West Nile Virus and Its Signs
by Beth A. Valentine, DVM, PhD

How it spreadsLike the more familiar Eastern Equine Encephalitis (EEE) and Western Equine Encephalitis (WEE), WNV is carried by mosquitoes. The natural cycle of infection is for mosquitoes to infect birds, which in turn infect more mosquitoes, resulting in a cycle of infection between mosquitoes and birds. Only occasionally does a mammal become infected from a mosquito bite, and then only humans and horses seem to be susceptible. People and horses are considered "dead­end hosts," meaning they cannot transmit the virus to mosquitoes or to other people or horses. So, in this sense, WNV in horses and people is not contagious, not even to mosquitoes. The risk comes only from the bite of a mosquito that has picked up the virus by biting an infected bird.

This virus is spreading more rapidly and farther than any other arbovirus known in North America for several reasons. One is that ours was a completely naive population—when the virus arrived, no person, animal, or bird had ever been exposed to it, and none had ever been vaccinated for it. That means we had no natural immunity to WNV.

Another reason is that WNV may be carried by several different types of mosquito, so it can readily move into different geographic areas. The kinds of mosquitoes that carry WNV exist in all 48 contiguous states. Other reasons likely involve characteristics of the virus itself and of the migratory birds that are infected, but I'm no virologist so I won't go into those factors.

Infection with WNV causes inflammation of the brain (encephalitis) and spinal cord (myelitis). Compared to individuals that develop clinical disease following exposure, many more animals and humans that are exposed to the virus do not become infected. Estimates of the percentage of exposed horses that develop disease range from 3% to 14%. In horses, the fatality rate of horses that develop disease is about 30% to 40%.

Birds infected with WNV are generally just found dead. Crows, magpies, and blue jays are particularly susceptible, although hawks and other birds also get infected. People with WNV report fever, blurred vision, nervous tics, and slurred speech. Severe cases can result in coma and death. Most people with this virus recover completely. Only about 10% of infected people die, and the elderly are the most susceptible.

Clinically affected horses, on the other hand, have ranged from young to old. In horses the signs of WNV are virtually identical to those of EEE, WEE), Venezuelan Equine Encephalitis (VEE, rabies, equine herpesvirus myelitis, and equine protozoal myeloencephalitis (EPM). Fever is one of the clinical signs, and may precede other signs by several days. Signs of brain and spinal cord inflammation include altered state of mind from listless to manic, seizures, paralysis of limbs, incoordination, tremors, difficulty walking, and inability to rise. A peculiar twitching of the muzzle has been reported in many infected horses.

The clinical signs of WNV in horses can be virtually identical to EEE, WEE, rabies, equine herpesvirus myelitis, and equine protozoal myeloencephalitis (EPM). Only a blood test can accurately determine if a horse is infected by WNV.

No specific treatment is available for WNV, but supportive care and medications to reduce inflammation are utilized while the infection runs its course. Death due to WNV occurs in horses within five to 10 days of the onset of signs. Horses that recover often show dramatic improvement within three weeks and have little or no residual problems.

West Nile Virus Prevention


Beth A. Valentine, DVM, PhD, is an equine pathologist at Oregon State University, Corvallis, and co-author of Draft Horses, an Owner's Manual.

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26 October 2011 last revision