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West Nile Virus: History, Signs, and Treatment
The West Nile Virus is a mosquito-borne virus that was first isolated
in a woman in the West Nile District of Uganda in 1937. Outbreaks
continued throughout Europe, Asia, Africa and the South Pacific. In
1999, the virus was discovered to be the cause of encephalitis
(inflammation of the brain) and death in humans, birds and horses in the
New York City area. Progression of the virus continued into Georgia and
Florida in June 2001. As of January 1, 2003, more than 14,000 cases of
equine West Nile Virus have been reported to the USDA in 40 states:
Alabama, Arkansas, Colorado, Connecticut, Delaware, Florida, Georgia,
Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maryland,
Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana,
Nebraska, New Jersey, New Mexico, New York, North Carolina, North
Dakota, Ohio, Oklahoma, Pennsylvania, South Carolina, South Dakota,
Tennessee, Texas, Vermont, Virginia, Washington, West Virginia,
Wisconsin and Wyoming. As of January 2003, the Canadian provinces of
Quebec, Ontario, Manitoba and Saskatchewan have over 350 presumed or
confirmed equine West Nile Virus cases.
Horses that have become infected with the West Nile Virus present with
sudden (sometimes overnight) or progressive ataxia (in coordination of the
limbs). The horse may appear to be “drunk” in the rear legs, stumble, or have
asymmetric movements of the limbs. In addition, affected horses may have a
fever, periods of hyper excitability, apprehension, periods of unnatural
sleepiness, listlessness, or depression. Other neurological signs have been
reported, including limb paralysis, fasciculation of the facial and neck
muscles, muscle rigidity, weakness of the tongue, and blindness.
The mortality rate (percent of horses that have been diagnosed with the WNV
that die) in 2000 was 30%. In the 70% that survived, little residual problems
were reported. However, at the University of Florida in 2001, in horses that
survived there was a recurrence of clinical signs 2-3 days after the acute
signs disappeared. These signs were usually mild to moderate.
Treatment of horses infected with the WNV is supportive; we can treat
the symptoms, i.e. fever, but we cannot eliminate the virus from the
horse’s body. Bute and Banamine are used to treat fever and discomfort.
DMSO is used to decrease the inflammation within the brain and spinal
cord, in addition to providing some pain relief and mild sedation. Other
drugs such as Acepromazine may be used for sedation, in addition to
easing apprehension. Hydration is maintained with intravenous (IV) or
oral fluid therapy. Horses that cannot stand can be supported in a
sling, and leg wraps and head protection can help eliminate any trauma
due to in coordination or recumbency.
Horses are infected by West Nile Virus through the bite of a mosquito
carrying the virus. The virus lives in the salivary glands of
mosquitoes. Here in Alaska we have four of the species of mosquitoes
known to carry West Nile Virus. When the virus enters the blood stream
of the horse it begins to multiply and then infects the brain and spinal
cord. Mosquitoes generally get the virus from birds, including barn-yard
poultry. Birds have a large number of virus particles in their
bloodstream and are the reservoir for the disease. Horses and humans are
generally considered “dead-end” hosts because they don’t usually have
enough of the virus in their bloodstream to be infective to mosquitoes.
Horses that have the disease are not directly infectious to other horses
but should be kept away from exposure to mosquitoes so there is no
possibility of transferring the virus on through mosquitoes. The virus
is transmitted by mosquitoes with no known animal to animal or animal to
human transmission. It is theoretically possible for a human handling an
infected horse to be infected through the bite of a mosquito that has
bitten the horse. Precautions should be taken
Fortunately, there is now a vaccine against West Nile Virus available
for use in horses. This is a new product and has received full approval
from the FDA. It’s use is restricted to licensed veterinarians. The
product is considered safe, pure and has an efficacy of 94% in
preventing illness from the virus when the vaccine is administered
initially in 2 doses, 3-6 weeks apart. The vaccine is then boostered
yearly in the spring. This protocol has been used with success in other
parts of the country. At this time there is not a vaccine available for
humans or other animals.
Other measures that should be taken to prevent the spread of West
Nile Virus are mosquito control. It is important to reduce the number of
mosquito breeding sites. This means eliminating any sources of stagnant
water on your property. Property owners should eliminate any
water-holding containers, including discarded tires. Any containers that
are left outside should have holes drilled in the bottom of them. Roof
gutters should be cleaned. Wading pools and wheelbarrows should be
turned over when not in use. Ornamental pools should be aerated and bird
bathes should not have stagnant water in them. Livestock troughs should
be cleaned monthly. Mosquitoes can potentially breed in any stagnant
water that persists more than four days.
Decreasing exposure to adult mosquitoes can be attempted by using
screened housing (taking care that mosquitoes are not trapped inside),
using insect repellants, and possibly keeping horses in stalls at night.
Additional information about West Nile Virus
and Horses can be found at the USDA web site www.usda.gov/oa/wnv as well
as the CDC site (www.cdc.gov/ncidod/dvbid/westnile).
If you have questions about vaccination or wish
to schedule vaccination for your horses please contact the clinic at
688-9303.
For More Information Contact:
Alaska Equine & Small Animal Hospital, LLC
PO Box 671512 Chugiak, Alaska 99567
Tel: 907-688-9303
FAX: 907-688-2520
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