It is a virus passed by mosquito bites to humans and mosquito bites to animals such as birds and horses. However, humans do not get West Nile Virus (WNV) from birds and horses. Human-to-human transmission of WNV generally does not occur. However, human WNV infection has been associated with blood transfusions and organ transplants from infected donors. Blood donations in the U.S. are now screened for the presence of WNV.
Many people who are infected with WNV have no symptoms. Approximately 15 percent of individuals who are infected develop an illness with fever, headache, nausea, body aches, skin rash, or swollen lymph nodes. In a smaller percent of individuals infected (<1%), a more severe illness (e.g. viral meningitis or encephalitis) may develop.
The time between the mosquito bite and the onset of illness, known as the incubation period, ranges from 5-15 days in humans. Of the 9,300 confirmed human cases of WNV in the U.S. in 2003, 3 percent died. The elderly and the immunocompromised are particularly susceptible to severe illness caused by WNV. There is no specific treatment for infection with WNV.
Stay away from places where there are a lot of mosquitoes such as lakes and ponds. Stay inside at dusk and dawn when mosquitoes are more active.
When around mosquitoes:
Mosquitoes must have standing water to breed. Get rid of water around your home and yard where mosquitoes can lay their eggs.
A large or continuous die-off of birds, especially crows, ravens, or jays, may be a sign that WNV is circulating between the birds and the mosquitoes in that area. If you notice an unusual number of dead jays or crows in your area, notify Marion County Environmental Health at 503-588-5346.
Since birds from the Corvid family (crows, ravens, jays, and magpies) are very sensitive to the virus, we want to target these species for WNV testing. Only birds in the Corvid family that have been dead no more than 24 hours will be tested by Marion County. Birds killed on the road, in parking lots, or deaths related to possible pesticide spraying or injury from other animals will not be tested.
As of May 1, 2006, Marion County is no longer submitting bird carcasses for WNV testing but is instead collecting oropharangeal swab secretions from dead birds that meet the above criteria.
Birds from the Silverton and Aumsville area have recently tested positive for West Nile Virus. We will no longer test dead birds from either of these areas, but are still testing birds in other parts of Marion County.
Bird Identification for West Nile Virus (USGS Link)
Like people, most horses bitten by mosquitos will not become sick with WNV. However, of those that do, signs may include stumbling, circling, hind leg weakness, inability to stand, muscle tremors, and death. A vaccine to prevent WNV illness in horses is available from your veternarian.
Dogs and cats generally do not become ill from WNV.
Oregon WNV Information Line, Toll Free 866-703-4636
CDC West Nile Virus Homepage
Oregon DHS ACD: West Nile Virus
USDA | APHIS | WNV
USGS:NWHC - West Nile Virus
EPA and Mosquito Control
OSU: NPIC: WNV Resource
West Nile Virus Overview - PowerPoint Presentation
Fight the Bite Posters