Fact SheetJanuary 2006No. 9Update 2West Nile Virus
West Nile Virus Fact Sheet
West Nile Virus (WNV) is a mosquito-borne virus,
which can cause a wide range of clinical illness from
mild “flu-like” symptoms to encephalitis (inflammation
of the brain. The virus may be fatal to both humans
and horses. While horses are susceptible to WNV,
many horses infected with the disease will not develop
any illness and will recover uneventfully. Currently,
there is no specific treatment for WNV.
Virus Life Cycle
WNV was first isolated from a woman in the West
Nile District of Uganda in 1937. In the 1950’s an
outbreak was recorded in Egypt, and another outbreak
in Israel in 1957. As the disease gained more
recognition, new cases were reported in France and
Egypt in the 1960’s, and in South Africa in 1974.
Romania reported an outbreak in 1996, and Russia
recorded a few cases in 1999.
The virus was first isolated in the United States from a
dead crow on September 14, 1999, in New York city.
The virus was later identified as WNV and was
confirmed as the cause of a human encephalitis case
that occurred in New York City in August in 1999. Since
then, WNV has been detected in humans, animals,
and mosquitoes from coast to coast.
There are currently two approved WNV vaccine
products available for horses. An initial series of at
least two vaccinations, followed by periodic ‘booster’
injections is required with each. Horses are not fully
protected until they have had two initial injections and
are up to date with booster vaccinations. The number
and timing of booster vaccinations may vary depending
on seasonal climate and geography.
Horse owners are urged to consult their veterinary
practitioner to ensure the vaccination status of all their
horses is current.
Consult your veterinarian if your horse
exhibits any of the following signs:
Incoordination, especially in rear limbs,
causing stumbling and falling
Drooping lips and lip smacking