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Northland mosquitoes carry fatal disease

BRIMSON, Minn. - Vince Nelson placed a net full of mosquitoes into a cooler, closed the lid and checked off on his clipboard exactly where they came from.

It was one of dozens of batches of bugs he's collected in the north woods this summer and packed off to a University of Minnesota laboratory. It's the grunt work, as Nelson calls it, as wildlife and human health experts work together to find out more about northern Minnesota mosquitoes carrying a rare disease deadly to people, horses and maybe moose.

It's called eastern equine encephalitis virus and, for the first time, it's showing up in northern Minnesota wildlife.

A multitude of state, tribal and federal agencies and the university have joined forces to see if they can find out if EEEV may be part of the region's steep decline in moose population over the past decade - a decline so rapid that some experts predict Minnesota may lose all of its moose within 50 years.

Nelson, a seasonal technician with the Minnesota Department of Natural Resources Wildlife Division, has been setting and checking mosquito traps on a circuit across Lake County since June. He'll keep looking until the first hard frost, logging lots of miles on forest roads where moose once were common.

"I haven't seen a moose all summer, or not a live one anyhow,'' Nelson said. "Maybe this work can help find out why all the moose are dying off."

But the virus also is fatal to humans, and experts are worried that if it's now showing up in moose (and wolves) that it may eventually show up in people in the Northland.

"I call it the evil twin sister of West Nile virus. It's got a much higher mortality rate,'' said Erika Butler, wildlife veterinarian for the DNR. "This disease had never been found in this region before. We know we're finding it out there now. We just don't know what it's doing out there. It could be part of the moose problem."

Researchers also hope to learn more about which mosquitos carry the disease in hopes that more information might prevent EEEV from infecting humans. On average fewer than 10 cases are reported in the U.S. each year. But they are usually fatal - more than half die - and survivors usually suffer significant brain damage.

"It's a priority to find out more about this. It's not widespread, it's very rare in people, but it has a very high mortality rate,'' said Dave Neitzel, an epidemiologist for the Minnesota Department of health and one of the state's leading experts on mosquito-borne disease in humans.

An outbreak of eastern equine encephalitis occurred across the Midwest in 2001 including 75 equine cases in Wisconsin, three in Minnesota and one in Iowa. It was the first time the disease had ever been confirmed in Minnesota. No human cases have been confirmed in Minnesota. But Neitzel said many physicians may not be testing to find it, and victims may have been listed as simply "encephalitis" or related "meningitis'' on hospital or death records.

"We need to get more information out there so we can get more information back,'" he said.

In the woods

Ten stations with two traps each are scattered across northeastern Minnesota with two in the northwest. (In northwestern Minnesota, wild elk have tested positive for EEEV.) DNR staff like Nelson and some volunteers place the traps on one day and then remove them the next, once each week all summer, usually near tamarack or spruce bogs where the disease seems to be most prevalent.

One trap at each site is high in the leaves of the trees; the other is near the ground.

Mosquitoes peak from mid-June into August "but it seems like they are winding down already,'' Nelson noted. Officials say they may not be finding as many mosquitoes this summer after a several-year dry spell in Minnesota's Arrowhead.

Still, nearly 4,000 different mosquitoes lured into traps with carbon dioxide and light already have been examined in the study. Of the 51 known species of mosquitoes found in Minnesota, the new study already has identified 19 in northern Minnesota, including four of the six species known to carry EEEV.

The disease is spread from infected birds to one species of mosquito, called culiseta melanura, which specializes in biting only birds. The virus then builds up or "amplifies" in the local bird population, Neitzel said. Once it amplifies to enough birds, other species of mosquito that bite both humans and birds begin to move EEEV around - species like Aedes and the so-called "cat tail mosquito.''

It's those mosquitoes that are biting moose and wolves and probably people, usually at dawn and dusk.

About 90 percent of horses that contract the virus perish. In some areas of Cook County, as many as 25 percent of all moose sampled are carrying antibodies for EEEV, Butler said, meaning they had the virus in their blood at one point.

"These levels were high enough that, if they had been in say, a horse, the animal would very likely be displaying clinical signs of illness'' or died, Butler said. "That being said, we don't know what the implications of these titer (antibody) levels are for moose."

That means no one knows how many moose are dying because of EEEV. That could change in 2013 when moose biologists fit GPS collars on 100 moose that will send instant text messages the minute the moose dies from any cause.

"The goal is to find it fast and find out what killed it,'' Butler said, before the animal decomposes or is eaten by other critters.

It appears the disease is most common near boggy areas where certain kinds of mosquito are found. But it's not even clear how many species of mosquito can carry EEEV, or even how many species call the bogs and forest of the region home.

"We know a lot about mosquitoes in the Twin Cities area thanks to the Metropolitan Mosquito Control District. But we really don't know that much about mosquitoes in northern Minnesota,'' Neitzel said. "We want to know which ones are out there and which ones are likely to be carrying the disease and are likely to pass it on to humans."

Organizers hope to get funding to keep looking for mosquitoes in 2013.

The effort involves the Minnesota Department of Natural Resources, Minnesota Department of Health, the 1854 Authority tribal resource agency, University of Minnesota's College of Veterinary Medicine and Department of Entomology, University of Minnesota Duluth, U.S. Fish and Wildlife Service and even the Twin Cities' Metropolitan Mosquito Control District.

John Myers writes for the Duluth News Tribune