To the bay, Asian oysters offer hope -- and parasites

The Asian oyster touted as a replacement for the vanishing Chesapeake Bay species is a host to parasites in Chinese and Japanese waters similar to one that has already decimated the native oyster, scientists reported yesterday.

Researchers from the Virginia Institute of Marine Science said genetic and microscopic studies of the Asian bivalves have identified two Perkinsus parasites related to the microbe that produces deadly dermo infections in bay oysters, plus a herpes virus that could threaten oyster hatcheries.


Scientists will now have to determine the extent of the risk to Chesapeake species before officials here and in Virginia can make any decision about releasing the Asian oysters.

"We want to be careful not to introduce any hitchhikers, pathogens or parasites as we bring in these stocks," said Ryan Carnegie, a member of the research team at the Virginia Institute of Marine Science (VIMS).


The Asian oysters in the samples (known formally as Crassostrea ariakensis) appeared to be healthy, he said. Not all were infected, and those that were had only light infections.

"The real issue isn't that they have a parasite. I think we assumed that we were going to find something," Carnegie said. "The real issue is, what are we going to do about it here so we don't introduce a new pathogen into the environment?"

Chris Judy, director of the shellfish program at the Maryland Department of Natural Resources, played down the finding.

U.S. scientists had already learned that the Asian oysters could be infected by a Perkinsus microbe native to Virginia waters without significant mortality, he said. "That's not really news. The news is that dermo -Perkinsus - is in China and Japan. That was news to everybody."

VIMS researchers presented their findings yesterday to more than 160 scientists gathered for a Chesapeake Bay fisheries research symposium at the Patuxent Wildlife Refuge in Laurel.

It comes as the federal government gathers comments for an environmental impact study to assess the risks of the proposed introduction of Asian oysters into Chesapeake waters.

Bay populations of the native oyster, Crassostrea virginica, have collapsed, victims of two parasitic diseases - dermo and MSX.

Last year's oyster harvest in Maryland was only 53,000 bushels, down from as many as 2.5 million bushels just two decades ago. This year's landings are expected to be even worse.


Hard-hit watermen have urged scientists to quickly assess the feasibility and safety of introducing the Asian species, which appear resistant to MSX and dermo.

Carnegie said yesterday that the VIMS study, led by Gene Burreson, Kimberly Reece and Stanley Allen, gathered live and preserved samples of the C. ariakensis oysters from five sites along the coast of China and one in Japan.

Genetic tests and microscopic examinations revealed the presence of two varieties of Perkinsus, a family of single-celled microbes.

The Perkinsus family includes the parasite that causes dermo, Carnegie said, but the Asian species are different. One was previously identified in Asia and the Atlantic coast of Europe. The other is a new species, never before described.

Though the Asian oysters appear to tolerate the infections and remain healthy, Carnegie said, "we still don't know what impact [the microbes] could have on oysters locally."

The Asian parasite might also infect native clams or other Atlantic species. Or it could sicken Asian oysters if they're placed in the Chesapeake environment.


The herpes virus the VIMS study found is not one of the types responsible for a variety of human diseases, but can be fatal to oyster larvae in hatchery environments, Carnegie said.

When it has appeared in French hatcheries, he said, it has been difficult to eliminate. "So it's something we need to be very careful with."

Bill Goldsborough, senior scientist at the Chesapeake Bay Foundation, said microbes such as Perkinsus are not automatically transmitted to offspring. Proper quarantine procedures in the hatcheries should keep them isolated and out of the bay.

Viruses such as herpes, however, can survive those procedures. That, he said, is "something you would need to evalu- ate before introducing that shellfish somewhere else."

Assessment of such risks will be a focus of study for the next two years at the Center of Marine Biotechnology (COMB) in Baltimore, under a contract with the state Department of Natural Resources.

A key issue will be determining whether the Asian oyster is really resistant to the Perkinsus microbe that causes dermo, said COMB's director, Yonathan Zohar.


"What happens if conditions become extreme, if temperatures or salinity go out of range or they're exposed to any kinds of contaminants?" he said. "That may result in compromising the immune response of the oysters, and then they become much more susceptible."

COMB has not been asked to test native species for vulnerability to the Asian strains of Perkinsus or herpes that VIMS has found.

"But our scientists are interested in oyster diseases in general, and if there is another potential risk to local oysters, we will be sure to look into it," Zohar said.