Fish Epidemic Traces to Novel Germ

Most angst over emerging diseases has focused on infections that suddenly strike people, the way AIDS appeared to have done in the early 1980s. However, a new study indicates that wildlife too are susceptible to new plagues.

In the late 1990s, a mysterious epidemic hit the Chesapeake Bay’s striped bass (Morone saxatilis), one of the eastern seaboard’s most prized fish. Scientists at the Virginia-Maryland Regional College of Veterinary Medicine (VMRCVM) in College Park, Md., have now pegged the cause to a new mycobacterium. In the February Journal of Clinical Microbiology, they dub the microbe Mycobacterium chesapeaki.

Genetically, it closely resembles Mycobacterium tuberculosis, which causes tuberculosis. Like that human disease, the new fish infection riddles the internal organs of its victims with tubercules–nodules filled with bacteria. In fish, the progressive, slow-growing disease can also trigger skin eruptions that come and go.

Researchers had previously found at least three mycobacteria that infect fish. Although none of these had been detected in the Chesapeake, they became prime suspects when fish ecologists opened striped bass and discovered mycobacteria’s characteristic nodules.

Ana Baya, Robert A. Heckert, and their VMRCVM colleagues found, however, that the biochemical traits of the mystery bug differed from those of the five most likely mycobacteria. So, the College Park researchers examined the microbe genetically, and their results confirm that it’s a species new to science.

Heckert’s team plans to analyze tissue and serum from striped bass collected during earlier periodic die-offs in the Chesapeake to see if the germ was present. He says such studies might help establish when the microbe entered the nation’s largest estuary.

It’s possible the agent evolved in or near this major nursery for striped bass and other marine fish. Alternatively, says A. Whitman Miller of the Smithsonian Environmental Research Center in Edgewater, Md., the germ might have been introduced by visiting freighters. Annually, he says, these ships release into the bay some 3.2 billion gallons of ballast water that they have picked up in distant ports.

About 5 years ago, routine game fish sampling by Harley Speir and his colleagues at the Maryland Department of Natural Resources in Annapolis began picking up large numbers of striped bass with external sores. The apparent disease has been most widespread during summers. Last June, Speir notes, 26 percent of the striped bass that his team netted had “external anomalies,” mostly lesions characteristic of mycobacterial infections in fish.

“What really struck us,” observes Anthony S. Overton, a fish ecologist with the University of Maryland Eastern Shore in Princess Anne, “is that the sores were appearing only on striped bass–the prettiest fish.” An extremely popular game species, these bass can live 30 years and exceed 80 pounds.

However, Overton’s data show that the unsightly lesions–which can heal and scar over-mark only a fraction of infected fish. Among the bay’s adults, he now finds that “almost 50 percent exhibit [internal mycobacterial] nodules.”

Fish infected with other mycobacteria tend to lose their appetite, weaken, and die–sometimes succumbing to secondary infections. Though Speir has no evidence that the new infection is killing striped bass, Overton finds that infected fish tend to be lean or emaciated, which is consistent with a flagging appetite.

Do the infected bass pose a human threat? “I don’t think we need to worry about that,” Baya says. To thrive, the new microbes need to incubate at a fish’s temperature, she finds. “We are too hot.”