If this exceptionally warm winter passes without an extended hard freeze, spring could bring an explosion of black-legged ticks, those tiny pests that transmit Lyme disease.
That promises trouble for outdoors-loving Minnesotans already being diagnosed in record numbers with tick-borne illnesses.
"The trend is that we're seeing more deer ticks at our sampling locations, and more sampling locations have deer ticks present," said Jim Stark, executive director of the Metropolitan Mosquito Control District (MMCD), which monitors tick populations year-round.
"Their mortality is much lower when the temperatures are more moderate."
Except for Thursday's subzero weather, black-legged ticks, also known as deer ticks, have slept in relative warmth below blankets of fallen leaves, Stark said. Their already substantial numbers could multiply this spring without an extended cold snap -- and in that regard, forecasters don't offer much hope.
"I suspect we've just gone through what will probably be the coldest of winter," said meteorologist Paul Douglas, who said the odds of colder days in February will diminish rapidly as the sun climbs higher in the sky.
Warnings about black-legged ticks have swept across the nation as warmer winters have allowed them to expand their habitat. Most states have issued alerts to hunters, hikers and other outdoor enthusiasts about the dangers of Lyme disease.
In Minnesota, black-legged ticks have expanded their territory since 2007. More people live in wooded areas where ticks thrive, more mice and other rodents that carry Lyme disease survive warmer winters, and more ticks "hitchhike" on birds and humans.
In the metro area, the black-legged tick has been most at home in the woodlands of Washington and Anoka counties. That's changing, Stark said, with more and more ticks found in collection traps in all seven metro counties.
Black-legged ticks don't have natural predators, and "we don't have any really good way to control tick populations," he said.
Less than 15 percent of black-legged ticks in Minnesota transmit disease, Stark said, but that rate has been sufficient to kick infections into high gear.
State Department of Health statistics show that from 1986 to 2008, more than 11,000 cases of tick-borne diseases were reported in the state. Most involved Lyme disease, a potentially serious bacterial infection.
More than 1,000 Lyme disease cases were reported in 2008; a record 1,239 confirmed cases were reported in 2007.
Last June, the health department reported the first known death in Minnesota from the tick-borne powassan virus that can cause encephalitis or meningitis. In 2010, record numbers of Minnesotans fell sick from two less common tick-borne illnesses, babesiosis and human anaplasmosis. Data from 2011 will be released in March.
"If enough ticks are able to find suitable conditions, they can survive from year to year and their populations expand," said David Neitzel, an epidemiologist at the Health Department.
But just as warm winters can keep ticks alive, so can heavy snowfalls that protect them from harsh temperatures, Neitzel said. And that's the contradiction -- unusual warm weather this winter has ensured that even modest snowfalls melt quickly. Ticks also need wet conditions to survive.
"They're more exposed so they can either freeze solid or dry out if they don't have enough leaf litter," he said. "Even if a bunch of them are killed, [their numbers] seem to recover."
It's that persistence that worries public health officials. Anyone who spends time in wooded and brushy areas should report symptoms such as fever and rashes to a doctor, Neitzel said.
"Given recent history, we definitely want folks to take precautions against ticks when they venture out in the woods this spring and summer," he said.
Black-legged ticks carry the biggest threat, he said. The American dog tick, also known as the wood tick, can cause Rocky Mountain spotted fever, but that's rare. Another tick, the Lone Star, is rare in Minnesota but also transmits disease, Stark said.
Winters in Minnesota have grown milder since 1998, Douglas said, with a steep reduction in Arctic air that was common even 20 or 30 years ago. "It's a slow-motion evolution," he said.
Kevin Giles • 651-925-5037 Twitter: @stribgiles
