|West Nile Virus and Its Signs|
by Beth A. Valentine, DVM, PhD
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 populationwhen 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
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
Valentine, DVM, PhD, is an equine pathologist at Oregon State University,
Corvallis, and co-author of
Horses, an Owner's Manual.