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The West Nile Virus cases in humans and horses have been on the
rise in 2012, and, according to the CDC, human cases are at their
highest levels since West Nile Virus was first detected in the
United States in 1999. Dr. Tracy Norman, clinical assistant
professor at the Texas A&M University College of Veterinary
Medicine & Biomedical Sciences (CVM) Large Animal Clinic,
suggests protecting horses by vaccinating against the disease and
taking measures to prevent mosquito bites.
The virus is transmitted by mosquitoes from avian hosts to
humans and horses. Both humans and horses are considered "dead-end"
hosts of West Nile Virus, which means it is not contagious from
horse to horse or horse to human. If bitten by an infectious
mosquito, the virus can multiply in the blood system, cross the
blood brain barrier, and infect the brain. There, it can cause
inflammation of the brain, interfering with central nervous system
Most horses infected with the virus do not exhibit signs of the
disease. For those that do, however, symptoms are similar to other
neurologic diseases and can include impairment of basic motor
skills (including loss of coordination or asymmetrical weakness, a
change in behavior, or drowsiness. Some horses with West Nile may
have a fever early in the disease and show symptoms such as
sensitivity to touch and sound, and muscle twitching in the face,
muzzle, and neck.
"These typical neurologic signs are not always present in
infected horses, sometimes infected horses just appear colicky,"
Norman said. "You should always consult with a veterinarian if you
suspect a horse of having West Nile Virus. Confirmation of
infection is easily diagnosed through a blood test. Then owners and
the veterinarian can plan a course of treatment."
Norman explained that the main treatment for West Nile is
supportive care. Often anti-inflammatory drugs (such as Banamine,
steroids, and DMSO) and intravenous fluids are used. If the horse
is having difficulty balancing, a sling can be used to support the
horse as it recovers.
"The idea is to keep the horse healthy so it can fight the
virus," Norman said.
"If the horse is down and cannot get back up on its own, the
outlook is pretty bad," Norman said. "The mortality rate for West
Nile is about 30 to 40 percent. Many infected horses will survive,
but many of those will have residual neurological impairment. Not
all horses will regain their previous performance levels."
Norman stressed that vaccines against West Nile are the best way
to prevent infection in horses. While the vaccine is not 100
percent effective at preventing clinical disease, the vaccine can
help reduce the severity of the symptoms (because of some
pre-existing immunity from the vaccine). Vaccinated horses that do
become sick with West Nile Virus are in general less sick,
requiring less intensive treatment, are sick for shorter periods of
time, and have a better chance at making a full recovery than
unvaccinated horses. Vaccination against West Nile Virus is
recommended by the American Association of Equine Practitioners as
one of the core vaccines that all horses should receive.
Additionally, horse owners can take steps to reduce the amount
of mosquitos around their facilities by eliminating standing water;
keeping stalls and pens clean; using equine mosquito repellents,
fly sheets, and fly masks; and placing fans inside stalls
(mosquitos have difficulty flying in wind).
The vaccines provide year-long coverage, but in some areas like
Texas, with long summers and short, mild winters, some
veterinarians will give the vaccine twice a year. Norman recommends
vaccinating horses that have not yet been vaccinated this year to
help protect horses through the fall mosquito months.
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