West Nile Encephalitis In Horses
by Gary Heusner, Extension Animal Scientist, University of Georgia
Anxiety levels have definitely increased with the confirmation of birds testing positive for West Nile virus in several counties in Georgia. West Nile virus (WNV) grows in birds and is transmitted from bird to bird and from birds to humans and horses (see diagram) by mosquitoes. Infected birds serve as the reservoir host of the West Nile virus. These birds develop a high level viremia - a large number of virus particles in their circulatory system. Mosquitoes become infected after taking a blood meal from an infected bird. They in turn infect horses by biting them. The incubation period for the first signs of the disease is between 5 and 15 days. Horses are considered "dead-end" or "terminal carriers". "Terminal carriers" have so few virus particles in their bloodstream that a mosquito cannot accumulate enough of the virus while taking a blood meal to subsequently transmit the infection. No evidence suggests that horses can transmit West Nile virus to other horses, birds or humans.
The clinical signs of West Nile virus disease in the horse vary. The virus can affect the central nervous system and cause encephalitis. Some horses exposed to the virus may not show any clinical signs or perhaps may exhibit only vague signs of the disease. Clinical signs may include one or more of the following: loss of appetite and depression, fever, weakness of hind limbs, paralysis of hind limbs, impaired vision, ataxia, head pressing, head tilt, aimless wandering, convulsion, inability to swallow, circling, hyper excitability, coma or acute death. Other diseases such as rabies, botulism, equine protozoal myeloencephalitis (EPM), Eastern (EEE), Western (WEE) and Venezuelan (VEE) encephalitis, will exhibit clinical symptoms similar to West Nile encephalitis. The blood of the infected horse must be tested.
Currently there is no vaccine available for the prevention of West Nile encephalitis in horses. Horses vaccinated against Eastern, Western and/or Venezuelan encephalitis will not be protected against West Nile encephalitis.
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