OSHKOSH, Wis. - Under Lake Winnebago's sparkling waters lurks an invader too tiny to see with the naked eye.

Wildlife officials detected the viral hemorrhagic septicemia virus in the lake system in May, alarming the state's fishing community.

Anglers fear it could devastate Wisconsin's billion- dollar-plus fishing industry, and if left unchecked, threaten waters to the west.

The virus poses no threat to humans, but can cause a wide range of trophy fish, including muskie, trout and bass, to bleed to death.

It already has spread through much of the Great Lakes, causing massive fish kills. Ohio wildlife officials estimate the virus wiped out hundreds of thousands of freshwater drum in Lake Erie last year.

The virus hasn't been detected in Lake Superior or the Mississippi River - yet - but it has appeared in inland waters in Michigan, New York and, now, Wisconsin.

"It could escalate all over the country eventually if left unchecked," said Steve Severing, assistant tournament director with Angler's Edge Premier Bass Series in Loves Park, Ill.

The North Dakota Game and Fish Department is monitoring the situation, said Scott Gangl, a fisheries management official. "We're concerned about it, though we don't currently have it here," he said of the virus.

Wisconsin wildlife officials are downplaying the virus' impact, stressing that it remains isolated in the Lake Winnebago system.

"Is it going to wipe out fishing? No," said Mike Staggs, Wisconsin's fisheries management director. "But this is a threat. A significant threat."

The virus first turned up in European fish in the 1930s and appeared on the U.S. Pacific coast in the 1980s. Scientists believe a mutant strain of the virus spread during the past few years into the Great Lakes through oceangoing ships' ballast water or imported bait.

The virus is insidious, Staggs said. It can infect more than 50 fish species - including walleye and northern pike - and is easily transmitted through fish fluids, bait fish and water.

The U.S. Department of Agriculture last year banned movement of virus-susceptible fish out of the Great Lake states as well as Ontario and Quebec. The agency has since modified the rules to allow movement of fish certified as healthy.

Because of the ban, North Dakota has been unable to bring in tiger muskie from Pennsylvania for stocking purposes, Gangl said.

Michigan, New York, Ohio, Pennsylvania and Wisconsin have adopted rules that generally prohibit moving live fish.