Prevention and Control of West Nile Virus Infection in Equine and Other Livestock or Poultry

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.

Reduction of Mosquito Breeding Sites

Reducing the population of mosquitoes, especially species that are apparently involved with bird-to-bird transmission of WNV, such as some Culex species, can help to reduce or eliminate the presence of virus in a given geographical area. The most important step any property owner can take to control such mosquito populations is to remove all man-made potential sources of stagnant water in which mosquitoes might breed. Dispose of any water-holding containers, including discarded tires. Drill holes in the bottom of containers that are left outdoors. Clean clogged roof gutters annually. Turn over plastic wading pools or wheelbarrows when not in use and do not allow water to stagnate in bird baths. Clean and chlorinate swimming pools that are not in use and be aware that mosquitoes can breed in the water that collects on swimming pool covers. Aerate ornamental pools and use landscaping to eliminate standing water that collects on your property; mosquitoes can potentially breed in any stagnant puddle that lasts more than 4 days. Thoroughly clean livestock watering troughs monthly. Local mosquito control authorities may be able to help in assessing the mosquito breeding risks associated with a specific property.

Decreasing Exposure to Adult Mosquitoes

It is also important to prevent horses from being exposed to adult mosquitoes. Several actions may help in that effort.

Screened housing

Housing animals in structures with well-maintained insect screening can be useful to reduce exposure to adult mosquitoes. Use of such mosquito-resistant structures may actually lead to mosquito exposure unless precautions are first taken to eliminate mosquitoes from inside the structure. This may be accomplished through a number of means including the use of mosquito adulticides. In addition, use of fans may reduce the potential ability of mosquitoes to feed on horses.

Insect repellents

Use of insect repellents may be of some value in decreasing exposure of horses to adult mosquitoes. Due to practical limitations in the coverage area that may be achieved on any given horse with a particular product formulation, and due to limited duration of effectiveness of some formulations under certain conditions (e.g., perspiration), repellents should not be solely relied upon to prevent mosquito exposure. Repellents should be used according to their label instructions regarding appropriate species, method of application, and other precautions. Topical application of a product containing a synthetic pyrethroid compound (e.g., permethrin) as the active ingredient may offer the best combination of safety and efficacy.

Outdoor exposure

Although some species of mosquitoes feed at dusk or dawn, others are daytime feeders or feed at any time of the day or night. As it is not yet clear which mosquitoes are responsible for the transmission of WNV to horses and other mammalian species, making recommendations as to when certain animals should avoid outdoor exposure may not be particularly useful at this time. However, a recently completed epidemiologic study of WNV suggests that keeping horses in stalls at night may be helpful in reducing their risk of infection.
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