Conditions ripe for comeback of West Nile virus

  • Article by: MAURA LERNER , Star Tribune
  • Updated: July 12, 2012 - 9:03 PM

Wet spring and hot summer are perfect for mosquitoes to breed. But nothing is certain, officials in the state say.

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Metropolitan Mosquito Control District lab is where mosquito populations are examined. The Aedes Vexans is the most common species of mosquito locally. The West Nile virus has not been tied to a certain species in Minnesota. This is a view through a microscope of dead mosquitos that had been recently captured from one trap.

Photo: Richard Sennott, Star Tribune

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Ever since the West Nile virus first showed up in Minnesota -- 10 years ago this month -- health officials have tracked it obsessively.

But for the past few years, the mosquito-borne infection has been largely missing in action. According to the official state tally, there were only eight Minnesota cases in 2010, and two last year.

Does that mean the threat is over?

Probably not, says Dave Neitzel, who tracks the virus for the Minnesota Department of Health. In fact, he says, this year's hot, dry weather might have set the stage for an upsurge -- and this is the time of year people are most likely to be infected.

"We don't have any evidence that this virus is going extinct anywhere in the U.S., much less Minnesota," Neitzel said.

So far this year, Minnesota has had one human case of West Nile illness, which developed in May in a St. Louis County man who visited south-central Minnesota.

The last big year for West Nile illness, which in rare cases can be deadly, was 2007 -- another hot, dry summer. That year, 101 cases were reported statewide. The number dropped to 10 in 2008 and hasn't hit double digits since.

"It's been a tricky one to pin down," said Kirk Johnson, an ecologist who specializes in mosquito-borne diseases at the Metropolitan Mosquito Control District. "The last couple of years I thought we would see an uptick in cases, and it just didn't occur at all."

Nationally, 712 cases were reported last year and 43 people died, including one in Minnesota, according to government reports. However, most people bitten by an infected mosquito have no symptoms, or a relatively mild fever, headache and rash. The most dangerous cases occur when infection spreads to the brain.

This year, experts say, the problem could get worse because of the wet spring and unusually hot, dry summer -- ideal breeding conditions for the particular species of mosquito (Culex) that carries West Nile. (Not all mosquitoes carry the virus.)

But that's no guarantee that more people will get sick, Neitzel said; there's also the "human side of the equation." Many people have been chased indoors by high temperatures, depriving mosquitoes of the chance to bite them.

Even the fact that mosquito populations are up this year might turn out to have a silver lining: When more people take precautions and use repellent, they keep infected mosquitoes from biting as well.

Scientists won't know how it turns out for some time; the peak infection period runs from mid-July through August.

Maura Lerner • 612-673-7384

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