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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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