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West Nile Virus in Horses:
Frequently Asked Questions
Carey A. Williams, Ph.D., Extension Specialist in Equine Management;
Wayne Crans, Ph.D., Research Professor in Entomology; and Jennifer
Gruener, Graduate Assistant in Entomology.
Published 8/25/2004
Fact
Sheet #526
Q. What is
West
Nile virus?
A. West
Nile
virus (WNV) is an old world mosquito-borne pathogen that appeared
unexpectantly in the New York metropolitan area in the fall of 1999.
The disease was first thought to be St. Louis encephalitis, a
closely related mosquito-borne virus that is relatively common in
the continental
United States.
Virus isolations from wild crows, which were dying from the disease
throughout the New York-metropolitan area, as well as captive birds
from the Bronx Zoo prompted the USDA’s National Veterinarian
Services Laboratories to investigate. In late September 1999, CDC
officials announced that the virus appeared to be West Nile virus, a
pathogen previously unseen in the
Western Hemisphere.
Avian cases were quickly linked to a series of human ailments in the
New York City area that ultimately produced 62 human cases of WNV
with 7 deaths. The virus was recognized as an important pathogen of
horses as it spread from its eastern focus, affecting large numbers
of animals as it moved westward. West Nile virus reached the west
coast of the
United States
in 2002 and was responsible for more than 14,000 diagnosed equine
cases that year.
Q. How does
West
Nile virus cycle in nature?
A.
West
Nile
virus is a disease of wild birds that is maintained in the avian
population by mosquitoes that use birds as their preferred blood
meal hosts. The Northern House Mosquito, Culex pipiens, is an
important bird-feeding species that perpetuates WNV in urban areas
as well as farm settings throughout the northeastern United States.
WNV is transmitted to humans and horses by mosquitoes that
occasionally bite birds but normally use mammals as their preferred
blood meal hosts. Culex pipiens may be directly responsible for some
human and equine infection, but mammal biters are thought to produce
the majority of WNV cases each year.
Q. How do horses
contract
West
Nile virus?
A.
Horses contract WNV when a mosquito that has
previously fed on an infected bird bites them. Birds circulate high
levels of the pathogen in their blood and serve as the sole source
of the virus for mosquitoes. Neither horses nor humans circulate
enough virus in their blood when they acquire the disease to pass
the virus back to mosquitoes.
West
Nile virus cannot be spread directly from horse to horse or from
horse to human.
A mosquito that has previously fed on an infected bird is required
in all cases.
Q.
What is the seasonal progression of
West
Nile virus?
A.
West
Nile
virus is at its lowest levels in early spring and builds in
intensity as the summer season progresses. Infected mosquitoes are
rarely found in large numbers until the month of August. Most
transmission to horses takes place during late summer into fall, the
same time that infection rates peak in mosquitoes and birds. Horses
that have not been protected through proper vaccination may contract
the infection at this time of year.
Q. Why should horse owners be concerned about
West
Nile virus?
A.
West
Nile
virus is a serious threat to horses. In
New Jersey,
even though WNV affected only a small percentage of the equine
population, the mortality rate of those infected from 2000 to 2003
was 34%.
Q. How can I tell if my horse is infected with
West Nile virus?
A.
Infected horses may display one or more of the
following symptoms: Lack of coordination and stumbling (most
commonly described symptom), depression or apprehension, anorexia
(off feed), weakness of the hind limbs, falling down, inability to
rise, flaccid paralysis of the lower lip (droopy lip), muscle
twitching, grinding teeth, inability to swallow, head pressing,
colicky appearance, aimless wandering, hypersensitivity and
excitability, excessive sweating, disorientation, convulsions, and
possible total paralysis.
Q. What
should I do if I see any of these signs?
A. Call your veterinarian immediately. Prompt
treatment may be life-saving!
Q. Is euthanasia of
West
Nile virus infected horses necessary?
A.
No. Horses are humanely euthanized only when
the viral infection is so severe they will not be able to recover.
For those that survive, a full recovery is likely. About two out of
every three horses that become ill will survive.
Q. Will horses and/or farms affected by the virus be quarantined?
A.
No. Since infected horses are not carriers for
the disease, quarantine is not necessary.
Q. How can I reduce the risk of
West Nile virus to my horses?
A. You can significantly reduce the risk of
West
Nile virus to your horses by taking the following steps:
1. Call your veterinarian:
-
Have your horses vaccinated!
-
Ask about proper nutrition and supplements that can be part of an
overall wellness plan to keep your horses healthy, and may
protect them from diseases such as
West Nile virus.
2.
Contact your local county mosquito control agency:
-
They are trained professionals that can help you learn to
recognize mosquito larvae and their habitats.
-
They can locate and assess mosquito breeding habitats on your
farm and in nearby areas.
-
This service is free of charge and available to county residents.
3.
Reduce local mosquito populations:
- Empty and clean watering troughs at least every four days.
- Consider purchasing watering troughs with bottom drainage plugs.
- Check automated watering systems at least once a week to make sure
they are working properly and not sustaining mosquito larvae.
- Turn over all unused containers such as buckets, feed pans, and
wheelbarrows.
- Drill holes in the bottom of tires and other containers that cannot
be discarded or readily emptied (call your county mosquito agency for
guidance).
- Check all indoor and outdoor drainage systems to make sure they are
free of debris and operating properly.
- Clean gutters on all outbuildings to make sure they are not holding
water.
- Grade pastures when possible to prevent rain water from collecting
in temporary pools.
- If you discover standing water on your property and are unsure how
to eliminate it, Call Your County Mosquito Control Agency!
|
New Jersey County
Mosquito Offices |
| |
|
Atlantic County Office of
Mosquito Control
www.aclink.org/PublicWorks/mosquito/
homepage.htm |
Phone:
(609) 645-7700 Ext. 4417 or
(609) 645-5948 |
Bergen County Division of Mosquito Control
www.co.bergen.nj.us/public_works/
mosquito.html |
Phone:
(201) 634-2880 or 2881 |
|
Burlington County Mosquito
Control |
Phone:
(609) 265-5064 |
Camden County Mosquito Extermination Commission
www.camdencounty.com/
government/offices/mosquito/index.html
|
Phone:
(856) 566-2945 |
Cape May County Mosquito
Extermination Commission
www.capemaycountygov.net/Cit-e Access/
webpage.cfm?TID=5&TPID=456 |
Phone:
(609) 465-9038, 9039 |
|
Cumberland County Mosquito Control Division |
Phone:
(856) 453-2195, 2197, 2170 |
|
Essex County Mosquito Control
|
Phone:
(973) 239-0342 |
|
Gloucester County Division of Mosquito Control |
Phone:
(856) 468-0100 |
|
Hudson County Mosquito Control
|
Phone:
(201) 915-1373, 1375 |
Hunterdon County Mosquito Vector Control
www.co.hunterdon.nj.us/health/westnile.htm |
Phone:
(908) 788-1351 |
|
Mercer County Division of
Mosquito Control |
Phone:
(609) 530-7501, 7529 |
|
Middlesex County Mosquito Extermination Commission
|
Phone:
(732) 549-0665 |
Monmouth County Mosquito
Extermination Commission
www.visitmonmouth.com/mosquito/ |
Phone:
(732) 542-3630, 3631 |
Morris County Mosquito Extermination Commission
www.morrismosquito.org/ |
Phone:
(973) 538-3200
|
|
Ocean County Mosquito
Extermination Commission |
Phone:
(609) 698-8271 |
|
Passaic County Division of Mosquito Extermination
|
Phone:
(973) 305-5754, 5759 |
|
Salem County Mosquito
Extermination Commission |
Phone:
(856) 769-3255 |
|
Somerset County Mosquito Extermination/Drainage Section
|
Phone:
(908) 722-0040, 2465 |
|
Sussex County Division of
Mosquito Control |
Phone:
(973) 948-4545 |
|
Union County Bureau of Mosquito Control |
Phone:
(908) 654-9834 |
|
Warren County Mosquito
Extermination Commission |
Phone:
(908) 453-3585 |
Concentrate on the recommendations above instead of using minimally
effective mosquito control measures: e.g., fly sheets and masks; repellents
for long term control; fly misters; bug zappers.
Q. Do mosquitoes
only bite at dusk and dawn?
A. No. Most species of mosquitoes seek blood meals at dusk and dawn, but
many species are day biters and some are nocturnal. Recommendations for
turnout cannot be made until the species that transmit WNV to horses have
been identified.
Q. Is there a
vaccine for West Nile virus?
A. Yes, there are two intramuscular vaccines available for horses. The
original vaccine and the most common is West Nile-INNOVATORTM vaccine,
manufactured by Fort Dodge Animal Health, a division of Wyeth. More recently
Merial, a division of Merck and Aventis Companies, has developed RECOMBITEK® Equine
West Nile Virus Vaccine, a vaccine using recombinant DNA.
Vaccination of all healthy horses is strongly recommended. The West Nile-INNOVATORTM vaccine
requires two injections, spaced three to six weeks apart. This part is
critical in order for the vaccine to take its full effect. Immunity may not
develop for four to six weeks after the second injection. A booster is
recommended every six months to continue protection. In New Jersey 44% of
the infected horses were not vaccinated, 21% had an unknown vaccination
record and 27% were vaccinated improperly (including missing the three to
six week window). The vaccination has been shown to be 93 to 95% effective.
Some early reports in 2002 suggested that the Fort Dodge West Nile virus
vaccine, approved by the U.S. Department of Agriculture (USDA), may cause
pregnant mares to abort or give birth to deformed foals. The misleading
information in those articles has sparked many anxious phone calls from
horse owners, veterinarians, and others involved with horses. As a result,
some horse owners choose not to use the effective preventive measure against
West Nile virus available to them.
The USDA continues to recommend use of the approved vaccine as protection
against West Nile virus. Millions of doses of the Fort Dodge vaccine have
been used since the USDA’s Center for Veterinary Biologics approved its use
in 2001.
The Center for Veterinary Biologics, within the USDA’s Animal and Plant
Health Inspection Service, maintains a toll-free telephone hotline
(800-752-6255) and a mailbox on its web site, www.aphis.usda.gov/vs/cvb, and
actively encourages veterinarians and other vaccine consumers to report
problems with vaccines. The Center and the vaccine manufacturer will
continue to collect, monitor, and track the performance of this vaccine.
Q. What is the proper method of vaccinating horses for West Nile virus?
A. Previously Unvaccinated Horses
- Vaccinate all previously unvaccinated adult horses in March/April
with a two-dose primary series, three to six weeks apart.
- Talk to your veterinarian about vaccinating pregnant mares during
the first trimester.
- Vaccinate all young horses (less than one year of age) in
March/April with a three-dose primary series. Allow three to four weeks
between doses one and two, and allow six to eight weeks between doses
two and three.
- There is no minimum age recommendation for the vaccine; however,
Fort Dodge Animal Health suggests that if a mare has been vaccinated for
WNV, the foal can be vaccinated when you would normally vaccinate for
eastern and western encephalomyelitis. If the foal is from a mare that
has not been vaccinated for WNV, the foal should be vaccinated at six to
eight weeks of age and receive a series of three injections spaced three
to six weeks apart (recommended by the NJ Department of Agriculture).
Previously Vaccinated Horses
- Should maintain their spring booster when encephalitis shot is given
(preferably before the start of mosquito season).
- Should be followed up with another booster six months later (aim for
before the peak mosquito season).
Q.
Where can I get more information about West Nile virus?
A. Local mosquito surveillance and control information is available from
your county mosquito control agency, and specific questions regarding WNV
may be addressed to your local health department or for any animal health
related issues contact the NJ Department of Agriculture.
References and additional sites of West Nile virus information:
The Animal and Plant Health inspection Service (APHIS):
www.aphis.usda.gov/lpa/issues/wnv/wnv.html.
New Jersey Department of Agriculture: www.state.nj.us/agriculture/westnile.htm
or call (609) 292-3965.
Centers for Disease Control: www.cdc.gov/ncidod/dvbid/westnile/index.htm.
Department of Health and Senior Services:
www.state.nj.us/health/cd/westnile/enceph.htm.
New Jersey Department of Environmental Protection: www.state.nj.us/dep/mosquito/.
New Jersey Mosquito Biology and Control (also links to County Mosquito
Control information): www.njmosquito.org.
Mention or display of a trademark, proprietary product, or
firm in text or figures does not constitute an endorsement by Rutgers
Cooperative Extension and does not imply approval to the exclusion of other
suitable products or firms.
© 2004 by Rutgers Cooperative Extension, New Jersey
Agricultural Experiment Station, Rutgers, The State University of New
Jersey. This material may be copied for educational purposes only by
not-for-profit accredited educational institutions.
Desktop publishing by RCE/Resource Center. Published: August
2004.
RUTGERS COOPERATIVE EXTENSION
N.J. AGRICULTURAL
EXPERIMENT STATION
RUTGERS, THE STATE
UNIVERSITY OF NEW JERSEY
Distributed in cooperation with U.S.
Department of Agriculture in furtherance of the Acts of Congress on May 8
and June 30, 1914. Rutgers Cooperative Extension works in agriculture,
family and consumer sciences, and 4-H. Dr. Karyn Malinowski, Director of
Extension. Rutgers Cooperative Extension provides information and
educational services to all people without regard to race, color, national
origin, gender, religion, age, disability, political beliefs, sexual
orientation, or marital or family status. (Not all prohibited bases apply to
all programs.) Rutgers Cooperative Extension is an Equal Opportunity Program
Provider and Employer.
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