Peak West Nile virus season is here again: Minnesota reports first West Nile virus case of 2010

The first human case of West Nile virus (WNV) disease for 2010 in a Minnesota resident has been confirmed. State health officials are reminding Minnesotans to protect themselves from mosquitoes from now through September.

The first human case of West Nile virus (WNV) disease for 2010 in a Minnesota resident has been confirmed. State health officials are reminding Minnesotans to protect themselves from mosquitoes from now through September.

A Yellow Medicine County man is the state's first laboratory-confirmed WNV case of the year. He was hospitalized with West Nile encephalitis in late July.

To protect themselves from this potentially life-threatening illness, Minnesota residents should routinely use one of the many mosquito repellent options available and take other simple precautions against mosquito bites this season, Minnesota Department of Health (MDH) 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.

"While Minnesotans have noticed lots of mosquitoes throughout the summer, August is typically the highest risk month for WNV," he said. "We've had a bumper crop of pest mosquitoes this summer, but the species that carry WNV are most abundant in late summer."


Illness from WNV 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. Elderly people face the highest risk of developing severe or even fatal illness from a WNV infection.

"They need to be especially diligent about protecting themselves from mosquitoes," Neitzel noted.

The risk of WNV-related disease 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.

"Repellent use from July through September is the key to avoiding WNV disease," Neitzel said.

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 applied to clothing (not skin). Treated clothing can be worn after the repellent dries.

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.

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, which carry WNV, prefer open, agricultural areas of central and western Minnesota and other Great Plains states. While the majority of Minnesota's WNV disease has occurred in people from these areas, cases are possible anywhere within the state. Since WNV was first found in Minnesota in 2002, 455 cases (including 14 fatalities) of WNV disease have been reported to MDH. In 2009, four cases were 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 and 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.

Information on WNV can be found at the MDH website: . Further information on repellents is at the CDC website: .

People who have questions about WNV can call MDH at 651-201-5414 or 877-676-5414 (outstate) between 8 a.m. and 4:30 p.m. Monday through Friday.

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