WEST NILE VIRUS INFECTION
West Nile virus (WNV) infection is more widespread throughout North America than any of the other equine encephalitides. WNV was originally identified in Europe, Africa (Uganda), the Middle East, and western Asia. In 1999 the first case of WNV infection was reported in the United States. How it got to North America is unknown.
WNV is the arbovirus (arthropod-borne virus) that causes WNV infections. Other arboviruses include the eastern equine encephalitis virus, western equine encephalitis virus, La Crosse encephalitis virus, and St. Louis encephalitis virus.
Migratory birds play an important role in the spread of the virus. Common sparrows are resistant to the effects of WNV and are therefore considered natural reservoirs for the virus. Crows and blue jays are severely affected by the virus, and the presence of dead crows or blue jays may indicate the presence of WNV in an area. WNV is spread by the bite of an infected mosquito (Figure 51). Horses and humans are infected when a mosquito takes a meal from an infected bird and subsequently bites a horse or human. Horses, humans, and other mammals do not develop a level of virus in the blood that is significant enough to make them a source of infection to other animals via a mosquito bite; they are, therefore, considered dead-end hosts.
WNV infection is indirectly zoonotic, meaning humans and horses cannot be infected directly from one another or from birds. The mosquitoes act as vectors that transmit the virus. WNV infection occurs most commonly from late spring through early fall, when mosquitoes are most active.
Most infected horses will not become clinically ill. If an infected horse becomes clinically ill, the signs are related to the inflammation in the brain, or encephalitis, caused by the virus. These include nonspecific signs such as loss of appetite, depression, and fever. The encephalitis can be manifested by difficulty swallowing, weakness or paralysis of a hind limb, impaired vision, incoordination, head pressing, aimless wandering, walking in circles, excitability, convulsions, coma, and death. These signs will usually appear between 5 and 15 days after infection. Animals usually recover in 4 to 7 days and do not show any signs of permanent brain damage. The mortality rate in horses can be up to 33%.