West Nile Virus
Introduction | Transmission Cycle | The Disease | Vaccines | WNV Links
Introduction
West Nile virus (WNV) is a flavivirus that is maintained in nature by a cycle involving mosquitoes and birds. However, it may also be transmitted to other vertebrates, including humans and horses by the bite of infected mosquitoes. There is no documented evidence that WNV is transmitted from horse-to-horse.
The virus occurs in Africa, Europe, the Middle East, West and Central Asia and appeared in the USA for the first time in 1999.
In the UK, serological surveillance shows antibodies present against WNV virus in birds, although actual virus has not been isolated. No evidence of clinical disease has been seen in birds, horses or humans in the UK.
Transmission Cycle
The Disease
WNV infection in horses is not usually accompanied by clinical illness.
However, in horses that do become clinically ill, the virus infects the central nervous system causing encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord). This has been observed to occur in about 10% of infected horses. The incubation period for WNV infection is usually 5-15 days.
Clinical signs reflect the neurological nature of the disease. These include:
- Fever
- Ataxia (Incoordination)
- Weakness or paralysis of the limbs
- Recumbency
- Impaired vision
- Inability to swallow
- Involuntary twitching of the skin
- Muscle tremours
- Muscle rigidity
- Facial nerve paralysis
Mortality rates among clinically affected horses range from 38-57%. Currently, there is no specific treatment for WNV in horses. Supportive veterinary care is recommended.
Vaccines
WNV vaccines for the horse are available in the USA, but are not licensed for use in the UK.
WNV Links
http://www.defra.gov.uk/animalh/diseases/notifiable/westnilevirus/index.htm
http://www.hpa.org.uk/infections/topics_az/west_nile/default.htm
