“We’re concerned that this virus could get out of the Great Lakes and affect other populations,” Jill Roland, a fish pathologist and assistant director for aquaculture for the USDA in Riverdale, Maryland, said in a telephone interview.

“The virus could potentially affect the catfish industry,” she said.

Catfish make up the largest sector of the $1 billion U.S. aquaculture industry, accounting for $462 million in sales, according to a 2005 USDA aquaculture census.

The public first began hearing about the virus after a die-off of fish in Lake Ontario, Lake Erie and the upper St. Lawrence River in May 2006 with dead fish washing up on beaches.

There is little the government can do to prevent the spread of the disease, other than limiting human movement of fish that may have the virus.

Fish caught in the Great Lakes may be used as bait in other parts of the country, with extra fish dumped into the water. Commercial farms sometimes get their breeding stock from wild fish.

However, fish migrate naturally and the Great Lakes does connect with the Mississippi River, a major waterway that runs to the Gulf of Mexico.

Scientists believe the VHS virus has been in the Great Lakes since 2003. Its origin is unknown but researchers think it may have come from bilge water released from one or more of the hundreds of ocean-going vessels plying the Lakes.

Another strain of the virus has affected trout and other freshwater fish raised commercially in Europe for several years.

“It has caused huge problems,” said Roland of the virus in Europe.