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Introduction
West Nile Virus (WNV) is a virus that can infect many different
types of animals but causes sickness only in a small number of
species. In North America, humans, horses, and especially members of
the crow family (crows, ravens, magpies, blue and gray jays) are at
most risk for serious illness. The main reservoir for the virus is
thought to be wild waterfowl. The WNV is spread from bird to bird,
and to other animals, by the bites of mosquitoes. The horse is a
dead-end host for WNV - the virus does not spread from horse to
horse, or to other animals, including humans.
WNV first appeared in North America in 1999 and has spread
steadily across the country over the past three years. The virus was
detected in birds in Manitoba in early July 2002, and has now been
found throughout Manitoba. Mosquito pools in Winnipeg have now
tested positive. No human cases have been confirmed to date.
Many equine cases of WNV have been confirmed in Minnesota and
North Dakota. In Manitoba, a number of cases of equine central
nervous system disease are currently being investigated. Initial
results on some of the animals are presumptively positive for WNV.
Confirmatory results are expected shortly.
What to Look for in your Horse
Horses and other members of the equine species become infected
following being bitten by a mosquito carrying WNV. The symptoms
relate to the central nervous system, because the virus causes an
encephalomyelitis or brain and spinal cord infection. Symptoms
include listlessness, depression, loss of appetite, stumbling and
incoordination, weakness of limbs, muscle twitching, partial
paralysis, inability to swallow, head pressing, recumbency,
convulsions and death. Fever may or may not be present. The symptoms
will progress from the very early, mild symptoms through to
recumbency and death in a period of five to seven days.
These symptoms are easily confused with other nervous system
disorders in the horse. Diseases to be considered when making a
diagnosis include rabies, sleeping sickness, equine viral
rhinopneumonitis, equine protozoal myeloencephalitis, and tetanus.
All ages of horses are susceptible to the disease, although very
young or very old horses, or horses already compromised by some
other disease condition, are more likely to be affected.
Treatment Options
WNV is a serious disease, with a case fatality rate approaching
35 - 40 %. In many cases, the affected horse is humanely euthanised
after becoming recumbent and exhibiting convulsions. However, if
symptoms are identified early, and supportive therapy is instituted,
the success rate can be improved. These treatments may include fluid
therapy, non-steroidal anti inflammatory drugs, antibiotics,
diuretics, and excellent nursing care.
Diagnosis
When a horse owner identifies any of the symptoms outlined, a
veterinarian should be consulted to make a differential diagnosis,
to collect samples for testing and to begin treatment. Blood samples
are collected from the horse to test for the presence of WNV
antibodies. Other tests will also be run to rule out other potential
causes of the symptoms. A second blood sample, 10 - 14 days later is
collected to confirm the diagnosis. If your horse dies, the
preferred option is to have the animal subjected to a post mortem at
the provincial veterinary laboratory. In any event, the horse's
brain and portions of the spinal cord should be collected for
analysis. The brain will be tested for rabies as well as for WNV.
Prevention
There is a conditional use vaccine available for WNV in horses.
The vaccine is available through veterinarians only and should be
administered by them. In some situations, if a very good
veterinary-client-patient relationship is in place, the vaccine may
be dispensed to the owner for that person to administer. The
veterinarian must keep detailed records on what horses are
vaccinated (breed, age, reproductive status, etc.), lot numbers,
results and reactions. A blood sample before vaccination may be
recommended especially if the horse could be exported. Some
countries require a negative WNV blood test and the vaccination
could hinder evaluation for previous infection.
The vaccine is a killed virus vaccine. It requires two doses in
the initial series to build up protective levels of antibodies. The
two doses are administered at an interval of 3-4 weeks, with
protective immunity developing in 2-4 weeks after the second
vaccination. Horse owners must realize that their horse will not be
protected form WNV infection for at least five weeks, and more
likely eight weeks, after the vaccination series is begun.
Consult with your veterinarian to set up a health program for
your horse. WNV is only one of many diseases to be considered in a
vaccination program. Consider all the factors -risk, cost, mosquito
levels, time of year, before embarking on a WNV vaccination program.
Other preventative measures center on mosquito control. Where
possible, stable horses at night in a barn with mosquito netting in
place. Use insect sprays and repellants. Where permitted, burn
smudges to reduce mosquito activity near your horses. Reduce the
amount of tall vegetation around corrals and barns. And get rid of
all sources of standing, stagnant water.
Final Comments
WNV has garnered a lot of media attention this summer. It is an
emerging disease, and it can be a serious, fatal disease in the
horse. Just bear in mind that we are not in the throes of an
epidemic. The risk to your horse of contracting WNV is low. If you
see signs of a central nervous system disorder in your horse, call
for veterinary assistance to rule out the more likely diseases such
as rabies, and to institute a treatment regimen.
For Further Information - Contact:
Your local veterinarian, or the
Veterinary Services Branch - 204-945-7650
Additional information is available at:
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