West Nile Virus

• WNV’s Mode of Action
• How Might West Nile Virus Affect Me?
• How Can I Protect Myself from WNV?

Background Information

West Nile Virus (WNV) is a disease transmitted primarily by mosquitoes. It was first isolated in the West Nile District of Uganda in Africa in 1937. The virus has been spreading around the world for many years. It can be found in Asia, Europe, Africa, the Middle East, India and Australia. WNV had not been known to exist in the United States or Canada until 1999 when the virus was first detected in New York. Birds, especially crows, began dying in unusually large numbers. The Culex pipiens mosquito was found to be the primary vector, but WNV can be transmitted by other mosquito species as well. As many as 36 different mosquito species from 10 genera have been identified as potential vectors. Research and testing has shown that only a few mosquito species act as the main vectors. WNV has spread to all states except Hawaii and Alaska. Since the virus came to the U.S. in 1999, nearly 42,000 human cases of WNV have been reported to the Centers for Disease Control (CDC) nation-wide. Of those, nearly 19,000 people have had neurologic disease (infections of the brain or spinal cord) and more than 1,700 people have died. Many more cases are not reported to CDC, which estimates that around 700,000 people have actually become ill due to WNV infection. From 1999-2020, there were 52,532 cases of WNV nationally, with the majority of them occurring in California and Texas.

California’s Surveillance Plan

In 2000, California expanded its well established mosquito-borne virus surveillance program to include WNV. This way, WNV was quickly detected when it arrived in southern California in 2003. The surveillance program is threefold. First, mosquito control districts and other public health agencies from around the state routinely collect mosquitoes and ship them a laboratory at the University of California, Davis, for WNV testing. Secondly, approximately 200 sentinel chicken flocks managed by mosquito control districts statewide are established each year for the purpose of testing their blood for mosquito-borne viruses. During the mosquito season, chicken blood samples are taken routinely and tested at a state public health laboratory for the presence of WNV antibodies. Lastly, the dead bird surveillance program coordinates the collecting and testing of dead birds for WNV. If a resident of California discovers a dead bird, he or she is encouraged to report the bird by telephoning toll-free, 1-877-WNV-BIRD or (1-877-968-2473). An operator will collect and screen your information. Arrangements, when warranted, will be made for trained District personnel to collect the bird and have it tested. (Never try to pick up or move a dead bird yourself and keep children and pets away). The District routinely places articles in the local paper and radio stations to make people aware of current WNV activity in our area.

WNV’s Mode of Action

WNV is transmitted to people and animals when bitten by infected mosquitoes. A mosquito acquires the infection after feeding on a bird that has virus in its blood. The virus then lives and amplifies in the mosquito’s body. Once a female mosquito is infected, she is essentially infected for life. When that mosquito subsequently takes a bloodmeal from another bird, the virus is transmitted to that bird, spreading it further through the bird population. This means birds are reservoirs or hosts for the virus. Occasionally, an infectious mosquito will not feed on a bird, but on a human, horse or other animal, resulting in possible illness. Humans and horses are considered accidental or dead end 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 commonly carry West Nile Virus in California and very few mosquitoes actually become infected. An infected human will not expose others to the disease. Human to human transmission does not normally happen with WNV. 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. Scrub Jays, Mapgpies, Crows and House Finches are hosts of the virus and are birds common in Sutter and Yuba Counties. These birds are the most commonly tested, but many more kinds of birds have been tested and found positive for the virus as well. The Cornell University of Ornithology estimates that millions of untested and unreported birds have died from WNV infections in the United States since 1999.

How Might West Nile Virus Affect Me?

Mosquito abundance and activity is at its highest between May and October in the Sutter-Yuba area. During these months, virus abundance is on the increase, making chances for disease transmission more likely. People with a higher risk of infection include the elderly, those with a weakened immune system and those with increased exposure to mosquitoes. During this time steps should be taken to avoid mosquito bites by wearing long-sleeved shirts and long pants, applying repellant and staying indoors when mosquitoes are biting. Unfortunately there is no vaccine for people as there is for horses. Hopefully a human vaccine will become available in the future.

Severe symptoms are rarely seen in children and young adults. Most people (70% – 80%) who become infected with WNV will have no symptoms. About 1 in 5 people who are infected will develop West Nile Fever with symptoms such as headache, body aches, joint pains, vomiting, diarrhea or rash. Most people recover completely, but fatigue and weakness can last for weeks or months. Less than 1% of people who are infected will develop a serious form of the disease called West Nile Neuroinvasive Disease. This includes much more serious symptoms such as bad headaches, high fever, neck stiffness, disorientation, memory loss, encephalitis, meningitis, coma, tremors, convulsion, seizures, paralysis and in approximately 10% of cases, death. Some people who have survived the neroinvasive form of the disease suffer from permanent mental or physical damage. However, the chances of becoming severely ill from any one mosquito bite are small. The information on this website is intended for educational and informational purposes only and shall not be considered medical advice, recommendation(s), diagnosis or treatment for illness. If you believe you may have contracted West Nile Virus you should contact your physician.

Number of Human Cases of West Nile Virus, 2003-2021 in Sutter and Yuba Counties

WNV activity in Horses

Horses are another concern as they are susceptible to WNV and can be protected from the disease by becoming vaccinated. The disease does not seem to be specific to or more pronounced in any particular breed of horse, but symptoms can be more severe in older horses. In 1999, there were 13 deaths in horses from WNV. As the virus spread across the United States, it is estimated that 4,500 horses died in 2002, 1,800 in 2003, and by 2004 the number had declined to 228. Fortunately a vaccine was developed to protect horses against WNV, and as a result of vaccination programs numbers of cases and deaths have continued to decline. West Nile Virus has also been associated with illness and death in other animals such as alligators, reindeer, mountain goats, squirrels. In 2002, a captive 12 year old harbor seal at the New Jersey State Aquarium died from WNV. It does not appear to cause extensive illness in dogs or cats.

Equine WNV vaccines are available from your local veterinarian. Regardless of where you obtain the vaccine, always consult with your veterinarian. Detailed vaccination records on your horses should be kept, especially if your horse travels. 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.

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 mosquito bites. Some things you can do include: drain all sources of standing water on your property, make sure your door and window screens fit tightly and are free of holes and tears, wear long sleeved shirts, long pants and shoes with socks when outside, place mosquito netting over your infant carrier or crib when outside, apply a mosquito repellant and avoid 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. 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 department for recommendations.

Some truths about mosquitoes

1. Mosquitoes breed in stagnant water that stands for more than 4 days.
2. Mosquitoes generally avoid people wearing mosquito repellant. Especially effective are repellents containing the active ingredient D.E.E.T.
3. Adult mosquitoes like to hide in dense vegetation to stay cool during the daytime. After sundown, they come out of hiding and search for people or animals to bite. Reducing vegetation around your home may reduce the number of mosquitoes there.
4. Mosquitoes bite mostly at dusk and dawn. Invasive species bite during the day (please see section on invasive mosquitoes in California).
5. Mosquitoes have a more difficult time biting through loose-fitting clothes.
6. 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 repellents 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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