West Nile virus: how to spot and report the disease

West Nile Virus (WNV) is a flavivirus related to the viruses that cause St. Louis encephalitis, Japanese encephalitis, and yellow fever.  

The disease caused by WNV is known as West Nile Fever. 

West Nile Virus can infect birds, reptiles, amphibians, mammals, mosquitoes and ticks. It causes disease in horses, several species of birds and, more rarely, in humans.  

Humans can be affected by West Nile virus, although often the infection does not lead to any symptoms. If present, the most common symptoms are a flu-like illness. 

Horse owners and equine vets should stay alert and report any concerns to the Animal and Plant Health Agency (APHA). It is an offence to not report a suspected notifiable disease. 

Equine vets can use the GB testing if they want to rule out the disease.

Clinical signs

In horses, WNV can affect the central nervous system causing encephalitis. The incubation period of the virus in horses is around 5 to 15 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.

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 lifelong 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 WNV. It is licensed by the European Medicines Agency. 

Vaccination is strongly recommended for horses travelling abroad to areas where WNV is present. Where possible, avoid taking horses to higher-risk areas. 

Horse owners should talk to their vet about vaccination as part of their horse's health plan.

Human health implications

Most 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.

Legislation

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).

Prevention

Prevention is better than cure. Reducing the chances of disease entering the animal population - and acting quickly if it does - is central to the government's approach to disease control. 

Horse owners and vets should stay alert, know the signs of WNV, and report any concerns to APHA straight away. 

Research and surveillance projects monitor for signs of disease in wild birds and mosquitoes in the UK.