Summer is coming to an end, but Minnesotans should still be careful to avoid mosquito bites, the Minnesota Department of Health said.

So far this year, the Minnesota Department of Health has reported 20 cases of illness from West Nile virus. Two of the affected people have died. According to data from the Centers for Disease Control and Prevention, several cases have been reported in southwest Minnesota.

“This is a high-risk time of year for West Nile virus transmission,” Elizabeth Schiffman, supervisor of MDH’s Vectorborne Diseases Unit, said in a news release. “It’s important to prevent mosquito bites when people are enjoying time outside while the weather is still nice. That’s the best way to avoid getting West Nile virus.”

West Nile virus is transmitted to people through the bite of an infected mosquito. The virus was first found in Minnesota in 2002, the MDH said.

Sept. 2 data from the CDC reported West Nile activity in humans and animals in a total of 32 Minnesota counties.

So far, there have been three human cases of West Nile virus in Lyon County this year, the CDC reported. Lincoln, Pipestone, Murray, Cottonwood and Renville counties each had one West Nile case reported this year.

West Nile activity can also be detected through blood donors, said Schiffman. People who donate blood may not have symptoms of West Nile, but the virus can be found in their blood when it is screened.

“We’re seeing a lot of activity in that group this year,” Schiffman said.

This year there have been two cases where West Nile virus was found in donated blood in Lac qui Parle County, and one in Watonwan County, according to CDC data.

Virus activity in animals has been reported in Sibley and Le Sueur Counties, the CDC said.

Western Minnesota residents are at greater risk from West Nile virus, because open areas like farmland and prairie provide good habitat for the type of mosquito that can carry the virus, the MDH said.

The number of cases of West Nile in Minnesota can be affected by a few different factors, Schiffman said.

“This has been a year with favorable conditions for mosquitoes,” Schiffman said.

Hot weather is one factor that can speed up the natural cycles that produce mosquitoes, she said. Having regular rainfall, like we did this summer, also supports mosquito populations.

August and September tend to be a time of year with increased West Nile activity.

“It’s not quite the fall season yet, so the mosquitoes are out,” Schiffman said. And while parts of Minnesota have had fall-like weather over the past couple of weeks, there’s still a risk of being bitten by mosquitoes, especially in the morning and evening. Schiffman said it’s a good idea to take precautions to avoid insect bites.

Most people who are bitten by mosquitoes infected with West Nile virus will either experience no symptoms, or have a mild flu-like illness. However, in some people West Nile can lead to severe symptoms like meningitis or encephalitis.

Elderly people and people with weakened immune systems are at greatest risk for severe disease, the MDH said.

People who work outside or participate in outdoor activities are also at higher risk of being exposed to mosquitoes, and to West Nile virus.

There are a few different steps people can take to protect themselves from mosquito bites, the MDH said. Using insect repellents that contain up to 30% DEET is one way, as is wearing loose-fitting, long-sleeved shirts and pants. People should also avoid outdoor activities at dawn and dusk, which are the peak feeding times for mosquitoes. Maintaining window screens can also help keep mosquitoes out of homes.

Schiffman said Minnesota typically sees up-and-down patterns in the number of West Nile cases reported from year to year. The last spike in West Nile cases was in 2023, she said.

“So far, patterns are looking like we would kind of expect,” Schiffman said. But data was still coming in, she said.

According to MDH data, a total of 35 West Nile cases were reported in Minnesota in 2024. That was down from 2023, when there were more than 60 reported cases.