Published on 27 August 2026

West Nile virus detected in a horse in Switzerland for the first time

On 26 August 2026, the IVI reported a case of West Nile virus in a horse in Switzerland. As the horse had never been abroad, this is the first case of an infection acquired in Switzerland. The confirmation comes as no surprise given the many cases in horses and humans in several European countries. The virus is transmitted by mosquitoes.

West Nile fever in horses is a monitored animal disease and is therefore subject to mandatory reporting. There is one authorised vaccine for horses in Switzerland.

Using a multi-stage diagnosis, the IVI identified the pathogen in a horse with neurological symptoms. IgM antibodies against the West Nile virus were detected in serum samples, indicating an immune response shortly after infection. To confirm the diagnosis, more samples were taken from the same horse a few days later, in which IgG antibodies against the West Nile virus were detected. This means that the immune response had matured, clearly confirming a recent infection. As it was not possible to detect the virus directly in the blood samples, and to rule out any potential cross-reactivity of antibodies against other flaviviruses, the samples were also sent to the Friedrich Loeffler Institut in Germany, where serum neutralisation tests were used to confirm specific WNV neutralising antibodies in all serum samples from the animal. The horse has since recovered from the disease.

Following the first case of the Shamonda virus in horses on 25 August, this is already the second emerging viral disease in horses in the space of a week. All samples from horses with neurological symptoms that are examined at the IVI are systematically tested for both pathogens.

Horses and humans are dead-end hosts
The West Nile virus is transmitted by mosquitoes of the genus Culex and occurs mainly in wild birds. However, mosquitoes can also transmit it to humans and to mammals (e.g. horses) and other vertebrates. No direct transmission from horse to horse or from horse to human has been detected. Horses and humans are therefore dead-end hosts.

Symptoms and disease progression
Most infected horses show no symptoms; however, around 10% of animals develop a severe neurological disease. In humans an infection is usually asymptomatic or involves a slight fever. In rare cases, an infection may lead to severe encephalitis or meningitis.

Unsurprising development
The confirmation of West Nile fever in a horse in Switzerland comes as no surprise given the many cases in horses and humans in various European countries. This year in particular, the number of cases in some European countries has risen significantly. In 2022, the West Nile virus was detected for the first time in Switzerland in local mosquitoes in Ticino. In 2025, Ticino also recorded Switzerland's first locally transmitted human case of West Nile fever.

Following the first confirmed case in a horse, there is now growing evidence to suggest that the virus is also increasingly becoming established in parts of Switzerland north of the Alps.

One Health
As the disease can be transmitted from animals to humans, West Nile fever is considered a zoonosis. The IVI, the Federal Food Safety and Veterinary Office (FSVO) and the Federal Office of Public Health (FOPH) have been working with several research institutes in Switzerland for a number of years to strengthen the surveillance of West Nile virus in terms of One Health. The ‘Vectors’ inter-agency working group coordinates the various activities. At the IVI itself, various research projects are being conducted on West Nile fever, both on surveillance in wild birds Blog Corinne Gerber – Migratory birds and viruses and on the infection mechanisms in humans Blog Amal Fahmi – Organoids.

Diagnosis

The West Nile fever is a vector-borne, monitored animal disease (Epizootic Diseases Ordinance EzDO). Diagnostic testing at the IVI is carried out in Mittelhäusern.
Please always submit sample material for simultaneous detection of the viral genome and antibodies.

Sample material
Viral genome test (RT-qPCR)

  • Live animal: serum, cerebrospinal fluid
  • Dead animal: brain, cerebrospinal fluid

Antibody test (important: state vaccination status)

  • Serum

Epizootic disease diagnostics – Equines

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