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Asian oysters revisited: a health threat?

Asian oysters may be able to repopulate and help clean up the Chesapeake Bay, but does that mean you'd want to eat them?  There are questions about whether the fast-growing imports would be more likely than the now-depleted native oysters to pick up and pass along along human disease pathogens or viruses.

Researchers at Johns Hopkins' Bloomberg School of Public Health have found that Crassostrea ariakensis, aka the Asian or Suminoe oyster, collected and retained viruses that cause gastro-intestinal illness in humans, such as norovirus and hepatitis A.  Their findings were published in last month's Applied and Environmental Microbiology.  The Johns Hopkins student newspaper, the News-Letter, featured the study last week, after I reported on the findings of a draft Environmental Impact Statement studying Asian oysters as a possible remedy to the decimation of the bay's native oysters.

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A couple years ago, the same researchers found that Asian oysters were more likely to pick up and accumulate spores of Cryptosporidium, a water-borne microorganism that also can cause gastro-intestinal illness in humans.  That study was published in 2006 in Applied and Environmental Microbiology.

The oysters tested by researchers had been chemically sterilized, so they grew faster than reproducing oysters.  Such sterile, or triploid, oysters are being considered for use by private aquaculture operations in Virginia and Maryland because they can reach market size in a year or less, compared with three years for wild-growing native oysters.

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Oysters of all types have long been recognized as a health threat when eaten raw because of their tendency to collect pathogens from contaminated water.  It's believed that the Asian oyster may pick up more pathogens than the bay's native Eastern oyster, Crassostrea virginica, perhaps because it filters more water to feed its faster growth.

That doesn't mean the imported oyster is a health threat in and of itself.  The Hopkins scientists note that the risk comes from eating oysters taken from polluted water.  Authorities in Maryland and other states attempt to protect the consuming public from water-borne illnesses by testing and closing oyster beds in waters contaminated with animal or human waste, the most likely sources of the disease-causing organisms.  Oysters grown in private aquaculture operations presumably would be subject to more scrutiny than those harvested from the wild.

Still, the draft Environmental Impact Statement looking at putting Asian oysters in the bay - due to be officially released this week - does acknowledge the potential risk.  The report's executive summary notes that "Suminoe oysters may bioconcentrate contaminants to greater levels than Eastern oysters."

Follow the link above to see all the pros and cons to restoring the bay's oysters that are being weighed by the U.S. Army Corps of Engineers, Maryland and Virginia.

(Maryland Sea Grant College photo)

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