Recent fish kills in Chesapeake Bay point away from Pfiesteria

Pfiesteria Outbreaks Questioned by State Scientists

The microorganism Pfiesteria piscicida has been blamed for killing large numbers of fish along the East Coast and causing human health problems for more than a decade. But a collaboration of Virginia researchers from Virginia Commonwealth University, the American Type Culture Collection and George Mason University have identified another pathogen, the effects of which may have been attributed wrongly to Pfiesteria. Unlike Pfiesteria, however, the pathogen poses no potential health threat to humans.

Two weeks ago, fishery biologists from VCU’s Center for Environmental Studies initiated the discovery after collecting and processing significant numbers of fish in the lower James River with ulcerative lesions exposing internal organs and other tissues, a symptom commonly associated with Pfiesteria’s toxic effects. The lesions were present on more than 90 percent of Atlantic menhaden, a common fish of the Chesapeake Bay.

"There was concern that this was a repeat of the 1997 Pfiesteria outbreak in Chesapeake Bay estuaries, but we were able to determine within 24 hours that that wasn’t the case," said Greg Garman, Ph.D., VCU associate professor of biology and director of the VCU Center for Environmental Studies.

Employing a relatively new molecular technique similar to DNA fingerprinting, the VCU team including Drs. Stanley Webb and Bonnie Brown, was able to rule out Pfiesteria as a possible cause for the fish kills. Pfiesteria is suspected by some of releasing potent toxins that kill fish, but no traces of the organism were found in water and sediment samples. The Virginia Department of Environmental Quality confirmed these negative results.

Using samples from the recent event, ATCC protistologist Thomas Nerad, Ph.D., identified a microscopic parasite – Kudoa clupeidae – from the lesions and muscle tissues of affected fish. Members of the VCU team confirmed the presence of this parasite in sections of muscle tissue underlying the lesions.

Kudoa is known to cause liquefaction of fish muscle tissue and as a result makes some fish harvests unsuitable for commercial use.

"Liquefaction of fish tissues by Kudoa may be the primary step in initiating many of the reported menhaden fish kills," Nerad said. "This parasite may have been overlooked in earlier studies because of the focus of the research community on Pfiesteria as well as on recent discoveries of other microorganisms in menhaden fish lesions."

The recent incident marks one of the few times Kudoa has been found in the Chesapeake Bay or its tributaries. The parasite is, however, frequently found in fish farms where high concentrations of fish, waste and nutrients make fish prone to infections.

"It may be that our coastal watersheds are creating conditions in rivers and bays that are similar to those found in a crowded fish farm," Garman said. "Non-point pollution sources such as farm runoff and multiple sewage inputs from cities may be causing Kudoa and other fish pathogens to proliferate, particularly where fish are in abundance."

Because of the high density of the pathogen in affected fish, the state researchers suspect that Kudoa may be one of the primary agents of infection in the recent event and may be responsible for previous outbreaks of dead and dying fish that had originally been attributed to Pfiesteria.

"We’ve thought for quite some time that fish kills may have been caused by a combination of environmental stress and alternative disease-causing organisms and not by Pfiesteria-produced toxins. These results support our hypotheses," said Patrick Gillevet, Ph.D., research associate professor at GMU.

The collaboration’s findings agree with a recent national report from the Centers for Disease Control that challenges early suppositions that Pfiesteria kills fish. The CDC report states that no toxins have been isolated from the organism and no known link has been made between Pfiesteria and either fish kills or human disease.

VCU, GMU and ATCC teams have been analyzing fish and sediment samples from the Chesapeake Bay for the last 12 months to evaluate the roles of environmental conditions and potential pathogens in contributing to an increase in reports of sick fish in Virginia tidal rivers.

The collaboration now will work to definitively determine pathology of Kudoa infections in menhaden and the cause of Kudoa outbreaks and conduct retrospective studies on samples from earlier Pfiesteria-like outbreaks.