West Nile Virus

West Nile is a mosquito–borne virus that was first

detected in the United States in 1999. The virus,

which causes encephalitis, or inflammation of the

brain, has been found in Africa, Western Asia, the

Middle East, the Mediterranean region of Europe,

and, most recently, in the United States.

Mosquitoes acquire the West Nile virus (WNV)

from birds and pass it on to other birds and animals.

While humans and horses may be infected by the

virus, there is no documentation that infected horses

can spread the virus to uninfected horses or other

animals. Migrating birds appear to play a role in

spreading the disease.

U.S. History

On September 14, 1999, the National Veterinary

Services Laboratories (NVSL) of the U.S. Department

of Agriculture’s (USDA) Animal and Plant Health

Inspection Service (APHIS) isolated a virus from neurological and other tissues of flamingos and

tragopans (pheasants) from the Bronx Zoo and crows

from the New York City area. Samples of the isolated

virus were sent to the U.S. Department of Health and

Human Services’ Centers for Disease Control and

Prevention (CDC) for identification. On September

27, 1999, CDC officials announced that the isolated

virus was very similar to that of WNV, previously

unseen in the Western Hemisphere. CDC later confirmed the virus as West Nile and connected it to an

encephalitis outbreak that killed 7 people and infected at least 55 others in the New York City area in

1999. The virus has since been identified in horses,

mosquitoes, or wild birds in all 48 continental States

and the District of Columbia.

Because the virus is transmitted by mosquitoes,

it has the potential to affect livestock and poultry. In

2003, more than 5,181 equines in 48 States were

diagnosed with cases of West Nile equine encephalitis.

Clinical signs of WNV infection in horses include

ataxia (stumbling or poor coordination), depression or

apprehension, weakness of limbs, partial paralysis,

muscle twitching, or death. Fever is not often

observed.

Although the role of commercial poultry in maintaining or transmitting the virus is not thoroughly

understood, no clinical signs have been reported in

U.S. poultry. According to scientific literature, chickens can develop a short–lived infection, but clinical

signs are not seen.

Federal Actions to Protect Agriculture

Any horses showing clinical signs of severe

encephalitis in an area without previous equine cases

that year and where other common causes such as

rabies can be ruled out, will be investigated by

APHIS’ Veterinary Services (VS) program as having a

potential foreign animal disease. VS uses reliable

diagnostic tests for WNV and is on the lookout for

virus activity across the United States.

USDA’s Agricultural Research Service has conducted WNV inoculation studies with turkeys. NVSL

did the same with chickens and studies were conducted with horses at the Department of Homeland

Security’s (DHS) Plum Island Animal Disease Center

(PIADC). Only chickens showed the likely potential

to produce enough of the virus in their blood to infect

mosquitoes. Each species developed antibodies

after being inoculated, and encephalitis was not

observed in any of the animals.

VS continues to treat all situations where birds

show neurological signs as having the potential for

hosting a foreign animal disease. Such a response

allows NVSL to rule out exotic poultry diseases, such

as Newcastle disease and highly pathogenic avian

influenza, before testing for WNV.

The CDC, the U.S. Geological Survey’s National

Wildlife Health Center, and APHIS are cooperating to

survey for the virus in a wide range of wild birds.

This Federal working group, in conjunction with relevant State agencies, gathers and analyzes surveillance data to define the extent to which the virus may

be distributed in mosquito and bird populations in the

United States.

How You Can Protect Your Animals

You can decrease the chances of your animals

being exposed to WNV by decreasing their exposure

to adult mosquitoes. The best way to do this is by

removing any potential sources of water in which

mosquitoes can breed. Dispose of any water–holding

containers, including discarded tires. Drill holes in

the bottom of containers that are left outside. Clean

clogged roof gutters on an annual basis. Turn over

wading pools or wheelbarrows when not in use, and

do not allow water to stagnate in birdbaths. Aerate

ornamental pools or stock them with fish. Clean and

chlorinate swimming pools that are not in use and be

aware that mosquitoes can breed in water that collects on your property; mosquitoes can breed in any

puddle that lasts more than 4 days. Thoroughly

clean livestock–watering troughs on a monthly basis.

Local mosquito–control authorities can help in

assessing the mosquito–breeding risks associated

with your property.

Factsheet

Veterinary Services October 2004

APHIS
Pet birds can also be protected by limiting their

exposure to mosquitoes. In areas reporting large

numbers of wild bird deaths, investigations are conducted and samples collected to determine the cause

of deaths. People finding dead wild birds should notify local health officials.

Currently, there is no treatment for WNV; however,

there are currently two licensed WNV vaccines available to aid in the prevention of the disease in horses.

In November 2002, VS granted a full license to Fort

Dodge Animal Health, of Fort Dodge, IA, for the manufacture of a killed WNV vaccine. A full license was

also granted to Merial, Inc., in December 2003 for the

manufacture of a live vectored recombinant vaccine.

Use of these vaccines is now no longer restricted to

licensed veterinarians.

Additional Information

For more information about WNV, contact:

USDA, APHIS, Veterinary Services

4700 River Road, Unit 41

Riverdale, MD 20737–1231

Phone: (301) 734–8073

Fax: (301) 734–7817

For more information about human health issues

related to WNV, check the CDC Web site at

www.cdc.gov.

The U.S. Department of Agriculture (USDA) prohibits discrimination

in all its programs and activities on the basis of race, color, national

origin, sex, religion, age, disability, political beliefs, sexual orientation, or marital or family status. (Not all prohibited bases apply to

all programs.) Persons with disabilities who require alternative

means for communication of program information (Braille, large

print, audiotape, etc.) should contact USDA’s TARGET Center at

(202) 720–2600 (voice and TDD).

To file a complaint of discrimination, write USDA, Director, Office of

Civil Rights, Room 326–W, Whitten Building, 14th and

Independence Avenue, SW, Washington, DC 20250–9410 or call

(202) 720–5964 (voice and TDD). USDA is an equal opportunity

provider and employer.

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