Questions and

Answers About West

Nile Virus

Q. What is West Nile virus? 

A. 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 Eastern United States.

Mosquitoes acquire the West Nile virus (WNV) from

birds and pass it on to other birds, animals, and people. 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.

Q. Why is the U.S. Department of Agriculture's

(USDA) Animal and Plant Health Inspection

Service (APHIS) involved?

A. APHIS is the agency within USDA responsible for

protecting the health of U.S. livestock and poultry,

which can both be affected by this virus. APHIS’

National Veterinary Services Laboratories (NVSL),

the only Federal facilities in the United States dedicated to the diagnosis of both domestic and foreign

animal diseases, provide support for agency programs designed to protect the health of the Nation's

livestock and poultry. NVSL uses state–of–the–art

diagnostic techniques to rapidly determine what disease agent is present and what risk it presents to

U.S. animal health. Because WNV was killing birds

at the Bronx Zoo in 1999, zoo officials went to NVSL

for assistance in isolating the agent causing the outbreak.

Q. How did APHIS help the U.S. Department of

Health and Human Services' Centers for Disease

Control and Prevention (CDC) identify the virus? 

A. On September 14, 1999, NVSL isolated an

unknown virus from neurological and other tissues of

flamingos and tragopans (pheasants) from the Bronx

Zoo and crows from the New York City area. NVSL

sent samples of the isolated virus to CDC for identification. On September 27, 1999, CDC officials

announced that the 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 the 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 at

least 43 States and the District of Columbia.

Q. What other monitoring activities is APHIS

undertaking? 

A. The CDC, the U.S. Geological Survey's National

Wildlife Health Center, and APHIS are cooperating to

survey for WNV 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. In addition, APHIS will continue to

monitor horses for encephalitis that could be caused

by WNV.

Q. What other actions is USDA taking? 

A. APHIS officials are working with Federal, State,

and local health and agricultural officials to coordinate efforts to ensure that future WNV outbreaks do

not become a significant animal health problem.

APHIS developed a diagnostic test for the virus, conducted inoculation studies to determine the effects on

U.S. livestock, and developed a virus surveillance

plan. USDA's Agricultural Research Service conducted WNV inoculation studies with turkeys. NVSL did

the same with chickens, and NVSL's Foreign Animal

Disease Diagnostic Laboratory conducted studies

with horses. Only chickens showed the likely potential to produce enough virus in their blood to infect

mosquitoes. Each species developed antibodies

after being inoculated, and encephalitis was not

observed in any of the animals. Although no clinical

signs of the virus have been reported in U.S. poultry,

APHIS will 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 diseases, such as Newcastle

disease and highly pathogenic avian influenza,

before testing for WNV.

Q. Was the WNV outbreak the result of a 

deliberately introduced attack? 

A. There is no reason to believe that WNV was

deliberately introduced into the United States.

Factsheet

Veterinary Services March 2003

APHIS
Q. Are animals other than birds and horses

affected by the virus? 

A. Experimental tests suggest that sheep, chickens,

and pigs could be affected by WNV. Two cases of illness caused by WNV were detected in sheep in the

United States in 2002. In tests, the virus caused

pregnant sheep to abort. Cows may show antibodies

to the virus, which means they have contracted it without showing any clinical signs or becoming ill.

Q. What signs of illness do horses exhibit when

infected with the virus? 

A. NVSL positively identified WNV as the cause in

horses showing signs of encephalitis. Clinical signs of

the virus in horses included ataxia (stumbling or incoordination), weakness of limbs, partial paralysis, or

death. A fever was not often observed.

Q. What precautions can be taken to protect animals from WNV? 

A. Preventing animals' exposure to mosquitoes is

essential. 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 bird baths. 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 the water that collects on swimming

pool covers. Use landscaping to eliminate standing

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.

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 the deaths. People finding dead wild birds should

notify local health officials. No treatment is currently

available for WNV; however, APHIS Veterinary

Services is working to assist all companies interested

in producing a vaccine. On August 1, 2001, USDA

issued a conditional license to Fort Dodge Animal

Health of Fort Dodge, IA, a division of Wyeth, for a

vaccine intended to aid in the prevention of the disease in horses. In November 2002, a full license was

granted for this product. Use of this product is

restricted to licensed veterinarians.

Q. Must horses affected by the virus be 

euthanized? 

A. No. Because horses are incidental hosts, it is

highly unlikely that mosquitoes feeding on an infected

horse could ingest enough of the virus to transmit it to

other animals. 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 affected by the virus be 

quarantined? 

A. No. Since infected horses do not appear to be

carriers for the disease, it is unlikely a quarantine

would be necessary.

Q. What was the horse mortality rate in the 

affected area?

A. In 2002, more than 15,000 equine were diagnosed

with cases of illness caused by WNV. Of those, it is

estimated that approximately 33 percent died or were

euthanized. This does not rule out the possibility that

other horses may have been infected with the virus. It

is likely that many horses recover from infection without clinical illness.

Q. Are dogs and cats affected by the virus? 

A. It is unlikely that dogs or cats will show signs of

clinical illness, although any mammal or bird could

potentially be exposed to the virus through mosquito

bites. A survey of blood samples from dogs and cats

in the New York City epidemic area showed a low

infection rate.

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