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West Nile virus (WNV) is a vector-borne
virus that was recognized in the Western Hemisphere for the fist 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 served 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 1999 and 2000 in the northeast
United States were mosquitoes. At least 14 species of mosquitoes
were found positive for WNV. No evidence has been found to suggest
that any invertebrate vectors other than mosquitoes were involved with
WNV transmission in either year.
Horses are affected by the WNV more
often than other domestic animals. Many horses infected with WNV
do not develop any illness, but of the 85 that did become ill in the
1999 - 2000 outbreak, 32 (38 percent) died or were
euthanized. Other livestock and poultry do not commonly show
illness if infected with WNV.
Given that mosquitoes are associated
with WNV transmission, the 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. The following recommendations would apply for other
livestock or poultry should illness due to WNV in those types of animals
come to be
commonly recognized.
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
population 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. Aerate
ornamental pools or stock them with fish, such as Bambusia, that
eat mosquito larvae. Clean chlorinate swimming pools that are not in use
and be aware that mosquitoes can breed in the water that collects on
swimming pool covers. Use landscaping to eliminate standing water
that collects on your property; mosquitoes can breed in in any
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.
Decrease
Exposure to Adult Mosquitoes
It is also important to prevent horses from being
exposed to adult mosquitoes.
Several actions may help in that
effort.
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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.
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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.
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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 WWNV to horses and other mammalian species, making recommendations
as to when certain animals should avoid outdoor exposure may no 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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Vaccination: In
addition to the mosquito-related prevention measures outlined above,
there is now one additional action that can be taken to prevent
illness in horses caused by WNV infection. 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 that initially has a one year license.
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 availability and use in your
area. If it is available for use in your area, the manufacturer
of the vaccine recommends giving two intramuscular doses of 1
milliliter each, 3 to 6 weeks apart.
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