You are here:

West Nile Virus

What is West Nile Virus

West Nile Virus (WNV) is a mosquito-borne virus distributed extensively around the world that can affect birds, humans and horses causing in some cases mild febrile illness, encephalitis, meningitis, or death. Outbreaks of WNV have occurred in Egypt, Asia, Israel, Africa and some parts of Europe and Australia. The virus is not present in the United Kingdom.

WNV in horses

The majority of infected horses do not display clinical signs and WNV encephalitis occurs in only a small proportion of infected horses. The incubation period for equine WNV encephalitis following mosquito transmission ranges from three to 15 days. Horses that do become ill may show mild to severe disease.

Typical signs include ataxia (incoordination, stumbling, limb weakness) that might appear suddenly or develop gradually and worsen. Other disease signs include:

  • sleepiness
  • dullness
  • listlessness
  • facial paralysis (droopy eyelids, lower lip), and
  • an inability to rise.

Some horses may develop a mild fever, blindness, muscle trembling and seizures. Treatment is supportive and signs may resolve or progress to terminal recumbency. The mortality rate in some outbreaks is approximately one in three clinically affected unvaccinated horses.

Horses vaccinated for WNV and foals of positive-testing mares are likely to give a positive blood test for the virus and veterinarians should consider the possibility of other neurological diseases in such circumstances. Differential diagnoses in horses include other arboviral encephalitis (e.g. Eastern, Western or Venezuelan equine encephalomyelitis, Japanese encephalitis), equine protozoal myeloencephalitis (Sarcocystis neurona), equine herpervirus-1, borna disease and rabies.

Testing services

Our laboratories undertake statutory diagnostic testing for WNV, however we also provide commercial WNV ELISA testing for the following diagnostic purposes:

  • Equine import/export
  • Test to exclude WNV infection in equines
  • Sera prevalence studies.

We offer the following diagnostic tests for WNV:

West Nile Virus (WNV) total Ab detection cELISA for use with equine serum TC0869.

  • This UKAS accredited test is designed to detect horse and avian antibodies against the WNV envelope protein by competitive ELISA and will provide evidence of current and past infections.
  • Maximum turnaround time is five working days upon reception of sample.
  • The monoclonal antibody used in this ELISA cross-reacts with viruses related to WNV including Japanese encephalitis virus and the tick-borne encephalitis virus.

West Nile Virus (WNV) IgM Ab detection ELISA for use with equine serum TC0868.

  • This UKAS accredited ELISA allows the detection of specific West Nile IgM antibodies in horses and it is very useful for the diagnosis of the disease as it reveals a recent infection of the host. Equine WNV-specific IgM antibodies are usually detectable from seven-ten days up to three months after infection.
  • Maximum turnaround time is five working days upon reception of sample.

Further Information

You can submit clotted blood samples or serum for WNV testing using the West Nile Virus Serology Submission form (WNV02).

For further advice on the West Nile Virus sample submission please see ourĀ guidance documents.

For more information on our lab tests please view our APHA Scientific Test Price list.

The WNV testing group can be contacted on

View our West Nile Virus page for further information.

WNV is a notifiable disease in horses. Any suspicion must be reported immediately by calling the Defra Rural Services Helpline on 03000 200 301.

Page last modified: 5 August, 2020