A deadly Ebola-like virus is killing fish of all types in the Great Lakes, a development some scientists fear could trigger disaster for the USA's freshwater fish.

Because of a lack of genetic resistance to viral hemorrhagic septicemia, fish populations could be damaged in the same way the smallpox virus struck Native Americans and Dutch elm disease decimated elm trees, says Jim Winton, chief of fish health at the U.S. Geological Survey in Seattle.

The disease has been found in Lake Erie, Lake Ontario, Lake Huron, the St. Lawrence Seaway, the Niagara River and an inland lake in New York. The aggressive virus, which causes fish to hemorrhage, was unexpectedly found in the Great Lakes in 2005. Last year, it resulted in large fish kills that struck at least 20 species. Scientists are watching to see whether the disease returns in mid-May when water in the lakes warms to temperatures at which the virus attacks.

"VHS is the most important and dangerous fish virus known worldwide," Winton says. "Its discovery in our fresh water is disturbing and potentially catastrophic."

The United States and Canada try to contain the virus by restricting the transporting of fish and live bait and telling boaters to wash their boats when moving them between lakes. Michigan's Department of Natural Resources has taken the most dramatic action: closing hatcheries that produce three important sport fish - walleye, northern pike and muskellunge.

"The last thing you want to do is get the virus into the hatcheries and become a vehicle for spreading the virus," says Gary Whelan, who runs the state's hatcheries and chairs the multistate Great Lakes Fish Health Committee.

"What's so disturbing is that it's killing fish from so many species and with amazingly high mortality levels," says Paul Bowser, professor of aquatic animal medicine at Cornell University. The virus does not threaten humans, Bowser says. "If you cook the fish, heat will kill the virus," he says.

How VHS got into the Great Lakes is unclear. The dumping of ocean water from an international cargo ship is a suspected cause. Also not ruled out: spawning fish swimming upstream or a bird carrying a diseased fish.

Genetic tests show that the strain of VHS found in the Great Lakes probably originated in the Atlantic Ocean, near New Brunswick, Canada. That places the virus' origin near the start of the St. Lawrence River shipping route that leads to the lakes.

VHS thrives in water of 40 to 59 degrees. Most water in the Great Lakes, which contain about 20% of the world's fresh water, has not hit that temperature yet this year.

"The best-case scenario is that the virus becomes something that lurks in the background and attacks only when conditions are ripe," Whelan says. "I'm not expecting that, but I am hoping."