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West Nile Virus Information


What is 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 -- an inflammation of the brain) in humans and other animals. People primarily get West Nile from the bite of a mosquito that is infected with WNV. Mosquitoes become infected with WNV when they feed on infected birds that carry the virus in their blood. In rare instances, WNV can be transmitted from person to person via blood transfusion, organ transplant, breast milk, or through the placenta.

We are discovering that the more that is learned about WNV, the more complex the disease cycle is found to be. However, there are several recurring WNV themes:

  • Corvid species of birds (crows, ravens, and blue jays) are sensitive indicators of viral presence in a particular geographic area.
  • Culex species of mosquitoes are important in the transmission of WNV to humans.
  • Hot, dry weather conditions are favorable for amplification of the virus cycle in birds and mosquitoes, particularly in urban/suburban areas.
  • 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.

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Transmission of West Nile VirusWest Nile Virus Transmission Cycle

Mosquitoes become infected with WNV when they feed on infected birds, which may circulate the virus in their blood for a few days. Infected mosquitoes can then transmit West Nile Virus to humans and animals while biting to take a blood meal. The virus is located in the mosquito's salivary glands. During blood feeding, the virus may be injected into the animal or human, where it may multiply, possibly causing illness.

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Symptoms of West Nile Virus

The risk of getting West Nile encephalitis is limited to persons in areas where virus activity occurs, and the risk is higher in persons 50 years of age or older. 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. 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. 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.

Persons with symptoms of encephalitis or meningitis should seek medical attention immediately!

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Treatment for West Nile Virus

There is no specific treatment for West Nile Virus. In more severe cases, intensive supportive therapy is needed, including hospitalization, intravenous (IV) fluids, respiratory support, prevention of secondary infections, and good nursing care.

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Prevention of West Nile Virus

There is no vaccine currently available for West Nile Virus. However, since mosquitoes need standing water in which to lay their eggs, you can reduce the risk of becoming infected by eliminating standing water and following these other tips:

  • Mosquito Life Cycle - Photo Courtesy of Leon County Mosquito Control, Tallahassee, FLGet 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. The draining and filling of wetlands for mosquito control is not an acceptable approach.

  • Keep drains, ditches, and culverts free of weeds and trash so water will drain properly.

  • Keep roof gutters free of leaves and other debris.

  • Cover trash containers to keep out rainwater.

  • Repair leaky pipes and outside faucets.

  • 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.

  • Fill in tree rot holes and hollow stumps that hold water.

  • Change the water in pet dishes, horse troughs, birdbaths and plant pots or drip trays at least once each week.

  • Store boats covered or upside down, or remove rainwater weekly.

  • Keep grass cut and shrubbery well trimmed around the house so adult mosquitoes will not hide there.

  • Make sure ornamental ponds have fish, which will eat mosquito larvae.

  • Repair window screens.

In addition to eliminating potential mosquito breeding places, it is important to protect yourself and family members from being bitten. When outdoors in the evening or when mosquitoes are biting, use personal protection measures to prevent mosquito bites, including:

  • Wear long sleeves, long pants, shoes and socks.

  • Wear insect repellent containing active ingredients approved by the EPA. This incluides DEET, picaridin (KBR 3023), oil of lemon eucalyptus (p-menthane 3,8-diol or PMD), catnip oil (Nepeta cataria, also known as catmint), IR 3535 or oil of citronella. Whenever you use an insect repellent, be sure to read and follow the label directions! Spray clothing with repellent because mosquitoes may bite through thin clothing. Wash repellent off after coming back indoors. Visit the EPA's Inspect Repellent Search Tool for help in finding the right insect repellent for your needs.

  • Use caution when using insect repellents on children. Spray the repellent on your hands and then rub it on the child. Also, avoid applying repellent to the hands of children because these products may irritate the eyes and mouth. Read the label carefully and follow instructions!

In addition, Washtenaw County encourages local municipalities to participate in larvaciding activities during the summer months. Larvaciding involves placing materials harmful to mosquito larvae into storm drains, catch basins, and other standing water sites. This procedure dramatically reduces the number of mosquito larvae, thereby reducing the number of adult mosquitoes that are able to transmit the virus. Contact your local municipality for more information on their mosquito control plans.

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Virus Surveillance

Federal, state, and local governments, along with other non-governmental organizations, are conducting disease monitoring efforts for West Nile virus infections among mosquito, bird, horse, and human populations. The Centers for Disease Control and Prevention (CDC) tracks all the data that is collected.

Dead Bird Mapping
In 1999, a connection was made between an outbreak of human West Nile virus infection and disease in birds, especially in American Crows, ravens, and blue jays. American Crows and ravens are large black birds, about 15-17 inches long from the tip of their beak to the tip of their tail. These particular birds appear to be very susceptible to the virus, and usually die within two weeks of becoming infected. Currently, the best indicator of West Nile virus activity in an area is the presence of dead crows, ravens, or blue jays. Visit the State of Michigan West Nile Virus Site for bird identification tips.

As part of the monitoring effort, citizens are asked to report dead crows, ravens, or blue jays to the State of Michigan Sick or Dead Mammal Reporting Site. Washtenaw County will not be testing any dead birds for West Nile virus.

There is no evidence that a person can get the virus from handling live or dead infected birds. However, persons should avoid bare-handed contact when handling any dead animals. If the dead bird must be handled, wear disposable rubber gloves or use a plastic shopping bag to scoop up the bird. Dispose of the bird by burying it in the ground or placing it in two plastic bags and putting it in the garbage.

Human Surveillance
Washtenaw County Public Health will partner with the Michigan Department of Health and Human Services to identify active cases of West Nile virus in Washtenaw County. Public Health will also provide information to physicians, urgent care facilities, hospitals and other caregivers regarding symptomatology, testing, reporting requirements, and treatment.

Equine Surveillance
Washtenaw County Environmental Health and MSU Extension will provide information on West Nile virus to equine veterinarians within Washtenaw County, encourage the use of the West Nile virus vaccine for horses, and monitor reports of cases of West Nile virus in horses and other animals.


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West Nile Virus & Animals

Although the vast majority of infections have been identified in birds, the national Centers for Disease Control and Prevention has received reports of West Nile virus infection in horses, cats, bats, chipmunks, skunks, squirrels, domestic rabbits, raccoons, and other creatures. Common household pets, such as dogs and cats, are unlikely to suffer from serious illness due to West Nile virus infection. Talk to your veterinarian if you are concerned about your pet becoming infected. Note: Do NOT use insect repellents containing DEET on pets!


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Michigan's History of Mosquito-Borne Illnesses

Michigan's history of mosquito-borne disease can be traced back before the arrival of the European settlers in the early 18th century. However, it wasn't until 1858 that the first documented mosquito-borne disease outbreak was identified. This occurred at the Michigan Agricultural School (now Michigan State University) where 70 students were diagnosed with malaria. Since that time, Michigan's history of mosquito-borne diseases has involved three separate concerns: malaria, dog heartworm, and encephalitis [St. Louis Encephalitis (SLE), Eastern Equine Encephalitis (EEE), and the California Group].


Locally acquired cases of malaria are rare in Michigan, but are still of potential concern because the disease is reintroduced each year by travelers and immigrants. Dog heartworm was first identified in Michigan during the early 1960's. It is currently found in the continental United States and in every county in Michigan's lower peninsula. It is of greater importance as a veterinary concern than as a human health threat.


It wasn't until the 1930's that scientific methods were able to discover mosquito-borne encephalitis viruses. EEE, SLE, and California Group viruses have been responsible for diseases requiring hospitalization of over 100 people in Michigan, with several deaths. Some of these viruses have also caused equine disease with over 200 confirmed horse deaths.



History of West Nile Virus


West Nile Virus was first discovered in Uganda, Africa, in 1937. The virus was then found in humans, birds, and other animals in Africa, West and Central Asia, and the Middle East. It is not known how West Nile virus was introduced to the United States, nor how long it has been here.

Scientists from the Centers for Disease Control and Prevention believe the virus probably entered the eastern United States during early summer of 1999, when New York City experienced numerous illnesses and deaths related to the virus. Since entering the United States, the virus has spread rapidly across the country. From 1999 through 2001, a total of 149 human cases of West Nile Virus had been detected in the United States, resulting in 18 deaths.

Michigan first experienced WNV activity in 2001, when 65 positive birds were identified in 10 counties in the Lower Peninsula. No human or equine cases were detected in Michigan in 2001.

In 2002, the United States saw a dramatic increase in WNV activity in birds, equines, and humans, with 4,156 human cases and 284 deaths. Michigan suffered the second highest number of WNV human cases in the nation with 614 laboratory positive cases, and 51 deaths. One unconfirmed human case occurred in Washtenaw County, with no deaths.

In 2003, WNV continued to spread throughout the country, resulting in 9,862 human cases and 264 human deaths. Colorado, Nebraska, and South Dakota were the hardest hit, comprising over 60% of all human cases in the United States. In Michigan, there were 19 human cases and 2 human deaths. Washtenaw County experienced no WNV-related human cases or deaths that year.

West Nile virus continued its westward-spreading trend in 2004, resulting in 2,539 human cases with 100 human deaths in the United States. California and Arizona were the hardest hit, reporting nearly half of all human cases nationwide. In contrast, many northeastern states experienced no human cases. Michigan experienced 16 human cases and no deaths, and Washtenaw County experienced no WNV-related human cases or deaths.

In 2005, the United States experienced 2,949 human cases, with 116 deaths. Most of the cases were in California, although Illinois accounted for a high percentage of cases. Michigan experienced 62 human cases with 4 deaths, with most of the cases occurring in Wayne and Kent Counties. Washtenaw County accounted for 3 of the human cases, with no deaths.

In 2006, there were increased cases nationally, with 4,256 human cases total and 165 deaths. Michigan accounted for 55 human cases, with 7 deaths. As in 2005, most of the human cases occurred in Wayne and Kent Counties. Washtenaw County had one human case, with no deaths.

In 2007, there were 3,623 human cases and 124 deaths in the United States. Michigan had 16 cases and 4 deaths. Washtenaw County had no human cases or deaths.

In 2008, there were 1,338 human cases and 43 deaths in the United States, most of which occurred in California. Michigan had 17 human cases with no deaths. Washtenaw County had no human cases or deaths.

In 2009, there were 720 human cases and 32 deaths. Michigan had one human case, with no deaths.  Washtenaw County had no human cases or deaths.

In 2010, there were 1,021 human cases and 57 deaths. Michigan had 29 human case, with 3 deaths.  Washtenaw County had 2 human cases and no deaths.

In 2011, there were 712 human cases and 43 deaths. Michigan had 34 human cases, with 2 deaths.  Washtenaw County had no human cases or deaths.

In 2012, the hot, dry weather likely contributed to the increase in West Nile activity in the United States.  There were 5,387 human cases and 243 deaths nationwide.  Michigan had 202 human cases, with 17 deaths.  Washtenaw County had 4 human cases and one death.

In 2013, there were 2,469 human cases and 119 deaths in the United States.  There were also 431 blood donors who tested positive for West Nile.  Michigan had 36 human cases and 2 deaths, and there were 4 positive blood donors.  Washtenaw County had one human case and no deaths.

In 2014, there were 2,122 human cases and 85 deaths in the United States. In addition, there were 337 positive blood donors. Michigan had 2 cases and no deaths; one of these cases was a blood donor. Washtenaw County had no cases or deaths.

In 2015, there were 2,060 human cases and 119 deaths in the United States. There were also 332 positive blood donors. Michigan experienced 18 cases and two deaths; three of the cases were blood donors. Washtenaw County had no human cases or deaths.

In 2016, there were 2,038 human cases in the United States, with 94 deaths. There were 275 blood donors who tested positive. Michigan had 43 cases, with three deaths. Washtenaw County had two human cases and no human deaths.


Washtenaw County's activities regarding WNV are a partnership involving the Washtenaw County Environmental Health Division, Washtenaw County Public Health, Michigan State University Extension, the University of Michigan, and volunteer entomologists. These units comprise the Washtenaw County West Nile Virus Task Force. The Task Force's approach to prevention focuses on communication, education, outreach activities, surveillance, personal protection and mosquito control recommendations.


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