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West Nile virus (WNV) was first recognized in the
United States in New York in 1999. WNV is considered an endemic
(permanent) disease of the United States. West Nile virus is
transmitted by mosquitoes, and produces clinical disease humans,
birds, and horses. Crows are particularly susceptible to infection
with WNV and experience high mortality. Many birds produce a large
number of virus particles in circulating blood, which allows them to
transmit virus to mosquitoes. Humans and horses are considered
accidental, dead-end hosts and do not participate in the life-cycle.
Many other mammals (cattle, dogs, cats, rabbits, deer) demonstrate
evidence of exposure to WNV (seroconversion), however, clinical
disease is not evident except in isolated cases associated with
dysfunction of the immune system. If an infected mosquito obtains a
blood meal from a WNV carrier bird, and takes a blood meal from a
naïve horse several days later, the horse will become infected.
Horses become ill 3 to 14 days after exposure to an infected
mosquito (incubation period). Horses may be affected at any age, and
there is no breed or gender predilection.
The
clinical signs of WNV infection in horses are
usually characteristic, recognizable signs that may wax-and-wane
dramatically at the onset of disease. The most common early signs of
infection include twitching of the muzzle and ears, frequent
chewing, aggression, and fine muscle twitching. Horses then develop
progressive incoordination, weakness, and listlessness. Severely
affected horses may develop paralysis of the limbs, seizures,
disorientation, coma, or death. Some horses may present as a single
limb, non-weight bearing lameness. The most difficult diseases to
differentiate from WNV on the bases of clinical signs are rabies,
eastern/western encephalitis, and equine protozoal myeloencephalitis.
It is important to obtain a definitive diagnosis of
WNV infection to document the number of cases in a region and to
rule-out diseases with similar clinical signs. Diagnosis of WNV is
determined by detection of antibody against WNV in serum.
Vaccination does not appear to interfere with testing. False
negative diagnosis may occur if the individual is tested early in
the disease process or if the horse fails to mount a typical immune
response to the virus. Retesting serum is recommended for horses
with clinical signs typical of WNV, but negative test results.
Treatment
of WNV is directed towards reducing edema
and inflammation in the central nervous system. Medications are used
that will decrease pain, reduce inflammation, and combat viral
infection. The duration of medical therapy is dependent on the
severity and duration of disease. In addition, protecting the horse
from self-inflected trauma is crucial to the management of WNV
encephalitis. Sedation, sling support, protective leg bandages, and
a helmet are recommended for horses with moderate to severe signs of
WNV.
Appoximately 30% of horses with WNV encephalitis
will die from their infection or complications associated with
encephalitis. However, the prognosis for an individual horse is
dependent upon the severity of clinical signs. The survival rate in
horses that remain standing is approximately 80 to 90%. The survival
rate in horses that cannot be supported with the use of a sling is
much lower. Horses cannot transmit WNV to humans through contact.
There is no reason to destroy a horse infected with WNV based on
concern for human disease.
The WNV vaccine is available for horses for
prevention of infection. An initial vaccination is followed by a
booster 3 to 6 weeks later. A single vaccination appears ineffective
for prevention. Preliminary data indicates protection may begin 30
days after the booster vaccination. Vaccination is recommended once,
twice, or three times per year, thereafter, dependent on geographic
location. Foals from vaccinated mares should begin their vaccination
program at 3 months of age, and should be followed with 2 booster
vaccines. Foals from unvaccinated mares should be vaccinated at 1,
2, 3, and 6 months of age.
Control of mosquitoes is just as important in the
prevention of disease for your horse as vaccination. To prevent
exposure to adult mosquitoes, horses should be maintained away from
wooded areas and standing water, particularly during times of
mosquito feeding. Repellent should be applied frequently. The most
effective method of controlling mosquito populations is targeting
the larval stage (sources of standing water). Once mosquitoes become
flying adults, control is more difficult and expensive. The
following recommendations to prevent larval development are
standard:
- Eliminate unnecessary standing water (buckets, cans, used tires).
- Empty livestock water sources once weekly.
- Eliminate trapped water in plastic covers on boats, swimming
pools.
- Make sure rain gutters are clean and do not hold water.
- Stock garden ponds with mosquito eating fish (e.g. minnows,
goldfish).
- Check water sources for developing mosquitoes. Obtain a water
sample using a white plastic container (cottage cheese container),
and observe for wigglers (larvae) and tumblers (pupae).
- Chemical products based on the bacterium Bacillus thuringiensis
are safe and effective, with selective pathogenicity against
mosquito and blackfly larvae. Trade names: Vectobac, Gnatrol,
Mosquito Dunks.
- No matter how a used tire is oriented, it always collects water
and is an ideal breeding site for several mosquito species. A
handful of common rock salt will prevent mosquito replication in old
tires.
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