West Nile Virus (WNV) is a member of a group of viruses known as arboviruses. It affects primarily birds, humans and horses, and to a lesser extent other domestic animals such as sheep, goats, dogs and cats. WNV was first detected in North America in 1999 in New York. Since then, WNV has steadily moved westward and the latest count includes California, Arizona and Utah. In Nevada, the Animal Disease Laboratory has been conducting surveillance testing for WNV on a variety of species since March of 2001. West Nile Virus was first detected in Nevada in crow found in Carson City on July 15th 2004.
West Nile virus like other arboviruses is spread through a bird-mosquito cycle and transmitted to mammals, including humans, through the bite of an infected mosquito. Mosquitoes are infected by taking a blood meal from infected birds, such as the members of the raven family. In North America, humans, horses, and many species of birds are susceptible to infection with WNV. However, only the Corvid species (crows, ravens, magpies, jays, and their relatives), raptors and owls are truly susceptible to WNV disease
and serve as useful sentinels of WNV activity.
According to the Center for Disease Control in Atlanta, there have been no documented cases of person-to-person (except for blood transfusion, organ donation and trans-placental), animal-to-person, or animal-to-animal transmission of WNV. The horse and other mammals are considered to be dead-end hosts, meaning that the virus doesn't multiply to high enough levels in these species to provide a source of infection for mosquitoes.
Currently, several vaccine candidates for humans are in different stages of clinical trials. Public availability is not expected for at least another two years. Currently there are two licensed vaccines for horses. Clinical signs of West Nile virus infection in horses include listlessness, stumbling and incoordination, weakness of limbs, ataxia, partial paralysis, and death. Because adequate protection requires nearly two months, the Nevada Department of Agriculture is encouraging Nevada horse owners to have their horses vaccinated for WNV in time for the mosquito season. The vaccination schedule consists of two intramuscular doses administered 3-6 weeks apart, and then annual revaccination. The two vaccines are significantly different from each other (Fort Dodge Animal Health's West Nile-Innovator™ is a killed whole virus vaccine, Merial's Recombitek® Equine WNV Vaccine is a live recombinant canary pox vaccine). Both companies have conducted vaccination trials to test if immunization with West Nile-Innovator™ can be boostered with Merial's Recombitek Equine WNV Vaccine. Results are contradictory. Please contact your veterinarian for more information.