DETROIT ? They call it muskie pox. It showed up in Lake St. Clair as red sores on muskellunge last year, and this spring anglers reported seeing hundreds of big muskies floating dead in the Detroit River, apparently killed by the disease.

A similar disease, P. salmonis, was first seen in farm-reared salmon in Chile in 1989 and identified in 1992. It has caused enormous losses on Pacific and Atlantic salmon farms in South America, Europe and Canada. But biologist Mike Thomas said the Lake St. Clair muskie outbreak “is the first that it has been seen in wild fish.”

“We don’t know much about it yet,” said Thomas, a researcher at the state Department of Natural Resources’ Lake St. Clair laboratory in Macomb County. “We know that in salmon it’s caused by a bacterium called Piscirickettsia salmonis, and we found a very similar organism in dead muskellunge from Lake St. Clair.”

The disease is not known to affect people, but it poses enough of a threat to the muskellunge population that Thomas has launched a study. He is encouraging anglers to bring live large muskellunge to research vessels on Lake St. Clair this summer and fall so biologists can try to determine how many fish are infected and how the disease spreads.

Muskellunge infected with the pox exhibit red wounds similar to the open sores left by sea lampreys. The sea lamprey wound is round and depressed, but these wounds usually are more irregularly shaped and raised, like a bumpy rash.

“I’ve caught 26 muskellunge since the season opened June 7, and three of them had those rickettsia sores on them,” said Kim Phillips, a St. Clair Shores angler who spends a lot of time on the lake. “I didn’t know about rickettsia when I saw the first one, so I thought a lamprey had got it. But it was a raised, lumpy spot, and after I heard about this new disease, I knew what to look for.

“The sick fish I caught were all in the first week the season opened. I haven’t seen one since. I’m hoping that the dead fish people were seeing back in May were more susceptible to the disease because they were really stressed by spawning.”

Nearly all of the infected muskellunge have been big fish, 36 inches or more, Thomas said, but not enough is known about the disease to determine if smaller fish are less susceptible.

Lake St. Clair is the premier muskellunge fishery in North America. It has an estimated population of more than 50,000 of these biggest members of the pike family, and anglers commonly land more muskellunge in a single day on Lake St. Clair than in an entire year on lakes in other parts of the country.

The disease apparently hasn’t had any effect on the lake’s muskie fishing, which many anglers say is the best they have seen.