More Questions & Answers About West Nile Virus
March 2003 - 2008 update
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
Q. Why is the U.S. Department of Agriculture's
(USDA) Animal and Plant Health Inspection Service (APHIS)
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
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
Q. What other actions is USDA taking?
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
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
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
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