Caution urged on Asian oysters

Hopkins study says shellfish could pose health threat to humans


A Johns Hopkins study released yesterday has concluded that Asian oysters being considered for introduction into the Chesapeake Bay could pose a health threat because the shellfish are more likely to harbor pathogens that cause intestinal illness.

"These oysters may present a public health threat upon entering the human food chain, if harvested from polluted water," Thaddeus Graczyk, an associate professor at the Johns Hopkins School of Public Health, wrote in an article published in a scientific journal.

The species, native to the China coast, could also have a positive impact on the bay, the study found. The oysters would probably improve water quality because they are better at filtering out pollution and grow faster than Chesapeake oysters, Graczyk said in an interview.

The administration of Gov. Robert L. Ehrlich Jr. is studying whether to release Asian oysters into the Chesapeake to bolster a native population that has been devastated by over-harvesting and disease.

Michael Slattery, assistant secretary of the Maryland Department of Natural Resources, said scientists will study the new report as part of their decision-making, which could continue until spring 2007.

"I don't find it terribly surprising that shellfish can at times cause human health concerns," Slattery said. "Pathogens in native oysters can also pose a human health risk. The fact is that, when collected from places where they are certified to be healthy, shellfish are safe to eat."

The study raised concerns about health threats if the oysters, Crassostrea ariakensis, are eaten raw from contaminated waters. Slattery said the state is vigilant about prohibiting oyster harvesting in unhealthful waters.

Roger Newell, a biologist who studies oysters at the University of Maryland Center for Environmental Science, said the Johns Hopkins study is interesting but of limited relevance.

"We shouldn't base our decision about whether to introduce the non-native oysters on this," Newell said. "The shellfish would only be coming from waters that are certified pure."

Graczyk said his analysis is important because it highlights the potential for shellfish contamination from protozoa that the state doesn't test for today.

The state monitors for fecal coliform, a bacteria found in sewage, and it bars watermen from harvesting shellfish in areas where it is found, Graczyk said. But the state does not test for pathogens such as the protozoa Cryptosporidium parvum and Giardia lamblia, which can also give people intestinal illnesses when eaten in raw oysters, Graczyk said.

Maryland officials don't necessarily keep watermen from harvesting shellfish in polluted areas with these pathogens, Graczyk said.

Graczyk's study, co-authored by four other Hopkins scientists, was funded by the National Oceanic and Atmospheric Administration and published in the May edition of Applied and Environmental Microbiology.

The team of scientists conducted experiments on the oysters in their lab. They added Asian oysters to tanks of water with low, medium and high levels of salt to represent the range of salinity in the Chesapeake Bay. The researchers then introduced pathogens into the tanks.

After waiting several weeks, they dissected the oysters to see how many were harboring the protozoa. The biologists compared their results with earlier studies of how long Chesapeake oysters harbored the same pathogens.

They concluded that Asian oysters harbored the pathogens up to 30 days, which is about 25 percent longer than Chesapeake oysters. In one experiment, the Asian oysters harbored the pathogens almost five times longer, Graczyk's study said.

The same protozoa used in the lab, including Cryptosporidium, live in polluted areas of the Chesapeake Bay today, Graczyk said. But few people become ill from eating raw native oysters contaminated with those protozoa, he said.

Along the Chinese coast, people sometimes become ill with gastroenteritis (diarrhea) from eating raw Asian oysters contaminated with these pathogens, Graczyk said.

Graczyk said the Asian oysters could have environmental benefits for the bay, however, by filtering out protozoa and pollution.

"The non-native oysters will really offer great ecological services, because they will remove particulates from the water. They will improve the water clarity" more than the native oysters, Graczyk said.

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