United States Department of Agriculture • Animal and Plant Health Inspection Service

ANIMAL DISEASE ALERT

APHIS 91–45–015 Slighly revised September 2004

West Nile Virus:

Protecting Your

Horses

How It Spreads

West Nile virus (WNV) is a mosquito-borne virus that

was first detected in this country in 1999. Since then,

evidence of the virus has been found in all 48 of the

Continental United States. The virus causes encephalitis or inflammation of the brain and has

previously been found in Africa, western Asia, the

Middle East, and the Mediterranean region of Europe.

Only birds are known to infect mosquitoes with West

Nile virus, and mosquitoes spread the disease to

horses and humans.

West Nile Virus Transmission Cycle

Mosquitoes become

infected by feeding on

infected birds and pass

the virus to other birds,

animals, and people.

During periods of blood

feeding there is

continual transmission

between mosquitoes

and their bird hosts.

Once a bird is infected,

it can transmit the

disease to mosquitoes

for 4–5 days. Mosquitoes may feed on the

bird’s blood, far from the

initial location, and

become infected with

the virus, furthering the

spread of the disease.

The newly infected

mosquito may pass the

virus to humans or

horses. Both horses

and people are

considered “dead-end”

hosts, meaning that

while they become

infected, they do not

spread the infection.

Infected horses are not

a risk to other horses.

West

Nile

Virus

West

Nile

Virus

Clinical Signs of West Nile Virus in Horses

• Ataxia or stumbling and incoordination

• Depression or apprehension

• Weakness of limbs, partial paralysis, or the

inability to stand

• Muscle twitching or

• Death

Horses may become infected without showing any

clinical signs. Fever is not a common sign.

Figure 1—Clinical signs of WNV include stumbling, incoordination, and weakness of limbs. However, horses may

become infected without exhibiting any clinical signs.

(Photo by Maureen T. Long, College of Veterinary Medicine,

University of Florida, and used with permission.)

Protecting Your Animals

It is important to take preventive actions early, prior to

the time of the year when mosquitoes are likely to bite

and infect horses.

Vaccinate Your Horses

In November 2002, a vaccine intended to aid in

the prevention of WNV in horses was licensed by the

Veterinary Services division of the U.S. Department of

West

Nile

Virus
The U.S. Department of Agriculture is an equal

opportunity provider and employer.

Agriculture’s Animal and Plant Health Inspection

Service. This is a killed vaccine product, and its use

is no longer restricted to licensed veterinarians.

For your horses to be protected by vaccination,

they should receive the second of two initial doses of

the currently licensed vaccine at least 2 weeks before

mosquitoes are likely to bite and infect them. The

vaccine label stipulates that horses should receive an

annual booster, although some State Veterinarians

are recommending more than one booster per year.

In December 2003, Veterinary Services licensed

a live vector recombinant vaccine product for use in

horses with the same dosage regime and recommendations. This product is also available over the

counter.

There is no treatment for WNV once a horse

becomes infected. About two out of every three

horses that become ill will survive. For horses that

survive, a full recovery is likely. Horses vaccinated

against eastern equine encephalitis, western equine

encephalitis, or Venezuelan equine encephalitis are

NOT protected against WNV.

Reduce Mosquito Breeding Sites

You can decrease the chance of your animals’

being exposed to the virus by limiting their exposure

to mosquitoes. The best way to do this is to reduce

mosquito breeding sites.

Mosquitoes can breed in any puddle that lasts

more than 4 days. The best way to reduce your risk is

to remove any potential sources of standing water in

which breeding can take place.

• Dispose of water-holding containers such as old

tires.

• Drill holes in the bottom of containers that are left

outside.

• Thoroughly clean watering troughs, bird baths,

etc., every few days.

• Clean clogged roof gutters every year.

• Turn over wading pools or wheelbarrows when

not in use, and do not let water stagnate in bird

baths.

• Aerate ornamental pools.

• Clean and chlorinate swimming pools that are not

in use and do not let water collect on pool covers.

• Use landscaping to eliminate low spots where

standing water can collect.

Screened Housing

Well-maintained insect screening can be useful to

reduce exposure to adult mosquitoes if precautions

are taken to first eliminate mosquitoes from inside the

structure. Fans may reduce the potential ability of

mosquitoes to feed on horses.

Insect Repellant

Using insect repellants may help decrease

exposure of horses to adult mosquitoes. Because

under certain conditions (e.g., perspiration) some

products have a limited duration of effectiveness, it is

not wise to rely solely on repellants to prevent

mosquito exposure. Use repellants according to label

instructions. Products containing a synthetic

pyrethroid compound (such as permethrin) as the

active ingredient serve two purposes: (1) they offer

superior safety and repellent efficacy and (2) they are

contact pesticides that kill mosquitoes.

Outdoor Exposure

Mosquito species vary in their feeding habits,

making transmission possible at any time of day or

night. However, a recent epidemiologic study of WNV

suggests that keeping horses in stalls at night may be

helpful in reducing their risk of infection.

Additional Information

For more information about West Nile virus, see http://

www.aphis.usda.gov/vs. For information about human

health issues related to West Nile virus, see http://

www.cdc.gov.

Figure 2—Vaccinate your horses before mosquitoes are

likely to bite and infect them. (Photo by Dr. Darien Feary,

Colorado State University, and used with permission.)