a virus neutralisation test.
Further investigation
The horse in question was imported from Ireland in June 2024 and had not been vaccinated against WNV. Since November 2024, it has been kept at the location, which means the infection was acquired locally.
Additional diagnostic testing of the infected horse and other animals at the same location has been initiated in consultation with the owner. No virus was detected in the tested horses by PCR, which was not surprising since horses are only briefly and weakly viremic. However, several animals at the same site showed antibodies against WNV, indicating that they have either been infected in the past or vaccinated. There is currently no evidence of other recent WNV infections. All collected samples have been sent to the European Reference Laboratory for WNV for confirmatory testing to rule out cross-reactivity with other flaviviruses, the group of viruses to which WNV belongs.
Spread of West Nile virus
WNV is spread by migratory birds, in which the virus can replicate. It is transmitted from birds to horses and humans via mosquitoes. Horses and humans are so-called dead-end hosts: the virus cannot spread further via horses or humans. Therefore, horses and humans cannot infect each other with WNV. At a distance of about 25 kilometres from the site were the infected horse is situated, the virus was recently detected in mosquitoes; see: Actualiteiten over westnijlkoorts | RIVM (in Dutch only).
Symptoms and consequences for horses
In most cases, WNV infection in horses is subclinical. When symptoms do occur, they are often non-specific, such as fever, anorexia and lethargy. In around 10 percent of cases, neurological signs develop, including ataxia, muscle tremors of the head, shoulders and flanks, and paralysis. In some of these cases, the symptoms are so severe that euthanasia is required. Horses can be protected against WNV through annual vaccination.
Zoonosis and notifiable disease
WNV is a notifiable zoonosis, but not subject to control measures. Confirmation of infection in a horse has no direct consequences for the animal or the premises, but it serves as a warning signal to human and veterinary health authorities regarding the presence of WNV in certain regions. The detection of the virus may lead to additional export requirements to third countries.
WNV in Europe
WNV has been present for some time in southeastern Europe and the Mediterranean region. In recent years, the virus has spread to central Europe, including Germany. Further spread to northwestern Europe was therefore expected. In 2023 and 2024, several WNV-positive horses were identified in Germany near the Dutch border. Recently, birds in Belgium tested positive for WNV for the first time.
Monitoring in Nederland
Since November 2018, a multidisciplinary working group has been active in developing recommendations for surveillance and preparedness for WNV infections. This group coordinates active WNV monitoring in the Netherlands, including in horses. The monitoring is funded by the Ministry. Its aim is to detect WNV introduction and circulation in the Netherlands as early as possible through syndrome surveillance in horses with neurological symptoms. During the vector season (May to November), veterinarians can submit serum samples from horses showing neurological signs free of charge to GD for exclusion diagnostics.