West Nile Virus and Horses
Q. Has West Nile Virus caused severe illness or death in horses?
A. Yes. While data suggest that most horses infected with West Nile Virus
recover, results of investigations indicate that West Nile Virus has caused
deaths in horses in the United States.
Q. How do the horses become infected with West Nile Virus?
A. The same way humans become infected—by the bite of infectious mosquitoes. The
virus is located in the mosquito's salivary glands. When mosquitoes bite or
"feed" on the horse, the virus is injected into its blood system. The virus then
multiplies and may cause illness. The mosquitoes become infected when they feed
on infected birds or other animals.
Q. How does the virus cause severe illness or death in horses?
A. Following transmission by an infected mosquito, West Nile Virus multiplies in
the horse's blood system, crosses the blood brain barrier, and infects the
brain. The virus interferes with normal central nervous system functioning and
causes inflammation of the brain.
Q. Can I get infected with West Nile Virus by caring for an infected
A. West Nile Virus is transmitted by infectious mosquitoes. There is no
documented evidence of person-to-person or animal-to-person transmission of West
Nile Virus. Normal veterinary infection-control precautions should be followed
when caring for a horse suspected to have this or any viral infection.
Q. Can a horse infected with West Nile Virus infect horses in neighboring
A. No. There is no documented evidence that West Nile Virus is transmitted
between horses. However, horses with suspected West Nile Virus should be
isolated from mosquito bites, if at all possible.
Q. My horse is vaccinated against eastern equine encephalitis (EEE),
western equine encephalitis (WEE), and Venezuelan equine encephalitis (VEE).
Will these vaccines protect my horse against West Nile Virus infection?
A. No. EEE, WEE, and VEE belong to another family of viruses for which there is
Q. Can I vaccinate my horse against West Nile Virus infection?
A. A West Nile Virus vaccine for horses was recently approved, but its
effectiveness is unknown.
Q. How long will a horse infected with West Nile Virus be infectious?
A. We do not know if an infected horse can be infectious (i.e., cause mosquitoes
feeding on it to become infected). However, previously published data suggest
that the virus is detectable in the blood for only a few days.
Q. What is the treatment for a horse infected with West Nile Virus? Should
it be destroyed?
A. There is no reason to destroy a horse just because it has been infected with
West Nile Virus. Data suggest that most horses recover from the infection.
Treatment would be supportive and consistent with standard veterinary practices
for animals infected with a viral agent.
The materials, information and answers provided through this website are not intended to replace the medical advice or services of a qualified veterinarian or other pet health care professional. Consult your own veterinarian for answers to specific medical questions, including diagnosis, treatment, therapy or medical attention.
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