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What is West Nile Virus?
West Nile Virus
(WNV) is a mosquito-transmitted disease that affects humans and animals
(mammals).
The virus' natural life cycle involves mosquitoes and birds. The cycle
starts with infected birds
which can travel long distances in
a short amount of time. When a mosquito bites a bird carrying
the West Nile virus, the mosquito then becomes infected. This
infected mosquito then feeds on
another bird, a horse, human or other mammal,
which then becomes infected with WNV. Once a
horse has been
bitten, it may take only 5 to 15 days for signs of West Nile virus to appear.
Horse
to
horse transmission does not occur. The virus is most prevalent from May to October when mosquitoes
are most
abundant.
West Nile Virus
History
The West Nile Virus
established itself in the United States
in 1999 and spread fast.
In 1999, WNV appeared in New York for the first time in the Western
Hemisphere and caused disease
and some deaths in humans, birds and animals including horses. Previously,
WNV was only reported in
Africa, the Middle East, Asia and Eastern Europe.
In 1999, WNV primarily affected New York, New Jersey
and Connecticut.
In 2000, the virus spread along the eastern seaboard from New Hampshire
to North Carolina.
In 2001 WNV has spread from Maine to Florida and westward to Wisconsin,
Iowa, Illinois and Louisiana.
The virus has also been detected in Canada.
By 2002 WNV was found in all but 4 states, Oregon,
Nevada, Utah and Arizona. (From 1999 through 2002.)
By 2004 WNV had been
detected in all continental 48 states in at least one year since 1999.
West Nile Virus
Signs In Horses
The most common signs of WNV infection in horses include stumbling, uncoordination,
weak limbs,
partial paralysis, muscle twitching and in some cases, death. Fever has occurred in less than one fourth
of all confirmed equine
cases.
From 1999 to present various clinical signs
of WNV have been observed. Each horse may exhibit any
combination
of these symptoms, which include:
-
Muzzle Tremors. The muzzle will twitch sometimes for
hours, sometimes for days.
There will be no apparent reason for it -- no flies. Just a constant
twitch.
-
Muscle Twitching. The horse looks like he is trying to get flies off
his body.
-
Behavior. Depression is common. Hyper-responsiveness
such as flinging around a stall,
and other anxieties.
Sweating has been noted,
but does not always occur.
-
Head
pressing, aimless wandering or circling,
-
Facial Paralysis. On one or both sides of face. The
right ear may droop but the left ear stand up.
The muzzle may be paralyzed on just one side or both sides so that the
lips won't seal and
the horse is constantly drooling.
-
Vertigo-Balance problems. The horse may actually lean
on a wall to stand up. He stumbles a lot
and is uncoordinated.
-
Tongue Paralysis. The tongue may lop out of the mouth.
The horse doesn't seem to be able
to pull its own tongue back into the mouth. And in some advanced
cases, the tongue has to be
pushed back into the mouth but if the muzzle is paralyzed, the tongue
will just drop back out.
-
Weak. Stiff walking. Tired.
-
Impaired
vision
-
Convulsions
-
Collapse. Usually in advanced cases. With some the horse
will fall to its front legs
as if praying and can't get up. In others, the horse may just sit down
and may be unable to
stand on the back legs. He'll dog sit. If collapse is on all four
legs, this case probably won't make it
because the horse can't stand on his own and these are the most
difficult cases to care for.
How long does it take for the
symptoms to show after they have bitten by the mosquito?
The incubation period is between 5 and 15
days from the date of the mosquito bite.
Treatment Recommendations
Treatment is vital for any
horse with WNV.
At this present time there is no specific
"medicine" for a horse
once he develops WNV,
although there are some promising advances being made in that area.
.
However, your veterinarian
will be able to provide supportive
therapy that can save your horse’s life
if he
does develop WNV.
For advanced cases, horses usually have to be
hospitalized. For mild
cases, home care may be adequate.
But regardless, nursing care and
treatment is important to the recovery
process and each animal is assessed
according to it's
age and health. And in cases of WNV, 33% will die, 50%
will fully recover and 17% will have
'problem recoveries' -- relapse or
incomplete recovery.
Over the last few years, it has also been
discovered that of those horses that recover, some will relapse within
a few
months
or a year. Some of these will die also.
The focus of therapy is to decrease brain inflammation:
1) Treat the fever
(if any), and provide supportive care, which may require 1-4
weeks of intermittent therapy.
Common medications include flunixin meglumine (Banaminẻ), dimethyl
sulfoxide (DMSO), and dexamethasone.
2) Sometimes fluid therapy is needed for
animals not able to drink.
3)
Oral or intravenous feeding may also be necessary for
horses unwilling to eat.
4)
For horses unable to rise slinging
is recommended 2 to 3 times per day to aid in
circulation and to try to
prevent pressure point sores. (bed sores).
5)
Head and leg protection is also frequently needed.
Recovery Times
Recovery times depend upon the health and age of the
affected horse. Many horses will improve within 5 to 7 days
of displaying
clinical signs, however about 20-30% can
exhibit severe neurological deficits for several weeks.
After 3 to 5 days, horses that are recovering may exhibit a
sudden recurrence of clinical signs.
This may be of short duration or horses may become suddenly recumbent
and either die or require prolonged
treatment. If horse has clinical signs that last a long time, intravenous
catheterization or hospitalization is
recommended.
Horses with WNV
sometimes develop other problems
because
they were weakened by WNV.
Joint and tendon
infections, sheath infections, pneumonia, and diarrhea can all
occur as secondary events.
Monitoring for appropriate
fecal output is also important and horses may
need to be treated
with mineral oil during the course of disease.
Will the horse return to its normal
health?
When affected, most horses
start to show improvement in 3-5 days with an
overall mortality rate of 30-35%.
With animals who are unable to rise, mortality
rate is over 65%.
Once the horse
has demonstrated significant improvement, full recovery within 1 to 6
months can be expected
in 90% of the patients. Residual weakness and ataxia appear to be the
main problems.
The long term effects of the disease in horses is not yet known.
West Nile Virus vaccine for horses.
Horse owners should consult their veterinarians regarding its use.
The vaccine shots are no value if they
aren't
given prior to exposure to the disease. If the horse develops WNV it is
too late for the shot.
The vaccines require two doses, administered three to six weeks apart,
and full protection doesn't develop
until four to six weeks after the second dose.
Sometimes a third does is recommended. Boosters are
recommended, but recommendations vary and
depend upon mosquito infestation where you live.
It can take from 7 to 12 weeks
for the horse to develop maximum resistance to infection.
Reducing West
Nile and Helping To Prevent Spreading Of The Disease.
1) Reduce mosquitoes and their breeding
grounds.
- Eliminate any unnecessary standing water on your property (tires,
wheelbarrows, old buckets, etc.).
- Check around faucets and air conditioner units and repair any leaks and
remove puddles.
- Make certain roof gutters drain properly and remove any standing water,
especially from flat roofs.
- Clean out waterers, birdbaths, plant saucers, etc., at least weekly.
- Schedule pasture irrigation to minimize standing water.
- Keep swimming pools clean and free of water on covers.
- Keep ponds aerated and/or stocked with fish and consider stocking water
tanks with fish that consume
mosquito larvae.
2) Reduce your horses' exposure to mosquitoes.
- Stable horses inside during active mosquito feeding times (dawn/dusk).
- Utilize fans, barrier cloths, screens, flysheets, repellent sprays (permethrin,
DEET), and
insecticide misting
systems.
- Turn off lights that attract mosquitoes at night,
or use fluorescent lights, which do not attract mosquitoes
.
- Keep screens in stable windows
if possible.
West Nile Virus
In Humans
In most human
cases, WNV infection causes flu-like symptoms. A small percentage of
infected individuals
develop encephalitis (inflammation of the brain).
Above: West Nile Virus in the
United States Oct 28, 2003
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Spread
of West Nile Virus in the United States 1999-2002
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