Web Zap*Germs Archive

March 20, 2006

Most Chesapeake Bay Rockfish Infected With Mycobacterium

Update 26 Mar 2006: WaPo outdoors scribbler Angus Phillips writes that the "front-page story on [mycobacteriosis] in The Post two weeks ago [see my story below] sent rockfish market prices plunging by half. It came at a rough time for the sportfishing industry."
While the numbers of rockfish (striped bass) in the Chesapeake Bay have surged in recent years -- leading to huge catches by anglers and netters -- the fish have also become sick with mycobacteriosis, a bacterial infection that can spread to humans that handle the sick fish. So reports WaPo in a recent front-page story.

Approximately three-quarters of all Bay rockfish are carrying the infection, say biologists. They believe the fishes' immune systems have been weakened by swimming in the Bay's increasingly polluted waters. And they really don't know how the epidemic developed, or what to do about it. Some interesting snips:
In humans who touch the fish, the microbe can cause a skin infection known as fish handler's disease, which is not life-threatening but can lead to arthritis-like joint problems if untreated. Watermen say the only sick fish they see are in small, overcrowded rivers and streams. The netting season that ended Feb. 28 "was a super-good season as far as catching, and a good season as far as the price," said Larry Simns, president of the Maryland Watermen's Association. With no evidence of health risk from eating the fish, watermen say, prices have remained stable.

But at Ristorante Tosca in downtown Washington, "some people ask, 'Is it safe?'" chef Massimo Fabbri said of the rockfish on the menu. Such questions have prompted Fabbri to buy the restaurant's wild rockfish from Northern Europe and Ecuador, paying about three times what he would for local bass. "Wouldn't you?" he asked.

As researchers test a long list of hypotheses, they say their search for the bacterium's source and implications highlights the limitations of modern science when pitted against the complexities of the wild.
To find out when fish become infected, researchers such as Mark Matsche of the Maryland [state Department of Natural Resources] visit rockfish spawning grounds in the upper bay and the Choptank and Potomac rivers, collecting eggs and young.

"The fish are exposed to the bacteria right from the start. . . . It's ubiquitous," he found. "It can survive in water or sediment or mucus."

An infected rockfish can appear outwardly healthy. But inside, the bacteria settle first in its spleen. The creature builds walls of scar tissue in fighting it, but the infection spreads to other organs. The rockfish loses weight, even as its insides swell, and it often develops sores. At some point -- researchers do not know exactly when -- it dies.

In the bay, "by age 1, 11 percent are infected. By age 2, it's 19 percent," Matsche said. But he cannot go beyond that -- by the third year, some fish have left the bay for open water. There is no way to see the infection's progress without dissecting the fish.

"We can't even say they die for sure," Matsche said. "The severely infected fish I catch . . . a lot of them die. Some moderately infected ones have some sign of healing going on. But I'm not able to see that same fish a year down the line."

About the same time the first diseased fish appeared, some researchers grew concerned about a possible link to fish handler's disease. In Maryland, 18 cases of the skin condition were reported in 2000. In 2004, there were 46.

The Mycobacteria strain that causes the skin disease has been found only in a small percentage of diseased fish.

Michele M. Monti, director of the Waterborne Hazards Control Program at the Virginia Department of Health, said the fish handler's bacterium can also lead to other problems, including swollen lymph glands or lung disease.

Tracking the potential effect on humans is more difficult because the states do not require that the disease be reported. So, Monti said, the low number of cases "could either be because there's not a lot of it out there . . . or they haven't gotten it diagnosed."
So far, [the team of researchers headed by fish pathologist Wolfgang K. Vogelbein of the Virginia Institute of Marine Science] has found 10 strains of the bacteria in diseased rockfish, including two so new that their effect on humans is unknown.


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