ARCHIVE: West Nile Virus (WNV)

If you suspect signs of any notifiable disease, you must immediately notify your local Animal Health office.


West Nile Virus (WNV) is a viral infection of birds, horses and humans, spread by the bite of infected mosquitos that can cause encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord).

Recent research by Natural Environment Research Council (NERC) found antibodies against the virus present in birds in Great Britain, suggesting past or present infection with WNV. WNV infection has never been identified in horses or humans in Great Britain. WNV is a flavivirus, one of a member of a group of Arthropod-Borne viruses (Arboviruses).

Spread of disease

WNV can be transmitted to humans and animals via the bite of an infected mosquito. A mosquito becomes infected by biting wild birds that carry the virus. The infection is a zoonosis, i.e. a disease the causative agent of which can be transmitted between animals and humans. In the case of WNV, the virus is transmitted between birds and man, though a wide range of other animal species can also become infected. The mosquito vectors primarily involved are Culex species which are known to occur in some parts of the UK. Ticks are also infected in Asia and Africa but their role in transmission is unknown.

Migrating birds are the most likely mechanism of the infection being introduced into the UK. Humans, horses and other animal species are believed to be dead-end hosts, i.e. there is no spread from them to other people or animals. It is possible that a recently infected horse or person, incubating the infection, could have the virus in their blood stream for a short time and during that time be bitten by a mosquito, but the risk of this is thought to be remote.

The virus historically occurs in Africa, Europe, the Middle East, West and Central Asia, and recent outbreaks have occurred in Morocco (1996), Romania (1996), Italy (1998), Russia (1999) and the South of France (2000). However, it appeared for the first time in the USA in 1999.

Since WNV was first recorded in the USA in 1999, it has spread throughout much of the country where it is now considered to be endemic.

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 (EMEA).

Animals it Affects

The horse seems the most susceptible to infection but most cases are sub-clinical with the horse showing no obvious signs of disease but becoming seropositive (i.e. positive to the blood test for antibodies to the virus). The ratio of clinical to sub-clinical cases is estimated at 1:2.5 to 1:5. The incubation period is 5-15 days and mortality rate in the USA is about 35% of the animals which show signs of disease. Affected animals develop a fever and often encephalitis with nervous signs. Whilst birds are the main carrier and most remain apparently unaffected, some species are susceptible to disease - especially corvids (crow family). Mass die-offs can occur in these species. Other animals that can be infected are cats, bats, chipmunks, skunks, squirrels, rabbits and dogs (rarely).

Poultry can be infected and have been used in the USA as "sentinels" to detect infection in areas thought to be at risk. They do not usually develop disease. WNV is primarily an infection of birds and although a range of other animal species, such as goats and sheep can be infected, these species only develop low levels of virus. To date there have been no reports that cattle have been affected by the virus. The main route of transmission of WNV is through mosquitoes and the risk of humans acquiring WNV through consumption of food is extremely low. The virus is destroyed by standard cooking methods and pasteurisation and there have been no reports of the virus infecting people following consumption of meat and milk from infected animals. However, a related virus, tick borne encephalitis has caused infection following ingestion in unpasteurised goats milk.

Human health

In 2004, the Department of Health published a comprehensive contingency plan (PDF 963 KB) for West Nile virus to protect the public's health.

Many infected people show no symptoms. When disease does occur, it is usually a flu-like illness with fever. A small proportion of cases (less than 1%) develop meningo-encephalitis which produces nervous signs and may be fatal. In the USA in 2002, 4,161 people were reported as infected with the disease, of whom 277 died.

There is currently no evidence that WNV can be spread directly from birds to people. However, dead birds can carry a variety of diseases and, therefore, should never be handled with bare hands. Always use gloves and carefully place dead birds in double-plastic bags.

Infected mosquitoes are thought to be the primary source of the disease. Ticks have been found to be infected in Asia and Africa but their role in spreading the infection is unknown.

There is currently no licensed vaccine to protect humans against WNV, although there are experimental vaccines in development.

Disease prevention

In the absence of a vector, there is no evidence of direct person to person, person to animal, or animal to animal transmission, under natural conditions.

  • Vector control - eliminate breeding sites of mosquitoes (stagnant water, rainbutts etc) with possible use of insecticides.
  • Vector avoidance - for animals keep them away from vector sites, apply insect repellent, house in insect proofed accommodation when mosquitoes are active.

The Department of Health (DH) has commissioned research to evaluate the population size, species, biting behaviour and temporal and spatial characteristics of mosquitoes in GB. Tests for the presence of WNV in mosquitoes will also be carried out by the Health Protection Agency (HPA) Porton Down.

DH are identifying multi-level control measures to be used should disease caused by WNV or infection in mosquitoes be confirmed in GB. These include source control (reducing breeding habitats), advice to the public on how to avoid mosquito bites and consideration of appropriate pesticide control measures. The Health and Safety Executive (HSE) have undertaken an evaluation of the approved pesticides for use in the control of mosquitoes.

In the UK suspect cases of equine encephalitis have to be reported under the Infectious Diseases of Horses Order. Each case would be dealt with on an individual basis but a draft contingency plan has been prepared, and is being developed further, to put the arrangements on a firmer basis. Defra is liaising with the DH on this contingency plan.


The Veterinary Laboratory Agency (VLA) has been carrying out a survey on deaths in wild birds on behalf of Defra since 2001. A large number of birds have been examined using specific tests for the presence of the virus with negative results. In GB there has been no evidence of the mass die-offs of wild birds - particularly corvids which have been a feature of the outbreaks in the USA.

Defra has been working closely with the Department of Health in developing a contingency management and control strategy for WNV. This strategy includes public health control measures to be taken in the event of a case, the provision of advice for healthcare professionals and the public, and the validation of laboratory methods for the identification of the disease.

Ongoing surveillance of dead birds, across a range of species including Corvidae, is managed by Defra and includes attempted virus isolation on avian cadavers by VLA. England's Chief Medical Officer (CMO) recently published his 2002 Annual Report in which he recommended enhanced and strengthened surveillance of the bird and horse populations in GB for the presence of WNV. Defra is considering further surveillance in light of the recommendations.

The situation in GB is that there would appear to be an apparent infection in birds but no evidence of clinical disease in birds, horses or humans. The virus has still not been isolated from birds but antigen has been detected and serum surveys confirm seroconversion. There is still a lot to learn about the behaviour of the virus in view of the different disease pictures seen in different parts of the world. In USA and Romania, the disease is found in horses and birds; whereas in France the disease affected horses but not humans or birds. Defra is liaising with the DH to understand the epidemiology of the infection and to develop a control strategy for WNV.

See also



Page last modified: October 26, 2009
Reviewed: 11 August 2006