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MPR News

West Nile virus arrives in Minnesota weeks ahead of schedule

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Trapped mosquitoes
Trapped mosquitoes Tuesday, July 9, 2013 at the Metropolitan Mosquito Control District lab in St. Paul.
Jennifer Simonson for MPR News 2013

The last time Kirk Johnson found mosquitoes infected with the West Nile virus this early, was back in 2006.

  "This virus does very well in warm weather," he said. "It's amplified more rapidly. We did have some unusually warm days this spring."  

Johnson is a vector ecologist at the Metropolitan Mosquito Control District. Every year crews set mosquito traps across the metro area and test the insects for West Nile.

Two mosquitoes collected on June 6 tested positive for the virus earlier this week. Johnson said it's as early as the virus has ever been found in Minnesota — tying the 2006 record.  

West Nile was first detected in Minnesota in 2002. It's transmitted between mosquitoes and birds, starting in the spring. Usually, Johnson says it's not common enough to be detected until mid to late June. And most human cases show up in mid-July through September.  

Last year 83 human cases of West Nile and five fatalities were reported in Minnesota.

  Minnesota epidemiologist Dave Neitzel says when the virus is found early in mosquitoes it could mean more, and earlier human cases.

  "Early transmission is a sign that the whole cycle has been sped up," he said. "The earlier that happens, the earlier the risk season can start."

  A human case of West Nile was recently diagnosed in South Dakota. Neitzel says early cases in people are fairly normal in prairie regions. The strain of mosquito — culex tarsalis — most commonly infected with the virus prefers grasslands to forest.

  Johnson says crews in the metro area are conducting their regular mosquito control programs, monitoring mosquito hatching in wetlands, and spraying larvicide from helicopters.

  "We want to stop the mosquitoes before they hatch," he said.    

  A few weeks of cooler weather could also slow down the spread of West Nile.

  West Nile virus symptoms include headache, body aches, vomiting, diarrhea, or rash. Most people recover, but some develop life-threatening infections involving the brain. People over 60 years of age are at the greatest risk for severe disease.