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West Nile Virus

West Nile virus (WNV) is a mosquito-borne virus that can cause mild illness (West Nile fever) or severe symptoms (encephalitis or meningitis - inflammation of the brain) in humans and other animals. People primarily get West Nile from the bite of an infected mosquito. Mosquitoes become infected when they feed on infected birds that carry the virus in their blood.

West Nile virus key points

  • Fight the Bite!The chances you will become severely ill from any one mosquito bite are extremely small. Even in areas where mosquitoes do carry the virus, very few mosquitoes are infected.

  • Corvid species of birds (crows, ravens, and blue jays) are sensitive to the virus. Large numbers of dead birds in a particular geographic area can be a predictor of human infection.
  • Culex species of mosquitoes are important in the transmission of WNV to humans. These mosquitoes must lay their eggs in water. They prefer water that is stagnant and nutrient-rich.
  • Hot, dry weather conditions can increase the virus cycle in birds and mosquitoes, particularly in urban/suburban areas. This can lead to a greater chance of human infection.
  • August and September are the months of greatest risk to humans for becoming infected with WNV in Michigan.
  • WNV will likely remain an issue in Michigan, causing the need for annual prevention efforts.
  • For more information, see our West Nile Virus Fact Sheet.

Dead bird reporting

Dead or sick birds, especially corvids (crows, ravens or blue jays), can indicate that West Nile virus is active in a particular area. The Washtenaw County Health Department does not test dead or sick birds for West Nile virus or any other diseases.

To report a dead bird, please enter the information on the State of Michigan Sick or Dead Bird & Mammal Reporting Website.

Symptoms of West Nile virus infection

Most people infected with West Nile virus have no symptoms of illness, but some may become ill 3-15 days after the bite from an infected mosquito. Studies have shown that about 1 in 4 infected persons will experience mild illness with fever, headache and body aches, sometimes with a skin rash and swollen lymph glands.

Less than 1% of people who get bitten and become infected with West Nile virus will develop a severe illness like encephalitis or meningitis. The risk of getting West Nile encephalitis is higher in persons 50 years of age or older. More severe infection may be marked by headache, high fever, stiff neck, disorientation, convulsions, muscle weakness and paralysis. In a few cases, mostly among the elderly, death may occur.

Prevention of West Nile virus

Follow these tips to keep yourself and your family safe:

  • Mosquitoes need water in which to lay their eggs, so eliminate standing water around your home and yard. Get rid of old tires, tin cans, buckets, drums, bottles or any water-holding containers. Fill in or drain any low places (puddles, ruts, etc.) in the yard. Keep drains, ditches and culverts free of weeds and trash so water will drain properly. Empty plastic wading pools at least once a week and store indoors when not in use. Unused swimming pools should be drained and kept dry during the mosquito season. Change the water in pet dishes, horse troughs, birdbaths and plant pots or drip trays at least once each week.
  • Use personal protection measures to prevent mosquito bites. Wear long sleeves, long pants, shoes and socks. Wear insect repellent containing active ingredients approved by the EPA -- visit the EPA's Inspect Repellent Search Tool for help in finding the right insect repellent for your needs. Spray clothing with repellent, as mosquitoes may bite through thin clothing. Read repellant labels carefully and follow instructions.
  • High numbers of dead birds can indicate that the virus is active in a particular area, so report any dead birds you find. Report dead birds at the State of Michigan Sick or Dead Bird & Mammal Reporting Website.

West Nile virus activity in Washtenaw County 


Human Cases:
WNV Fever

Human Cases:

Human Deaths

Positive Blood Donors

Horse Cases

Other Animal Cases

(updated 9/28/17)
0 0 0 0 0 5*
2016 0 2 0 0 0 1
2015 0 0 0 0 0 2
2014 0 0 0 0 0 0
2013 0 1 0 0 0 0
2012 2 2 1 2 0 1
2011 0 0 0 N/A 0 0
2010 1 1 0 N/A 1 0
2009 0 0 0 N/A 0 0
2008 0 0 0 N/A 0 5
2007 0 0 0 N/A 0 1
2006 1 0 0 N/A 2 12
2005 2 1 0 N/A 0 7
2004 0 0 0 N/A 3 7
2003 0 0 0 N/A 0 3
2002 1 0 0 N/A 6 1

*green heron in Ypsilanti area; crow in Ann Arbor area; red-tailed hawk in Saline area; house sparrow in Saline area; American kestrel in Ypsilanti area 

For more information

  • CDC: West Nile Virus - General information, national cases, local outbreak information, control issues and more

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