Recent fish kills in Chesapeake Bay point away from Pfiesteria
Pfiesteria Outbreaks Questioned by State Scientists
Tuesday, Nov. 9, 1999
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
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
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.