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The Cape Fear Equestrian
West Nile Virus


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By Jan Luquire, DVM
May 2002

Mosquito season is upon us! Since its first appearance in this country in 1999, West Nile Virus(WNV) has found its way into the everyday vocabulary of the horse community and was found in six horses in North Carolina in 2001.  The following are questions I am frequently asked concerning WNV and how to protect you horses.  Prevention is the key!
Q.     What is the West Nile Virus?
         A.   West Nile Virus(WNV) is a mosquito-borne virus that causes encephalitis, or inflammation of the brain.  Previously restricted to Africa, West Asia and the Middle East, it was found in New York City at the Bronx Zoo in 1999.  Horses are affected by WNV more often than other domestic animals.
Q.     How is WNV transmitted?
         A.   WNV is transmitted to horses, humans(and other mammals) after being bitten by a mosquito that is infected by WNV.  The infections are characterized by a bird-mosquito cycle.  Birds are capable of long-term infection.  Consequently, migratory birds are considered to be instrumental in transporting the virus to new areas.  Crows, chickens, ducks, gulls and pigeons have been reported to show illness ranging from encephaltitis to death.  Other wild birds serve as natural reservoirs of the virus and do not normally show any symptoms of WNV infection.  Mosquitoes become infected after a blood meal from an infected bird.  Horses have not been shown to be capable of passing WNV to uninfected mosquitoes during the blood meal.
Q.     How do people get WNV?
         A.   WNV can only be contacted through the bite of an infected mosquito.  There is no evidence that a person can get the virus from handling infected horses, birds or other mammals.
Q.    What are the symptoms of WNV in horses?
         A.   From the time a horse is bitten by an infected mosquito until signs of clinical illness begins ranges from 7 days to 2 weeks.  Signs include listnessness, stumbling and incoordination, weakness of limbs, ataxia, partial paralysis or death.  A fever may or may not be observed.
Q.     How many horses are affected in 2001 and what age range?
         A.   In 2001 there were 640 confirmed cases of WNV in horses and 11 probable cases.  Of 470 horses for which an outcome has been reported, 33-2% died or were euthanized.  Ages of horses affected ranged from 4 months to 38 years of age.
Q.     How many cases of clinical WNV were reported in North Carolina in 2001?
         A.   Four counties reported a total of 6 horses.  Camden County(1), Hyde County(2), Pasquotank County(2), Tyrrell County(1).
Q.     Will horses affected by the virus be quarantined?
         A.   No.  Since infected horses do not appear to be carriers for the disease, it is unlikely a quarantine would be necessary.
Q.     What precautions can be taken to protect horses from this virus?
         A.   1)Reduce mosquito breeding sites.  Dispose of water-holding containers, thoroghly clean livestock watering troughs, and eliminate standing water.  Mosquitoes can breed in any puddle or water that stands more than 4 days.
               2)Decrease exposure to adult mosquitoes.  Insect repellents may help but have limited duration of effectiveness under certain conditions, such as prespiration.  A recently completed study suggests that keeping horses in stalls at night may be helpful in reducing the risk of infection. 
Q.     Is the WNV vaccination available for horses?
         A.   Yes.  On August 1,2001, a conditional license was issued by the USDA-APHIS Center for Veterinary Biologics for an equine WNV vaccine, "conditional" licensing means that the product has been shown to be safe, pure, and have a reasonable expectaion of efficiency in preventing illness cause by WNV.  The manuafacturer of the vaccine recommends giving two intramuscularly doses, 3 to 6 weeks apart, for the initial vaccination series.  Because use of this vaccine is restricted to veterinarians, you need to contact your veterinarian to schedule avaccinations to protect your horse this season.