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First Case of West Nile in MN

Friday, July 17th, 2009 at 6:08 am

The first human case of West Nile virus (WNV) disease for 2009 has been confirmed in a Minnesota resident. State health officials are reminding Minnesotans to protect themselves from mosquitoes from now through September.

A Stevens County woman is the state’s first laboratory confirmed WNV case of the year. She became ill with West Nile meningitis in late June, was hospitalized and is recovering.

To protect themselves from this potentially life-threatening illness, Minnesota residents should be routinely using one of the many mosquito repellent options available and taking other simple precautions against mosquito bites this season, Minnesota Department of Health officials said.

David Neitzel, an MDH epidemiologist specializing in diseases carried by mosquitoes and ticks, said Minnesota has entered the period of highest risk for West Nile virus in humans. “Mid-July through September is the time of year when you’re most likely to be bitten by a mosquito carrying WNV,” he said. “While our cooler than normal weather this summer should help to slow the growth of the virus in mosquitoes and birds, we expect West Nile virus cases every year.”

Illness from West Nile virus can occur in residents throughout Minnesota and among all age groups. However, WNV risk is greatest in western and central counties, which typically have the greatest number of Culex tarsalis mosquitoes, the primary mosquito carrier of the virus in Minnesota. Also, people who are elderly or who have weakened immune systems face the highest risk of developing severe illness from a WNV infection. “They need to be especially diligent about protecting themselves from mosquitoes,” Neitzel noted.

The risk of severe or even fatal WNV-related illness typically increases during late July and August as more Culex tarsalis mosquitoes begin to feed on people instead of birds, according to Neitzel. People can reduce their chance of WNV infection by using mosquito repellents at dusk and dawn when this mosquito is most active. Several safe and effective options are now available:

* Repellents containing DEET (up to 30 percent concentration) are still the most widely used and can provide long-lasting protection against mosquito bites.
* Permethrin is a strong repellent and will kill mosquitoes that come into contact with treated clothing. Repellents containing permethrin are recommended for use on clothing (not skin).
* Effective alternatives to DEET or permethrin include repellents containing picaridin, oil of lemon eucalyptus, or IR3535.
* For maximum effectiveness and safety, all mosquito repellents should be used according to label directions.

“The key to avoiding WNV illness is to keep repellent on hand from July through September, Neitzel said.

Other important steps to prevent mosquito bites and serious illness from WNV include:

* Minimize outdoor activities at dusk and dawn, as this is prime feeding time for WNV-carrying mosquitoes. If you go outside at these times, take precautions even if mosquito numbers seem low; it only takes one bite from an infected mosquito to transmit the virus.
* Wear loose-fitting, long-sleeve shirts and long pants if you must spend time in an area where mosquitoes are biting.

Culex tarsalis mosquitoes prefer the open, agricultural areas of the state. While the majority of Minnesota’s West Nile disease has occurred in people from these areas, cases are possible anywhere within the state. Since WNV was first found in 2002 in Minnesota, 451 cases (including 14 deaths) of West Nile disease have been reported to MDH. In 2008, there were 10 cases reported in Minnesota, with no deaths.

Of those who become infected with WNV, most people will fight off the virus without any symptoms or will develop West Nile fever, the less severe form of the disease. Approximately one out of 150 people bitten by infected mosquitoes will develop severe central nervous system disease (encephalitis or meningitis). Approximately 10 percent of people with this severe form of WNV infection die from their illness, and many more suffer from long-term nervous system problems.

Symptoms usually show up three to 15 days after being bitten. They can include headache, high fever, rash, muscle weakness, stiff neck, disorientation, convulsions, paralysis and coma. Severe cases tend to occur most often in the elderly and people with immune system problems.

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