The sights around the Great Lakes this year have been shocking, with massive fish kills due to the latest foreign invader to hit the water, a deadly virus known to scientists as viral hemorrhagic septicemia.

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The virus, which doesn't harm humans or birds, has had an effect akin to a piscine plague, killing fish from Lake Erie to the St. Lawrence River. The toll is in the tens of thousands, and perhaps far higher.

In May, at Ohio's Sandusky Bay along Lake Erie, so many fish were dying that it "was described as like windrows of fish washing up on the beach," says Geoffrey Groocock, a veterinarian at Cornell University in New York State who is tracking the virus. In the St. Lawrence River, he said, divers saw "dead fish all over the bottom of the river."

VHS has stumped scientists. It was first detected in the Bay of Quinte area of Lake Ontario in 2005, although researchers have now found that a sick fish caught in Lake St. Clair in 2003 and preserved as a laboratory specimen had the disease. They don't know how the virus got into the lakes, the world's largest body of fresh water, when it arrived, or how pathogenic it ultimately will be. But they do know it is spreading rapidly, now affects at least 12 types of fish and is found across wide areas of the Great Lakes basin.

For scientists, there is dismay that yet another foreign species has become established here, and one that has a worrisome characteristic; VHS is one of the few invaders that is a pathogenic organism, or one that causes an infectious disease.

"This now puts us up to 183 established invaders in the Great Lakes, although the number of pathogenic organisms is relatively small," says Hugh MacIsaac, director of the Canadian Aquatic Invasive Species Network. The virus has also been confirmed in Lake Ontario and Lake St. Clair, and has been detected in such important sports fish as muskellunge, northern pike and walleye, as well as smallmouth bass and yellow perch.

Scientists don't know enough about the virus to determine how big a swath it will cut into fish stocks.

Another big worry is that VHS will spread throughout the inland waters of North America.

One possible route for infection is Lake Chautauqua, in New York State. It lies just 10 kilometres south of Lake Erie, but drains into a tributary of the Mississippi. "That's a direct route straight into the heartland of the U.S.," Dr. Groocock says.

Fisheries and Oceans Canada and the Canadian Food Inspection Agency are working with the Ontario Ministry of Natural Resources, as well as their counterparts in the United States, to determine the extent of the infection. Until this outbreak, the virus has been found around Japan, Northern Europe, and in northern Pacific and Atlantic coastal areas.

Although not all fish are hurt by the virus, the picture of those that are is not pretty. It destroys the lining of blood vessels, causing internal bleeding; infected fish often have bulging eyes with bleeding around the sockets, pale gills, distended, fluid-filled bellies and corkscrew swimming behaviour.

Fisheries and Oceans Canada says it is safe to handle infected fish, even if they are dying or dead, but it doesn't recommend eating them.Experts say that with the virus firmly established, it cannot be eradicated. Government agencies are urging anglers not to move live fish around from one part of the Great Lakes to another, in an attempt to slow the spread of the disease.