| West
Nile
(Prevention
and Control of West Nile Virus Infection in Equine and Others)
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
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