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West Nile Virus
Background
West Nile Virus (WNV) is a pathogen transmitted primarily by mosquitoes. It was first isolated in the West Nile District of Uganda in 1937. The virus has caused disease and death in parts of Asia, eastern Europe, Africa and the Middle East for many years. It had not been known to exist in the United States until 1999. The virus was first detected in New York when birds, especially crows, began dying in unusually large numbers. The Culex pipiens mosquito was the primary vector, but WNV is transmitted by other mosquito species as well. Recently, 36 different mosquito species from 10 genera have been identified as infectious or potentially infectious. As of November 7, 2006, 23,383 human cases of WNV have been reported.
California's Surveillance Plan
In 2000, California expanded its well established mosquito-borne virus detection program to include surveillance for WNV. The detection program is threefold. First, mosquito control districts and other public health agencies from around the state routinely collect mosquitoes and ship them to University of California labs for WNV testing. Secondly, approximately 200 sentinel chicken flocks managed by mosquito control districts statewide are established for the purpose of testing their blood for mosquito-borne viruses. During the mosquito season, chicken blood samples are taken routinely and examined for the presence of WNV. Lastly, the state has adopted a dead bird surveillance program. If a resident of California discovers a dead bird, he or she is encouraged to telephone toll-free, 1-877-WNV-BIRD. An operator will collect and screen your information. Arrangements, when warranted, will be made for a trained government official to remove the bird and send it out for testing. (Never try to pick up or move a dead bird and keep children and pets away as live virus may be present.) In other states, unusually large numbers of dead birds and WNV positive birds have preceded WNV activity in humans.
The virus is transmitted to people and animals by infected mosquitoes. A mosquito acquires the infection after feeding on a bird with virus in its blood. The virus then lives and amplifies in the mosquito's body. When that mosquito subsequently takes a bloodmeal from another bird, the virus is transmitted to the next host. Occasionally, an infectious mosquito will feed not on a bird, but on a human or horse. Humans and horses are considered accidental hosts for WNV, as they are not important in virus maintenance in the transmission cycle and do not significantly increase virus abundance. Only certain mosquito species can carry West Nile Virus and very few mosquitoes actually become infected. An infected human will not expose others to the disease. Human to human transmission does not occur. (However, it is possible that breast-fed infants, unborn fetuses and blood and organ recipients could become infected without being bitten by a mosquito. It may also be possible to acquire the virus by contacting the fecal material of an infected bird or while gutting or cleaning an infected game bird or other animal with unprotected hands.)
The virus has been detected in more than 138 different species of birds. This number will likely increase as more species are submitted for testing. Jays, Magpies, Crows and the House Finch are all carriers and are all common to Sutter and Yuba counties. These birds also seem to suffer greatly from the virus, as it kills high percentages of them.
| Positive WNV Birds in the U.S., 1999-2002 | |
|---|---|
| Total | over 14,000 |
| American Crow | just under 8,000 |
| Blue Jay | about 5,000 |
| Other | over 1,000 |
Mosquito abundance and activity is at it highest between May and October. At the same time, virus abundance is on the increase, making chances for disease transmission more likely during this period. People with a higher risk of infection include the elderly, those with a weakened immune system and those with increased exposure to mosquitoes. Severe symptoms are rarely seen in children and younger adults.
How Might West Nile Virus Affect Me?
Most people who become infected with WNV will have no symptoms. About 20% will exhibit symptoms and most will be mild. For this reason the disease has been divided into 2 clinical categories: West Nile Fever and West Nile Meningoencephalitis (or neuroinvasive disease). In West Nile Fever, symptoms surface 3 to 15 days after the infected bite and will likely include fever, headache, nausea, body aches, mild skin rash and swollen lymph nodes. In a few cases, 1 out of 150, the disease will progress, causing meningoencephalitis. The neuroinvasive form causes a serious medical condition in which the brain and membranes covering the brain and spinal cord (called the meninges) become inflamed. This may cause the central nervous system function to become severely impaired. Symptoms of West Nile neuroinvasive disease will likely include severe headache, high fever, stiff neck, disorientation, tremors, stupor, convulsion, coma and rarely, death. However, the chances of becoming severely ill from any one mosquito bite are small. The information on this website is intended for educational purposes only and shall not be considered medical advice, recommendation(s), diagnosis or treatment for illness. If you believe you may have contracted the West Nile Virus contact your physician.
| West Nile Disease by Age and Clinical Category United States, 2002 | ||||
|---|---|---|---|---|
| Age Group | West Nile Neuro-Invasive Disease | West Nile Fever | ||
| (in years) | (number of cases) | (percentage) | (number of cases) | (percentage) |
| 0-9 | 31 | 1 | 12 | 2 |
| 19-19 | 58 | 3 | 18 | 3 |
| 20-29 | 138 | 6 | 52 | 7 |
| 30-39 | 265 | 11 | 121 | 17 |
| 40-49 | 355 | 15 | 184 | 26 |
| 50-59 | 334 | 14 | 118 | 17 |
| 60-69 | 349 | 15 | 101 | 14 |
| 70-79 | 489 | 21 | 67 | 10 |
| 80-89 | 295 | 13 | 27 | 4 |
| 90-99 | 33 | 1 | 3 | 0 |
| Total | 2,354 | 69.5 | 704 | 20.8 |
| Median Age | 59 yrs. old | 48 yrs. old | ||
Horses are another concern as they are susceptible to WNV. The disease does not seem to be specific to or more pronounced in any particular breed of horse, but symptoms seem to be more severe in older horses. Nationally, equine death rates have been about 30%. In California however, death rates have hovered below 45%.
| Annual Equine Case Counts | ||
|---|---|---|
| (year) | (number of cases) | (number of deaths) |
| 1999 | 25 | 8 |
| 2000 | 66 | N/A |
| 2001 | 738 | N/A |
| 2002 | 14,717 | 4,500 |
| 2003 | 4,426 | N/A |
| 2004 | 540 | 228 |
Equine WNV vaccines are available from your local veterinarian and recently from other suppliers, although generally in large quantities. Regardless of where you obtain the vaccine, always consult with your veterinarian. In some cases, vaccine has caused ill effects including abortion. Your vet can make specific recommendations regarding currently unvaccinated horses, pregnant mares, or newborns and foals. Interstate and international competition horses may face restrictions on where they may travel if they have high titers to WNV, which can be caused by a vaccination. Detailed vaccination records on your horses must be kept. The U.C. Davis Center for Equine Health website offers some good information on equine health and updated WNV statistics. The California Department of Food and Agriculture Animal Health Branch has a good informational website as well.
The information on this website is intended for educational purposes only. Always consult your veterinarian regarding the health of your animals.
WNV has also been associated with illness and death in other animals such as alligators, reindeer, mountain goats, squirrels and in Sept. 2002 took the life of a captive, 12 year old harbor seal at the New Jersey State Aquarium. West Nile Virus does not appear to cause extensive illness in dogs or cats.
WNV makes its presence known
West Nile Virus has created a large wake in its short run in the United States compared to either Saint Louis Encephalitis or Western Equine Encephalitis in a 40 year span.
| U.S. Human Cases | U.S. Equine Cases | ||
|---|---|---|---|
| Saint Louis Encephalitis | 4,600 cases 1963-2003 | Saint Lous Encephalitis | 0 cases 1963-2003 |
| Western Equine Encephalitis | 640 cases 1963-2003 | Western Equine Encephalitis | 2,000 cases 1963-2003 |
| West Nile Virus | 4,161 cases 1999-2003 | West Nile Virus | 15,480 cases 1999-2003 |
How Can I Protect Myself from WNV?
The best way to reduce your chances of being infected with WNV is to reduce your exposure to mosquitoes and their bites. Some things you can do include: draining all sources of standing water on your property; making sure your door and window screens fit tightly and are free of holes and tears; wearing long sleeved shirts, long pants and shoes with socks when outside; placing mosquito netting over your infant carrier or crib when outside; applying a mosquito repellant; and avoiding outside activity at dawn and dusk when mosquito activity is highest. Even small amounts of standing water can provide breeding habitat for large numbers of mosquitoes. So let your neighbors know too. Some commonly neglected or overlooked breeding sources for mosquitoes include: plugged roof gutters, tires, poorly maintained pools and spas, bird baths, pet water dishes, animal watering troughs, uncovered boats, flooded basements, uncapped metal fence posts, leaky swamp coolers, uncovered dumpsters, landscape lighting, hollowed out areas in trees, ornamental ponds, plant pots, lawn sculptures or ornaments, trash cans, and any miscellaneous containers. Any of these sources holding water for more than a few days can produce mosquitoes. If you do find a breeding source simply dump or flush the water out regularly. If you need help, call your local mosquito control district or health dept. for their recommendation.
Some truths about mosquitoes
- Mosquitoes can breed in stagnant water that lasts more than 4 days.
- Mosquitoes generally do avoid people wearing mosquito repellant. Especially effective are repellants containing the active ingredient D.E.E.T. Click the following link to see a detailed fact sheet on D.E.E.T.
- Adult mosquitoes like hiding or day resting in heavy vegetation during the daytime. Later, at feeding times, they come out looking for a bloodmeal. Reducing dense vegetation around your home may reduce the number of mosquitoes around your home.
- Mosquitoes bite the most at dusk and dawn.
- Mosquitoes have a more difficult time biting through loose-fitting clothing.
- Certain scents are attractive to mosquitoes. Some perfumes may draw mosquitoes in. Body sweat and bare, smelly feet are both particularly irresistible to them.
Check the Centers for Disease Control and Prevention website for valuable WNV information. Details about WNV and pregnancy, breast-feeding, blood transfusions, handling game animals, recommended insect repellants and other FAQ's are just a click away. CDC has also recently produced a helpful fact sheet- What you need to know.
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