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Prevention and Control of West
Nile Virus Infection in Equine and Other Livestock
or Poultry 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
these 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 euthanized. 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 has 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 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 weekly. 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.,
sweating), 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. For more
information, go to the EPA
Website.
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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