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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
Bittern, Least
Blackbird, Red-winged
Bluebird, Eastern
Cardinal, Northern
Catbird, Gray
Chickadee, Black-capped
Cormorant, Double-crested
Cowbird, Brown-headed
Crow, American
Crow, Fish
Dove, Mourning
Duck, Mallard
Finch, House
Flicker, Northern
Goldfinch, American
Goose, Canada
Grackle, Common
Grouse, Ruffed
Gull, Great Black-backed
Gull, Herring
Gull, Ring-billed
Hawk, Broad-winged
Hawk, Cooper's
Hawk, Red-tailed
Hawk, Sharp-shinned
Heron, Great Blue
Heron, Green
Hummingbird, Ruby-throated
Jay, Blue
Kestrel, American
Kingfisher, Belted
Mockingbird, Northern
Nighthawk, Common
Owl, Great Horned
Phoebe, Eastern
Rail, Virginia
Raven, Common
Robin, American
Skimmer, Black
Sparrow, Song
Titmouse, Tufted
Thrush, Hermit
Thrush, Wood
Turkey, Wild
Turnstone, Ruddy
Vulture, Black
Warbler, Blackpoll
Warbler, Canada
Warbler, Yellow-rumped
Warbler, Black-throated Blue
Waxwing, Cedar
Captive North American Bird Species Affected by WNV
Crane, Sandhill
Eagle, Bald
Gull, Laughing
Magpie, Black-billed
Night-Heron, Black-crowned
Owl, Snowy
Other Free-Ranging Bird Species Affected by WNV
Dove, Rock (pigeon)
Pheasant, Ring-necked
Sparrow, House
Starling, European
Swan, Mute
Free-Ranging Mammal Species Affected by WNV
Bat, Big brown
Bat, Little brown
Chipmunk, Eastern
Skunk, Striped
Pets and Other Domesticated Species Affected by WNV
Finch, Zebra
Goose, domestic
Rabbit, domestic
Turkey, domestic
Exotic Species Housed in Zoos Affected by WNV
Cormorant, Guanay
Duck, Bronze-winged
Flamingo, Chilean
Pheasant, Himalayan Impeyan
Tragopan, Blythe's

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 clinical signs.

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 victims

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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