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Public Health Fact Sheet - West Nile Virus in
Horses in Massachusetts
What is West Nile virus (WNV)?
West Nile virus (WNV) can cause illness in humans varying from a mild fever to
more serious disease like encephalitis or meningitis. WNV grows in birds and is
spread from bird to bird by infected mosquitoes. If mosquitoes infected with the
virus bite horses or humans, the animal or person can become sick. In the United
States, WNV was first identified in New York City during the summer of 1999.
Since then it has spread throughout much of the continental United States. WNV
has occurred naturally in Europe, Africa and Asia for many years.
How common is WNV in horses?
During the 1999 outbreak in New York City, 25 horses had clinical signs of WNV
disease and 9 of these (36%) died or were euthanized. During 2001, 738 cases of
WNV disease were reported in horses in the United States, with 45 of those cases
in Massachusetts. In 2002, WNV disease spread across the United States and was
reported in 14,900 horses from 40 states. While 45 horse cases were reported in
Massachusetts in 2001, only 2 cases were reported in 2002, both in horses not
vaccinated against WNV. Current WNV surveillance information for horses as well
as humans, birds, and mosquitoes can be found on the MDPH website at
www.state.ma.us/dph.
What are the signs of WNV infection in horses?
Clinical signs in horses with West Nile virus infection vary. Some horses may
show few or no signs of illness. The most common signs include weakness, ataxia
(an unsteady gait), depression or apprehension, fever, muscle fasciculation
(tremors) and decrease in appetite. In a small number of horses, muscle weakness
may progress to paralysis of all four limbs, recumbency (inability to rise) and
death within 2 to 9 days. If you suspect your horse to be ill for any reason,
have it examined by a veterinarian.
If my horse is infected with WNV, do I have to euthanize or quarantine
it?
No, horses affected by WNV do not need to be euthanized as many horses display
mild clinical signs and can recover from the disease with supportive care.
Horses are humanely euthanized only when they are suffering from illness from
which they will not be able to recover. Of the horses infected with WNV that
show clinical signs, approximately 30% are euthanized or die because of the
infection. Horses with WNV do not need to be quarantined since infected horses
cannot transmit the disease to others horses or humans.
Is there treatment for West Nile virus disease in horses?
Other than symptomatic and supportive veterinary care, there is no specific
treatment for horses with WNV.
Is there a vaccine for West Nile virus disease in horses?
In February 2003, the United States Department of Agriculture (USDA) fully
licensed a vaccine for WNV. Administration of the vaccine requires two doses 3-6
weeks apart, with an annual booster. Horse owners should discuss the risks and
benefits of the vaccine for preventing WNV infection with their veterinarians.
Is the Eastern Equine Encephalitis (EEE) vaccine protective against WNV?
No. If your horse is vaccinated against EEE virus, it will not be protected
against WNV infection. Although EEE virus is also carried by birds and
transmitted by mosquitoes, it belongs to another family of viruses.
If my horse is infected with WNV, can I or my other horses become
infected?
WNV is spread by the bite of a mosquito infected with the virus. There is no
documented evidence of horse-to-human or horse-to-horse transmission of WNV.
Caring for an infected horse is not considered a risk factor for developing WNV.
However, you and your other horses could be also be bitten by infected
mosquitoes.
What steps can I take to protect my horse from infection with WNV?
 | Consult with your veterinarian about having your horse vaccinated against
WNV.
| Remove potential mosquito breeding sites. Mosquitoes can begin to breed in
any puddle or standing water that lasts for more than 4 days. Therefore you
should:
| Dispose of or regularly empty any water-holding containers on your
property
| Dispose of old tires
| Drill holes in the bottom of recycling containers that are left outdoors
| Clean clogged roof gutters and eliminate standing water on flat roofs
| Do not leave garbage can lids upside down or allow water to collect in the
bottom of garbage cans
| Keep horses indoors during peak periods of mosquito activity (dusk to
dawn).
| Thoroughly clean watering troughs on a monthly basis.
| Avoid turning on lights inside the stable during the evening and
overnight. Many mosquitoes are attracted to incandescent bulbs that glow
yellow.
| If feasible, remove all birds, including chickens, that are in or close to
the stable.
| If your local health department considers your area to be high risk,
consider using topical preparations containing mosquito repellents that are
available for horses. Contact your veterinarian before using such a
preparation and follow all instructions on the label for proper use.
| Consider fogging of stable premises, with approved pesticides, in the
evening to reduce mosquito numbers. Contact your veterinarian before doing
this, and read directions carefully before using foggers. |
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What should I do if I suspect my horse is infected with WNV?
WNV is only one of many possible causes of central nervous system disease in
horses. Other diseases with clinical signs similar to WNV include rabies, equine
protozoal myeloencephalitis (EPM), EEE, botulism, and bacterial infections. If
your horse has any of the signs associated with WNV infection, you should
contact your veterinarian immediately. The Massachusetts Department of Public
Health (MDPH) will test specimens from horses with signs suggestive of WNV
disease. Veterinarians who suspect WNV disease in horses should contact the
Massachusetts Department of Food and Agriculture, Bureau of Animal Health at
(617) 626-1795.
For more information on WNV in horses:
| Massachusetts Department of Food and Agriculture (617) 626-1795
| Massachusetts Department of Public Health-Viral Serology Lab (617)
983-6396 |
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Websites with information about WNV and people and horses:
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