Viewpoint: Non-native oysters may endanger human health

By Sharon Nappier

January 15, 2009

As federal and state officials move toward a decision next year on whether to introduce non-native oysters into the Chesapeake Bay, I have some advice: Don't do it.

Action is clearly needed to restore the bay's oyster population, which is estimated to have fallen to less than 1 percent of its size during the 1800s. But recently published research I conducted while studying at the Johns Hopkins Bloomberg School of Public Health strongly suggests that the introduction of non-native oysters into the bay may present greater public health consequences for consumers than native oysters.

The Army Corps of Engineers and officials in Maryland and Virginia, who are starting work on a final environmental impact statement that will be published this spring, need to consider those consequences as they weigh various strategies for oyster restoration in the bay.

There are risks associated with eating oysters - especially oysters that have been exposed to fecally contaminated water. Oysters are filter-feeding bivalves that accumulate and retain pathogens, such as viruses, bacteria and protozoa, for long periods. Studies have shown that non-native Crassostrea ariakensis oysters can help to filter or clean the bay's waters. However, it has been proposed that these oysters be harvested for human consumption. If so, the ecological benefits provided by the oyster's filtration efficiency may have harmful repercussions for the health of consumers.

The large bay region receives fecal pollution, capable of contaminating oyster-harvesting waters, from various sources - including leaky septic systems, sewage overflows and numerous nearby chicken and cattle farms.

Our study aimed to assess these questions: If the oyster-harvesting waters became contaminated, would non-native oysters accumulate viruses? And if so, would the oysters retain the viruses for long periods? We looked at viruses, such as the norovirus and the hepatitis A virus, because they contribute to more than 80 percent of oyster-associated intestinal illnesses.

We exposed both the non-native and native oyster species to human and nonhuman viruses for 24 hours at varying salinities found in the bay's oyster-harvesting waters. After that, oysters were placed in clean water tanks for one month to simulate the process frequently used to purge oysters of harmful pathogens.

Our analysis showed that non-native oysters were statistically more likely to harbor human viruses than native oysters, and if exposed to contaminated water, non-native oysters may contain multiple virus types. Unlike native oysters, the ability of non-native oysters to harbor viruses was not driven by the salinity of the water. Additionally, we found that non-native oysters may have a wider range of habitat in the bay, and in those habitats it is possible the non-native oyster will be able to accumulate and retain human pathogenic viruses.

The Army Corps of Engineers and state officials need to consider these findings - and the questions they raise about the public health risks associated with consumption of non-native oysters - as they decide whether to introduce the species into the bay.

In their draft environmental impact statement, those officials acknowledged such a move "is very controversial and deviates significantly" from all prior oyster restoration efforts in the bay. As a citizen and a scientist, I believe it is simply not worth the risk.

Sharon Nappier is a research assistant professor at Drexel University in Philadelphia. The oyster research was published in the November issue of the American Society of Microbiology's peer-reviewed research journal Applied and Environmental Microbiology. Her e-mail is snappier@drexel.edu.