West Nile Virus In Horses
   Diagnosis and Prevention Tips



What is West Nile Virus?
West Nile Virus (WNV) is a mosquito-borne virus that causes encephalitis (inflammation of the brain) and/or meningitis (inflammation of the lining of the brain and spinal cord). Outbreaks of WNV have occurred in Egypt, Asia, Israel, Africa, and some parts of Europe and Australia. The virus was first found in the U.S. in the fall of 1999 in New York City and has since spread nationwide.

Birds: the Primary Reservoir Species for WNV
WNV infects and multiplies in birds, which then serve as the reservoir species for the virus. The impact of the disease in birds varies, with American Crows frequently dying from the infection. Many other bird species survive infection with mild or no indication of disease. WNV is spread from bird to bird by mosquitoes when they bite, or take a blood meal, from birds that are infected with the virus. Mosquitoes are also capable of spreading the virus to horses, humans, and other mammals. Birds have a significant impact on the spread of the virus across the U.S. Birds that are infected with WNV may show signs such as the inability to fly, incoordination, abnormal movement, and death.

WNV in Horses
Most horses bitten by carrier mosquitoes do not develop disease. Of those that do, approximately one-third develop severe disease and die or are so affected that euthanasia is required. The incubation period - or the time between the bite of an infected mosquito and when clinical signs appear ­ranges from three to 14 days. Although most horses do not usually develop clinical signs of WNV infection, horses that do become ill vary in symptoms from mild signs to serious and near death. Typical signs include muscle trembling; skin twitching; ataxia (incoordination, stumbling, limb weakness) that either appears suddenly or appears gradually and worsens; sleepiness; dullness; listlessness; facial paralysis (droopy eyelids, lower lip); difficulty with urination and defecation; and an inability to rise. Some horses may develop mild fevers, blindness, seizures, and other signs. WNV may cross the placenta from mother to gestating foal. Horses can not spread the disease to humans, but humans are susceptible to the disease if bitten by a carrier mosquito. No transfusion related horse illnesses have been reported. However, human to human transmission via blood transfusions has been confirmed, so this method of transmission is possible in horses.

WNV in Other Mammals
Animals other than horses or humans may be susceptible to WNV reaction. Antibody has been found in blood samples from bats, dogs, cats, chipmunks, gray squirrels, domestic rabbits, eastern striped skunks, crocodiles, cows, sheep, reindeer, alpacas, wolves, dogs and pigs. Although most of these animals do not become ill, unlike horses and humans, those with immune system problems or other illnesses may show signs or develop encephalitis. There is no evidence that infected horses, humans, or other animals are able to transmit the virus to other animals, people, or mosquitoes. Only a wild bird/mosquito transmission cycle has been proven as a means of transmitting WNV.

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