West Nile virus: how to spot and report the disease

Advice on what to do if you suspect there is an outbreak of this infectious disease.

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West Nile Virus (WNV) is an infection of birds, horses and humans that can cause encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal chord).

The disease caused by WNV is known as West Nile fever or West Nile encephalitis.

Although the reservoir of WNV is birds, the virus can be transmitted to humans and horses via mosquitoes.


Latest situation: the infection has never been identified in horses or humans in Scotland.

Recent research has found antibodies against the virus in birds in Great Britain, suggesting past or present infection with WNV.

Historically the virus occurred in Africa, Europe, the Middle East, West and Central Asia. Although it appeared for the first time in the USA as recently as 1999, it has since spread throughout much of the country and is now considered endemic.

Clinical signs

In horses, WNV can affect the central nervous system causing encephalitis. The incubation period of the virus in horses is around five to fifteen days, some may show no clinical signs of disease but it can be serious and sometimes fatal.

Affected horses may develop:

  • fever
  • weakness
  • muscle tremors
  • ataxia
  • head shaking
  • flaccid paralysis of the lower lip
  • hind limb paralysis
  • recumbence

In wild birds the impact will range from no clinical signs to severe disease and death.

Infection in domestic poultry and other livestock does not usually cause illness.

If you suspect signs of any notifiable diseases, you must immediately notify your Scotland: field service local office at the Animal and Plant Health Agency (APHA). Failure to do so is an offence. 

How West Nile virus is spread

The virus is transmitted through the bite of an infected mosquito after a blood meal from an infected bird. If they survive, birds carry an infectious form of WNV for one to four days, after which they develop life long immunity. Many bird species act as 'maintenance' hosts and provide sources of infection.

Although birds are the main carrier and most remain apparently unaffected, some species are more susceptible to disease, particularly crows, and mass die-offs can occur in those species.

A range of mammals can be affected, but they usually do not have the virus in their blood long enough or in sufficient concentrations to pass the virus on. These species are considered to be 'dead-end hosts' and include horses and humans.

Vaccination of horses

A vaccine is now available in the UK to protect horses against the emerging disease threat of WNV. It has been licensed via the European Medicines Agency.

Human health implications

The majority of people who become infected do not suffer from any illness. Around 20% of infected people develop a 'flu like' disease; a small number (less than 1% of the total) suffer serious disease with potentially fatal meningitis.

How to control the disease

If the disease is confirmed, the outbreak will be controlled in line with the contingency plan for exotic notifiable diseases.


In Scotland, the 1987 Infectious Diseases of Horses Order requires suspected cases of equine encephalitis to be reported to the regional office of the Animal and Plant Health Agency (APHA).


The Government's policy on disease control is that prevention is better than cure. This approach works by reducing the chances of a disease entering the animal population, and if it does then it can be quickly spotted and dealt with through the preventative measures.

Biosecurity is about being aware of the ways disease can spread, and taking every practical measure to minimise the risk of disease spreading.


If you suspect signs of any notifiable diseases, you must immediately notify your Scotland: field service local office at the Animal and Plant Health Agency (APHA). Failure to do so is an offence. 

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