Minnesota: Peak West Nile Virus Season Is Here

Ruzanna Harutyunyan's picture

With the first human cases of West Nile virus (WNV) disease for 2008 confirmed in three Minnesota residents, state health officials today urged Minnesotans to protect themselves from mosquitoes from now through September.

The first three cases of the year are residents of Becker, Carver, and Clay counties. The Clay County resident is a man in his sixties and the Becker and Carver county individuals are both under the age of 20. They all experienced onset of symptoms in early to mid-July and were later diagnosed with West Nile fever, a milder form of WNV-related illness. All three people are now recovering.

To limit further cases of this potentially life-threatening illness, Minnesota Department of Health (MDH) officials today strongly urged people to use one of the many mosquito repellent options now available and to take other simple precautions against mosquito bites this season.

“Culex tarsalis, the primary mosquito carrier of West Nile virus in Minnesota, was at low levels earlier this summer when weather was cooler than normal, but its numbers have begun to rise with recent warm weather,” said David Neitzel, an MDH epidemiologist specializing in diseases carried by mosquitoes and ticks. “Rising numbers of this mosquito species, the first human cases, and recent findings from mosquito and bird monitoring mean that the peak West Nile virus risk season has begun.”

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, where Culex tarsalis mosquitoes are most common. 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.

“We expect the risk of WNV-related illness to increase during late July and August as more Culex tarsalis mosquitoes are feeding,” said Neitzel. “This puts Minnesotans at risk for severe or even fatal illness,” Neitzel added, “but people can reduce their chance of WNV infection by using mosquito repellents at dusk and dawn.”

Mosquito repellent provides Minnesotans with an important first line of defense against mosquito bites and WNV. 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.


* For people who prefer not to use DEET, effective alternatives include repellents containing picaridin, oil of lemon eucalyptus, or IR3535.

* Repellents containing permethrin are recommended for use on clothing (not skin). Permethrin is a strong repellent and will kill mosquitoes that come into contact with treated clothing.

* 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, and to use it diligently,” 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, which carry WNV, prefer agricultural areas of central and western Minnesota. 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 Minnesota in 2002, 441 cases (including 14 fatalities) of West Nile disease have been reported to MDH. Of those, 101 occurred in 2007.

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 1 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 3 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.