September 2002

West Nile virus (WNV) is a vector-borne virus that was recognized in the Western Hemisphere for the first time in 1999. Invertebrate vectors, such as mosquitoes, circulate the virus among wild birds. Occasionally the virus is introduced into other vertebrate populations, such as humans or horses that serve as incidental hosts. Incidental hosts are infected animals that do not pass the virus on to vectors or other animals.

The only vectors found to be associated with outbreaks of WNV in the United States since 1999 are mosquitoes. At least 30 species of mosquitoes have been found positive for WNV, although several of those species are likely not involved in active transmission of the virus from bird-to-bird or from bird-to-mammal.

Horses are affected by WNV much more often than any other domestic animals. Many horses infected with WNV do not develop any illness, but of horses that become ill about one-third (33 percent) die or need to be euthanatized. Other livestock and poultry do not commonly show any illness if infected with WNV.

Given that mosquitoes are associated with WNV transmission, one key to preventing or controlling future outbreaks of WNV among horses is to control mosquito populations and to prevent horses from being exposed to any adult mosquitoes that may be present. Similar recommendations would apply for other livestock or poultry should illness due to WNV in those types of animals ever come to be recognized.

In addition to the mosquito-related prevention measures discussed below, there is now an additional action that can be taken to help prevent illness in horses caused by WNV infection: vaccination. On August 1, 2001, a conditional license was issued by the USDA-APHIS' Center for Veterinary Biologics for an equine WNV vaccine. The vaccine is a killed virus product. Conditional licensing means that the product has been shown to be safe, pure, and have a reasonable expectation of efficacy in preventing illness caused by WNV. Each state veterinary authority must also approve the use of the product in their state. Because use of this vaccine is restricted to veterinarians, you need to contact your veterinarian to find out more about its use in your area. The manufacturer of the vaccine recommends giving two intramuscular doses of 1 milliliter each, 3 to 6 weeks apart, followed by an annual booster. The booster should be given just prior to the start of the mosquito season in your area.