June’s Hot Weather And Record West Nile Mosquito Numbers Put Western Minnesotans At Risk

State health officials urge people to keep mosquito repellent handy

July 11, 2007 -- Warmer than normal weather conditions in June may put more western and central Minnesota residents at risk for West Nile virus (WNV) this summer than in previous years. While no human WNV cases in Minnesota have been reported to date, Minnesota Department of Health (MDH) officials are concerned that people may be exposed to the virus early in the WNV season, which typically begins in mid-July.

MDH monitors the abundance of Culex tarsalis, the primary mosquito carrier of WNV in Minnesota, in western and southwestern Minnesota. At most monitoring sites, June numbers of Cx. tarsalis mosquitoes were the highest recorded since 2002 when WNV was first detected in the state.

“Unusually warm weather in June led to an early spike in the numbers of the mosquito that transmits West Nile virus in Minnesota,” said David Neitzel, an MDH epidemiologist specializing in diseases carried by mosquitoes and ticks. “Warm weather also speeds up the development of WNV in mosquitoes, so there could be a greater than usual number of infected mosquitoes able to transmit WNV when they feed in July.”

In many areas, numbers of other pest mosquitoes have been relatively low with this summer’s hot and dry weather, Neitzel added. People are not being bitten as often, so they may believe their disease risk is non-existent. However, Cx. tarsalis mosquitoes and WNV thrive in this type of weather. “You may not have noticed many mosquitoes when you’ve been out at dusk this year, but unfortunately the few that are out there may be carrying WNV,” he said.

The Culex tarsalis mosquito prefers open, agricultural areas of Minnesota. In past years, the majority of Minnesota’s West Nile disease has occurred in people from these areas. Approximately 10 percent of people with the severe form of WNV infection die from their illness, and many more suffer from long-term nervous system problems after their illness. It is extremely important, therefore, that residents of western and central Minnesota take simple precautions to prevent mosquito bites.

To reduce your risk of being bitten by mosquitoes:

* While outside among mosquitoes, keep mosquito repellent handy and use it! The most effective repellents contain the active ingredients DEET (up to 30 percent), picaridin, or oil lemon of eucalyptus.
* Minimize outdoor activities at dusk or dawn, when the mosquitoes that transmit WNV are most actively feeding.
* Wear long-sleeve shirts and long pants if you must spend time in an area where mosquitoes are biting.
* Eliminate mosquito-breeding sites on and around your property, including items such as old tires, buckets, clogged rain gutters, cans, and other containers that can hold small amounts of water. Change water in birdbaths and horse troughs at least weekly.

August is usually the peak month for WNV cases in Minnesota, but people are at risk of disease from July until frost occurs. Freezing conditions kill mosquitoes or force them to find protected locations to over-winter. Thus, it is important to keep preventing mosquito bites through most of September.

Of those who become infected with WNV, most people will have no symptoms or display only mild symptoms. Approximately 1 out of 150 people who become infected will develop severe central nervous system disease (i.e., encephalitis or meningitis). 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. Since 2002, 340 Minnesotans have been reported with WNV, and 12 have died from their illness.

Information on WNV can be found at the MDH Web site: www.health.state.mn.us. People who have questions about West Nile virus can call MDH at 651-201-5414 or 1-877-676-5414 (outstate) between 8 a.m. and 4:30 p.m. Monday through Friday.

Source: Minnesota Department of Health