West Nile Virus (WNV) is a mosquito-borne virus that can cause encephalitis or meningitis (an inflammation of the brain) in humans and other animals. People get West Nile from the bite of a mosquito that is infected with West Nile virus. Mosquitoes become infected with West Nile virus when they feed on infected birds that carry the virus in their blood. West Nile virus is NOT transmitted from person to person. For example, you cannot get West Nile virus from touching or kissing a person who has the infection.
West Nile virus is closely related to two other mosquito-borne encephalitis viruses in the United States: St. Louis Encephalitis (SLE) and Eastern Equine Encephalitis (EEE). These viruses have been responsible for illnesses requiring hospitalization of over a hundred people in Michigan, with several deaths. Some of these viruses have also caused equine illness with over 200 horse deaths confirmed.
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 a hundred 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. In 2002, 644 human cases and 51 human deaths occurred in Michigan alone.
The first infected bird in Livingston County was detected in the summer of 2001. Since then, the virus has been detected in more birds, but no human cases have yet occurred in Livingston County.
Transmission of West Nile Virus
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 blood. 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. Click here to see an image of the West Nile Virus transmission cycle. The virus attacks the central nervous system and can weaken the immune system and muscles. Once the virus is in the bloodstream, it can multiply and inflame brain tissue, which can lead to encephalitis or meningitis.
Likelihood of Illness From 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 older than 50 years of age or in persons with compromised immune systems. 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 much less than 1% are infected. 80% of people who become infected with West Nile Virus experience no symptoms. Only 20% of those infected will experience mild flu-like symptoms such as sore throat, fever, lymph node swelling, headache, vomiting, abdominal pain, conjunctivitis, and/or rash. This mild illness is termed West Nile Fever. Less than 1% of people who get bitten and become infected with West Nile Virus will develop a severe illness like encephalitis or meningitis.
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. More severe infection (encephalitis) is less common and 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.
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.
There is no vaccine currently available for West Nile Virus. However, you can reduce the risk of becoming infected in these ways:
- Avoid activities in areas where large numbers of mosquitoes are present.
- Wear long-sleeved shirts and long pants whenever in areas with large numbers of mosquitoes.
- Apply insect repellent to exposed skin and clothing when in areas with large numbers of mosquitoes. Avoid applying repellent to children under two years of age, and to the hands of older children.
- Maintain window and door screens to keep mosquitoes out of buildings.
- To limit breeding sites for mosquitoes, drain standing water in your yard. Empty standing water from flower pots, clogged rain gutters, swimming pool covers, discarded tires, buckets, plastic wading pools, and similar items that can collect water in which mosquitoes can lay eggs. Keep swimming pools, wading pools, and bird baths clean and filled with fresh water.
In addition, Livingston County will be encouraging local municipalities to participate in larviciding activities during the summer of 2003. Larviciding 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 or adult mosquitoes that are able to transmit the virus. Contact your local municipality for more information on their mosquito control plans.
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.
West Nile Virus was first detected in Michigan in August 2001. The virus was found in dead crows and mosquitoes; no human cases were detected. Human cases were detected in 2002. Bird surveillance during the summer of 2003 will include testing of dead birds. Once a particular zip-code has tested positive, no more birds will be accepted from that zip-code area. Mosquito trapping and surveillance for West Nile virus (WNV), St. Louis Encephalitis (SLE), and Eastern Equine Encephalitis (EEE) will also occur. Active surveillance of suspect horse and human cases will be conducted in collaboration with physicians and veterinarians across the state.
Data from mosquitoes, birds, and veterinary cases are being collected and reported on a weekly basis. Go to the State of Michigan website at www.michigan.gov/westnilevirus to view the latest information, data and area maps on West Nile Virus Activity.
In the past few years, a connection was made between an outbreak of human West Nile virus infection and disease in the corvid species of birds (specifically crows, blue jays and ravens.) Crows, blue jays and ravens 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 these dead birds.
As part of the monitoring effort, citizens are asked to report dead birds to the State of Michigan. This can be done by one of the following means:
- Call the Livingston County Health Department directly at (517) 546-9858.
- Call the State of Michigan West Nile Virus Toll-Free Hotline at (888) 668-0869.
- Use the State of Michigan Web – Based Reporting System at www.michigan.gov/westnilevirus
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. Dead birds should be placed in a double plastic bag and disposed of in the outdoor trash.
The purpose of the mosquito surveillance is to identify species of mosquitoes carrying WNV, quantify mosquito numbers, determine the proportion of which species are infected and make an assessment of potential human risk. In conjunction with the Michigan Department of Agriculture, the Livingston County Health Department has been trapping and testing mosquitoes for SLE and EEE for the past 10 years. This year the State will also be testing mosquitoes for West Nile virus. From July through September, live mosquitoes will be collected and tested on a weekly basis by LCHD personnel.
West Nile Virus & Other 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, and raccoons. 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. Also, you can visit the USGS National Wildlife Health Center website for more information on West Nile Virus in other animals.
Last Modified June 22, 2023