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May 26, 2006

Nonnative Oysters in Chesapeake Bay Offer Benefits and Risks

The introduction of Asian oysters (Crassostrea ariakensis) to the waters of the Chesapeake Bay holds potential benefits to the environment as well as public health risks, according to a study by researchers at the Johns Hopkins Bloomberg School of Public Health. Findings showed that C. ariakensis, when grown in aquacultures, effectively and efficiently remove human pathogens from the water. However, the researchers also found that some human pathogens accumulated and persisted in Asian oysters up to five times longer than in native oysters. They concluded that the nonnative oysters could pose a health threat if harvested from polluted waters and consumed raw. The study is the first to examine the public health impact of nonnative oysters in the Chesapeake Bay and is published in the May 2006 edition of Applied and Environmental Microbiology.

Some policymakers have proposed introducing the nonnative oyster, C. ariakensis, to replace the dwindling population of the native oyster, C. virginica. The study authors note that C. ariakensis holds promise for rebuilding the oyster harvesting industry because it grows to market size in one year, half the time it takes native oysters to reach market size.

“Introducing nonnative oysters to the Chesapeake Bay is filled with unknowns and may have unintended consequences,” said Thaddeus K. Graczyk, PhD, associate professor in the Bloomberg School’s departments of Environmental Health Sciences and Molecular Microbiology and Immunology. “Our study indicated that nonnative oysters could provide excellent ecological services in regard to cleaning the Bay of infectious agents. However, these environmental benefits are associated with public health risk when these oysters are intended for human consumption.”

The study was conducted by observing commercial-sized C. ariakensis in a 30-gallon tank of water. The oysters were exposed to varying levels of water salinity to simulate conditions in the Chesapeake Bay and to several human pathogens, including: Cryptosporidium, Giardia lamblia, which are common causes of diarrhea, and several microsporidian parasites known to cause illness in humans.

According to the study, Cryptosporidium continued to be detected in the nonnative oysters up to 33 days after exposure in the water. Giardia lamblia cysts were detectable up to 14 days following exposure.

“Recovery, Bioaccumulation, and Inactivation of Human Waterborne Pathogens by the Chesapeake Bay Nonnative Oyster, Crassostrea ariakensis” was written by Thaddeus K. Graczyk, Autumn S. Girouard, Leena Tamang, Sharon P. Nappier and Kellogg Schwab.

The study was supported by grants from the NOAA Chesapeake Bay Office, the NATO Collaborative Linkage Grant, the Johns Hopkins Center in Urban Environmental Health and the Alternatives Research and Development Foundation.

Public Affairs media contacts for the Johns Hopkins Bloomberg School of Public Health: Tim Parsons or Kenna Lowe at 410-955-6878 or