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Questions &Answers About West Nile Virus
Veterinary Services
March 2003
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.
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.
The U.S. Department of Agriculture (USDA) prohibits discrimination
in all its programs and activities on the basis of 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.) 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.
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