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West Nile Virus FAQ for horses
OC Animal Care
What is West Nile encephalitis?
How do people or animals become infected
with West Nile Virus?
Does infection always lead to illness?
Is treatment available for West Nile encephalitis
in horses?
Is a vaccine available to protect against
infection with West Nile Virus?
How can I protect my horse against infection
with West Nile Virus?
What additional steps can be taken to reduce the likelihood of exposure of horses to adult mosquitoes?
Can a horse infected with West Nile Virus
infect other horses?
Other Resources
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What is West Nile encephalitis?
West Nile encephalitis describes an inflammation of
the central nervous system, which is caused by infection
with West Nile Virus. Prior to 1999, West Nile Virus was
found only in Africa, Eastern Europe, and West Asia. In August
of 1999, it was first identified in the United States. The first
human case in the state was reported in 2002, with the first equine case in
California reported in 2003.
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How do people or animals become infected
with West Nile Virus?
People and animals most commonly become infected
from the bite of certain kinds of mosquitoes that are infected
with the virus. Mosquitoes may pick up the virus when they
bite, or take a blood meal, from wild birds that are infected
with West Nile Virus. Those mosquitoes may then transmit
the virus to people and other animals when biting.
Infection occurs primarily in the late summer or early fall.
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Does infection always lead to illness?
Infection with West Nile Virus does not always lead to
signs of illness in people or animals. Horses are affected by
WNV more often than other domestic animals. Many
horses infected with WNV do not develop any illness, but of
the almost 15,000 equine cases reported in 2002, about
one-third died or were euthanized. Other livestock and poultry
do not commonly show illness if infected with WNV. In
horses that do become clinically ill, the virus infects the
central nervous system and may cause symptoms of
encephalitis. Clinical signs of encephalitis in horses may include
a general loss of appetite and depression, in addition to
any combination of the following signs:
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Ataxia (stumbling, staggering, wobbly gait or incoordination)
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weakness or paralysis of limbs
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inability to stand
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circling
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fever (in 25% of affected horses)
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muscle fasciculation
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paralysis
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head tilt
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knuckling over
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muscle twitches or tremors
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apparent blindness
It is important to note that not all horses with clinical
signs of encephalitis have West Nile encephalitis. There are
other diseases that can cause a horse to have symptoms
similar to those resulting from infection with West Nile Virus. If
you are concerned that your horse may be exhibiting signs
of encephalitis, please contact your veterinarian.
Laboratory tests are necessary to confirm a diagnosis.
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Is treatment available for West Nile encephalitis
in horses?
There is no specific treatment for West Nile encephalitis
in horses. Supportive veterinary care is recommended. It
is important to diagnose WNV because infection is an
indication that mosquitoes carrying the virus are in the area
and need to be eliminated.
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Is a vaccine available to protect against
infection with West Nile Virus?
There are now two
WNV vaccines available for horses. Ask
your veterinarian for their recommendation on vaccinating
your horse for WNV. Because it is important to
distinguish between vaccinated and infected horses, accurate
vaccination records should be kept. Horses vaccinated
against Eastern, Western, and Venezuelan equine encephalitis
are not protected against infection with West Nile Virus.
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How can I protect my horse against infection
with West Nile Virus?
Vaccination of horses is not a guarantee of
protection against infection, and does not offer any protection for
other animals or people. The best method of prevention
of infection with West Nile Virus for people and animals
is to reduce the risk of exposure to the mosquitoes
that may carry the virus. Reducing the risk involves
eliminating mosquito-breeding sites to reduce the number of
hatching mosquitoes, and to reduce exposure to adult
mosquitoes. Mosquitoes breed in stagnant water, so reduction of
breeding sites involves eliminating stagnant water sources.
To reduce the number of mosquito breeding sites:
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Dispose of tin cans, plastic containers,
buckets, ceramic pots or other unwanted
water-holding containers on your property.
-
Pay special attention to discarded tires.
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Tires
are important mosquito breeding sites.
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Drill holes in the bottom of recycling containers
left outdoors. Containers with drainage holes
located only on the sides collect enough water to act
as mosquito breeding sites.
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Clean clogged roof gutters every year. Millions
of mosquitoes can breed in roof gutters each season.
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Turn over plastic wading pools when not in use.
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Turn over wheelbarrows and don't let water
stagnate in birdbaths.
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Empty and refill outdoor water troughs or
buckets every few days.
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Aerate ornamental pools or stock them with
fish. Water gardens can become major mosquito producers if they are allowed to stagnate.
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Clean and chlorinate swimming pools when not
in use. Mosquitoes may even breed in the water
that collects on pool covers.
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Use landscaping to eliminate standing water
that collects on your property, especially near
manure storage areas. Mosquitoes may breed in any puddle that lasts for more than four days.
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Additional steps can be taken to reduce
the likelihood of exposure of horses to adult mosquitoes:
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Avoid turning on lights inside the stable during
the evening and overnight hours. Mosquitoes are attracted to yellow incandescent bulbs.
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If light is needed near the stable, place
incandescent bulbs outside the stable to attract
mosquitoes away from the horses. Black lights (bug
zappers) don't attract mosquitoes well.
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Reduce the number of birds in and around
the stable area. Eliminate roosting areas in the
rafters of the stable. Certain species of wild birds
are thought to be the main reservoir for the virus.
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Periodically look around the property for dead
birds, such as crows. Use gloves to handle dead birds
and place the birds in plastic bags.
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Use topical preparations containing
mosquito repellents. Use of insect repellents may be of
some value in decreasing exposure of horses to
adult mosquitoes. Repellents should not be solely
relied upon to prevent mosquito exposure and should
be used according to their label instructions
regarding appropriate species, method of application,
and other precautions.
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Fogging of stable premises can be done in
the evening to reduce mosquitoes; read directions carefully before using.
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House animals in structures with
well-maintained insect screening. Take precautions first to
eliminate mosquitoes from inside the structure, which may
be accomplished through a number of means
including the use of mosquito adulticides. In addition, use
of fans may reduce the potential ability of
mosquitoes to feed on horses.
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Reduce outdoor exposure. A recently
completed epidemiologic study of WNV suggests that
keeping horses in stalls at night may be helpful in
reducing their risk of infection.
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Can a horse infected with West Nile Virus
infect other horses?
There is no evidence that infected horses can transmit
the virus to other animals, people, or mosquitoes through
routine contact. Only a wild bird-mosquito transmission cycle
has been proven as a means of transmitting West Nile Virus.
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