West Nile Virus

Common Raven / Jeanette Tasey / Great Backyard Bird Count

What is West Nile Virus?

West Nile Virus (WNV) is a flavivirus, transmitted by mosquitoes and commonly found in Africa, West Asia, Australia, and the Middle East, and is closely related to St. Louis encephalitis virus, which is also found in the United States. The virus can infect humans, birds, mosquitoes, horses and some other mammals. In birds and mammals it can cause encephalitis (inflammation of brain tissue) and/or meningitis (swelling of the tissue that encloses the brain and spinal cord). West Nile Virus was first detected in North America in the summer of 1999, in New York City, with clusters of encephalitis diagnosed in humans simultaneously with the occurrence of scores of dead birds, most notably crows, throughout several New York City neighborhoods. Within a few months, WNV had spread to Connecticut and New Jersey, and caused the death of thousands of birds. During the subsequent nine years, it has continued to spread across the continent; by the end of 2007 it had been detected in all but two U.S. states (Alaska, Hawaii) and in the District of Columbia, in seven Canadian provinces (Nova Scotia, New Brunswick, Quebec, Ontario, Manitoba, Saskatchewan, and Alberta), and in Mexico and the Caribbean.

How is West Nile Virus Spread?

West Nile Virus is primarily spread between avian and mammalian hosts by mosquitoes. More than 40 species of mosquitoes have been linked to WNV transmission, of which the most important (competent) belong to the genus Culex.  Many of these mosquito species, while mainly feeding on birds, will also facultatively feed on mammals, including humans. Culex species tend to be active between dusk and dawn, while some other species (eg. Aedes and Ochlerotatus) are active during the day (i.e., diurnal). Once the virus has been taken up in a blood-meal from an infected host (usually birds; mammals tend to have a very brief and low-level viremia, making them poor “amplification-hosts”), it multiplies in the cells of the mosquito gut. From there the virus spreads to the rest of the body, most notably the salivary glands, where large numbers of virus particles are produced. At a later blood-meal the mosquito can then transmit the virus, via the saliva, to a new host (bird or mammal). Other blood-sucking insects have lately come under scrutiny for a possible role in virus transmission. New studies in Colorado suggest that cimicid swallow bugs might be able to harbor the virus during the winter and pass it on to nestling cliff swallows, thus ensuring early seasonal amplification of the WNV.

Is West Nile Virus Seasonal?

In the temperate zone of the world (i.e., between latitudes 23.5° and 66.5° north and south), West Nile encephalitis cases occur primarily in the late summer or early fall. In the southern climates where temperatures are milder, WNV can be transmitted year round.  From its first occurrence in the USA in 1999 the WNV season has increased from a few months in late summer to almost year-round occurrence by 2004, now spanning February through December in the Southern parts of the USA. The first WNV-positive dead bird of 2005 was found in California in early February.

Has West Nile Virus been Found in Washington State?

During 2002 in Washington State, West Nile Virus was detected in six counties. A raven in Pend Oreille County, crows in Snohomish, Thurston and Pierce counties, and two horses in Island and Whatcom counties tested positive for the virus. However, there were no reports of human cases that year, nor were there any human or veterinary cases of WNV detected in Washington State in 2003, 2004 and 2005, despite extensive monitoring. In 2006 three human cases from Washington State were reported to the CDC, while none were reported in 2007. In 2008 there were again three human cases reported, while 2009 saw 38 human cases, with high activity also reported in Oregon and Idaho.  Click for up-to-date information from Seattle & King County or Washington State.

Who is at Risk for Contracting West Nile Disease?

People of any age can contract WNV, but the risk for more serious illness starts to increase after about age 50. In 2003, the median age of persons who developed the more serious neuroinvasive form of the disease was 54 years, and the median age of the 264 people who died was 77 years (median age means that half the individuals were older and half were younger). Children are not at greater risk than young or middle-age adults. People of any age on immunosuppressive treatment (such as transplant patients) or suffering other debilitating diseases may be very susceptible to WNV, and should take extra precautions against mosquito-bites.

What are the Symptoms in Humans?

Most persons who become infected with WNV develop no clinical illness or symptoms. Of the approximately 20% of infected people who do develop symptoms, most develop what has been termed West Nile fever, which presents with fever, headache, fatigue, and occasionally a skin rash on the trunk, swollen lymph nodes or eye pain.West Nile fever ranges from a mild illness to a more severe flu-like syndrome that may last from 1 to 3 weeks.

Less than 1 % of people who contract WNV infection develop a serious neurological form of illness. When the central nervous system (CNS) is affected, clinical symptoms ranging from febrile headache to aseptic meningitis to encephalitis may occur, and these are usually indistinguishable from similar syndromes caused by other viruses, such as St. Louis encephalitis or polioviruses. There is usually an alteration of consciousness, which can be mild, resulting in lethargy, or which may progress to confusion or coma. Limb or flaccid paralysis or cranial nerve palsies (as in a stroke) may be observed. Tremors and movement disorders can also occur. Respiratory failure appears to be common in patients with CNS-involvement.

What Bird Species are Vulnerable to West Nile Virus?

To date, the WNV strain circulating in North America (the New York [NY] strain) has been isolated from more than 300 bird species, with the bulk (80 %) of positive cases found amongst corvids (crows, jays, ravens, and magpies). In areas with low population densities of corvids, other species such as Cooper’s and red-tailed hawks, house sparrows and house finches are amongst the more commonly infected bird species. Of note is that in contrast to chickens, galliform birds of the family Phasianidae (game birds) appear to be highly susceptible with the greater sage grouse experiencing as much as 100 % fatality upon infection with the NY strain of WNV.

What are the Symptoms of West Nile Virus in Birds?

As in humans the majority of individual birds infected do not get sick, although the species differences in susceptibility are very marked. Most birds that do get sick die within 24-48 hours of becoming lethargic, unbalanced, and unable to maintain normal body posture. In infected raptors the following symptoms have been observed: initially the birds are depressed and anorexic with consequent rapid weight loss. This is followed by head tremors, green waste products (indicating liver damage), blindness, ataxia and weakness in legs, and general lack of awareness of surroundings. In the final phase preceding death there are severe tremors and seizures. Many of these symptoms reflect virus infection of and inflammation in the brain and spinal cord, but many other tissues are also damaged, including heart, lungs, kidneys, spleen and liver, explaining the virtual inevitability of a fatal outcome in susceptible species. Under experimental conditions, house sparrows may clear the infection, but it remains unknown whether that happens in the wild.

Are Pets and Domestic Animals at Risk of West Nile Virus?

Fortunately, clinical illness due to WNV is rare in dogs and cats. Chickens are resistant as well. Persons with pet birds should be careful to protect them from mosquito bites. Horses and other equines like mules and donkeys are susceptible to WNV infection and severe illness and death can result. Several efficacious equine WNV vaccines are now available, and horse owners are strongly encouraged to consult with their veterinarian about immunization. Horses will also benefit from mosquito control efforts.

What can I do to Avoid Exposure to West Nile Virus?

Protect yourself and your family in two ways: (1) by reducing mosquito breeding habitat and, (2) by taking personal protection measures. Help elderly neighbors and relatives, too, since they are the most at risk for serious WNV infection or death.

To reduce mosquito breeding opportunities:
  • Reduce standing water in the vicinity of residences as much as possible by removing unused containers, and emptying water from flowerpots, barrels, pools, birdbaths and boat covers every few days.
  • Clean out gutters and remove tarps, tires, and other objects where water can collect.
  • Turn over wheelbarrows and plastic wading pools when not in use.
  • Keep swimming pools clean and chlorinated.
  • Aerate ornamental pools or stock them with mosquito-eating fish
  • Check trees for cavities that hold water and fill them with soil or sand.
To protect against mosquito bites:
  • Avoid getting bitten by mosquitoes, by (i) reducing outdoor activity at dawn and dusk, when most mosquitoes are most active, (ii) wearing long-sleeved shirts and other clothing that provides protective coverage, and (iii) using insect repellents. In addition to DEET and citronella there are several new products coming on the market (eg., picaridin which is widely used in Europe and Australia and is purported to be longer-lasting in effect than DEET). Read more from the Centers for Disease Control and Prevention.
  • Install screens on your windows and outside doors and check them regularly for defects.
  • Please do not use black light “bug zappers”: they destroy moths and other beneficial insects.  And remember that alternatives to spraying are preferred because many birds require insects for food.  Mosquito “suckers” such as the Mosquito MagnetTM, which emits CO2 and heat thereby attracting biting insects and subsequently vacuuming them into a bag, have received mixed reviews. They are expensive (approximately $ 500 and up) and may actually attract more mosquitoes to the area because of the CO2emission.
What can I do if I Find a Fresh, Dead Bird?

You can help by reporting dead birds, especially crows, jays, magpies, ravens or raptors, to your local health department (for birds found in Seattle-King County, call Public health at 206-205-4394 or use their web-based reporting. In other counties, find the reporting location nearest you. You will be advised on whether the bird should be submitted for testing. Keep in mind, however, that although it is important for public health officials to track dead bird sightings, most dead birds will not be collected or tested.

In the meantime, place the bird in a plastic bag (eg. a Ziploc™ bag) with a listing of the location, time and date and put it in the fridge.

Although there is no evidence that you contract WNV by handling dead birds, avoid bare-handed contact. Use gloves or double plastic bags to secure the carcass for collection by health officials or to dispose of it in a garbage can, if it is not collected.

Additional information relevant to Washington State residents can be found on the following websites:

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