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West Nile Virus
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 nerve and other tissues of flamingos and 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 Center for Disease Control and
Prevention (CDC) for identification. On September 27, CDC officials
announced that the isolated virus was very similar to that of the West
Nile Virus, 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 earlier that same year.
West Nile Virus (WNV) was first isolated in Uganda
in 1937 and has been known to cause asymptomatic infections and fevers
in humans in Africa, West Asia, and the Middle East. In 1957, the
virus was blamed for the deaths of several elderly patients in Israel.
In 1960, it was observed in horses in Egypt and France. Human and
animal infections were not documented in the Western Hemisphere until
the New York City outbreak in 1999. During 1999 and 2000, 83 humans
(with 9 deaths) and 85 horses were diagnosed with encephalitis caused
by West Nile Virus. The infections were located in numerous states
along the east coast.
In 2001, human infection with WNV occurred in 10
states with 66 cases and 9 deaths. In 2002, WNV activity has spread to
most eastern and mid-western states, with several hundred cases
reported and many deaths. According to the National Center for
Infectious Diseases, most of the WNV infections have occurred in
Louisiana, Texas and Mississippi.
2003 West Nile Virus Activity in the United States
(reported as of March 24, 2004)
Encephalitis is an inflammation of the brain and can
be caused by viruses and bacteria, including viruses transmitted by
mosquito bites. West Nile encephalitis is caused by the West Nile
Virus, a flavivirus (type of virus) commonly found in Africa, West
Asia, and the Middle East. It is closely related to the St. Louis
encephalitis virus found in the United States. It is not known from
where the U.S. virus originated, but it is most closely related
genetically to strains found in the Middle East.
Animals are also infected by West Nile Virus. Birds
are most susceptible, but horses, cats, bats, chipmunks, skunks,
squirrels, and domestic rabbits can also show symptoms of the disease.
More recently, a dog in Illinois died of the disease. Among birds, the
virus has had the greatest impact among crows. In 1999, in the New
York area, the crow population crashed by about 90 per cent in a few
months due to West Nile Virus infections.
Animals Affected by West Nile Virus
Free-Ranging Native North American Bird
Species Affected by WNV
Gull, Great Black-backed
Heron, Great Blue
Owl, Great Horned
Warbler, Black-throated Blue
Captive North American Bird Species
Affected by WNV
Other Free-Ranging Bird Species Affected by
Dove, Rock (pigeon)
Free-Ranging Mammal Species Affected by WNV
Bat, Big brown
Bat, Little brown
Pets and Other Domesticated Species
Affected by WNV
Exotic Species Housed in Zoos Affected by
Pheasant, Himalayan Impeyan
WNV is transmitted to humans and other animals
through mosquito bites. Culex pipiens, or common household mosquitos,
become infected when they feed on infected birds that have high levels
of WNV in their blood. Scientists believe that the most likely
"reservoir" for the virus in North America is the common sparrow,
which can tolerate the infection. Ten days to two weeks after the
mosquito's initial blood meal, the West Nile Virus reaches it's
salivary glands and can then be transmitted to birds, animals or
humans. When the mosquito bites a human or an animal, it injects the
virus. Once the virus is in the human or animal, it multiplies in the
blood system and crosses the blood-brain barrier to reach the brain.
The virus causes inflammation of brain tissue and interferes with
normal central nervous system functioning.
Cycle of West Nile Virus Infection
On September 4, 2002 the Centers for Disease Control
and Prevention announced that organ transplants may have been the
cause of West Nile Virus infections found in 3 transplant recipients.
Since the organ donor was positive for the West Nile Virus, they claim
that the virus could have been transmitted through the donated organs.
The CDC also confirmed that infected blood may also be a source of the
West Nile Virus. There is no evidence; however, that a person can get
infected by handling live or dead infected birds. But, to add a
further level of safety, if birds or other potentially infected
animals must be handled, a protective barrier (e.g., gloves, inverted
plastic bags) should be used.
Most WNV infected humans have no symptoms. A small
proportion develops mild symptoms that include fever, headache, body
aches, skin rash and swollen lymph glands. Less than 1% of infected
people develop more severe illness that includes meningitis
(inflammation of the spinal cord) or encephalitis. The symptoms of
these illnesses can include headache, high fever, neck stiffness,
stupor, disorientation, coma, tremors, convulsions, muscle weakness,
and paralysis. Of the few people that develop encephalitis, a small
proportion die but, overall, this is estimated to occur in less than 1
out of 1000 infections. At highest risk for becoming infected with the
mosquito-borne virus are the elderly, those with compromised immune
systems and young children.
Horses appear to be the only domestic animal
adversely affected by West Nile Virus. The virus can infect other
domestic animals, but most do not show any clinical signs.
Clinical signs of West Nile Virus infection in
horses include ataxia (stumbling and incoordination), depression or
apprehension, weakness of limbs, partial paralysis, muscle twitching,
recumbency and death. Most horses infected with WNV; however, show no
A vaccine is available for the prevention of West
Nile Virus infection in horses.
It is presently believed that West Nile Virus does
not pose a particular threat to the nation's dog population. The dog
that died (an Irish Setter / retriever mix) in Illinois was
particularly susceptible as it already had a disease that weakened its
immune system allowing West Nile to take hold.
Dr. Connie Lindley of the Texas Department of Health
says, "A lot of dogs tested have shown they've been exposed to West
Nile and never develop clinical signs. A number of other species are
exposed to it, but never show signs." Lindley says pet owners don't
need to worry. Dogs won't be the next animal at risk for the virus.
"This is not a virus mutation. This is what we've been seeing. It's
not a mutation to the virus that allowed this dog to get it. There was
something wrong with the dog itself."
Pet owners should look for unusual head bobbing,
stumbling and weakness -- but those are also symptoms of rabies -- and
health officials say that is a bigger reason to worry. If your pet
shows any unusual symptoms, it should be taken to the vet immediately.
Health experts say there is almost no chance it's West Nile -- but
that doesn't mean it's not something else.
For more information about West Nile Virus in birds
and mammals, click on the
CDC's Vertebrate Ecology Website.
There is no specific treatment for WNV infection.
Treatment of severe illnesses includes hospitalization, use of
intravenous fluids and nutrition, respiratory support, prevention of
secondary infections, and good nursing care.
On August 20, 2002, the U.S. Food and Drug
Administration cleared the way for Intron A, an interferon-type of
drug, to be used as a trial treatment for hospitalized patients with
the West Nile Virus. Intron A, sold by New Jersey-based Schering
Plough, is FDA approved for the treatment of hepatitis C.
Intron A, normally used to treat hepatitis C, may help West Nile
Based on lab research, doctors believe Intron A will
suppress the West Nile Virus and increase the immune response in sick
patients, which will minimize neurological effects. Doctors also hope
the drug will result in fewer respiratory problems and comas.
The National Institutes of Health is looking at
drugs to treat West Nile Virus in humans. Presently, there are nine
chemicals that could potentially be turned into drugs if they show
promise and the National Center for Vaccine Research is exploring the
possibility of a vaccine to prevent the disease.
At the present time mosquito protection is the way
to go. Individuals can reduce their contacts with mosquitoes by taking
these actions -- When outdoors, wear clothing that covers the skin
such as long sleeve shirts and pants, apply effective insect repellent
to clothing and exposed skin, and curb outside activity during the
hours that mosquitoes are feeding which often includes dawn and dusk.
In addition, screens should be applied to doors and windows and
regularly maintained to keep mosquitoes from entering the home.
You can decrease the chance of you and your animals
being exposed to the virus by reducing the mosquito population. 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 plastic 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 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.
Birds can also be protected by limiting their
exposure to mosquitoes. In areas reporting large numbers of bird
deaths, investigations are conducted and samples collected to
determine the cause of the deaths. People finding dead birds should
notify local health officials.
West Nile Virus Update
Click Here For 2003 West Nile Virus Activity in the
United States (reported as of March 24, 2004)
These are the human case totals that have
been reported to ArboNet from January 1, 2003 - December 31, 2003.
ArboNet is the national, electronic surveillance
system established by CDC to assist states in tracking West Nile and
other mosquito-borne viruses.
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