Could State-of-the-Art Management Make Shellfish Disease Problems Worse?

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Could State-of-the-Art Management Make Shellfish Disease Problems Worse?

Current shellfish disease management might let new diseases slip by.

A microscope image reveals destruction of an oyster's stomach lining caused by Dermo disease. ©Ryan Carnegie/VIMS

A microscope image reveals destruction of an oyster’s stomach lining caused by Dermo disease. ©Ryan Carnegie/VIMS

By Chris Patrick, staff writer

When diseases infect enough shellfish, it cripples these lucrative industries. It’s happened before.

Oysters in Chesapeake Bay are at less than one percent of their historical population. This is partially due to a disease known as Dermo. Although it doesn’t make people sick, Dermo devastated Chesapeake Bay oyster populations in the 1980s and 90s, helping bring annual oyster harvests in Virginia down from 8,196,500 pounds in 1979 to 163,066 pounds in 2000, according to landings data by the National Marine Fisheries Service.

To prevent this from happening again, managers frequently use DNA-based methods to check shellfish for a list of well-known diseases prior to transport.

“The problem is that we spend too much time looking for stuff that we know is there,” says Virginia Institute of Marine Science researcher Ryan Carnegie. In a recently published report with colleagues from Rutgers University and the French Laboratory of Genetics and Pathology of Marine Molluscs, he says that shellfish managers need to look beyond the list of known threats if they hope to prevent new diseases from slipping through unnoticed.

Today, managers often search shellfish for the DNA of pathogens, disease-causing micro critters. These methods are faster and easier than traditional, microscope-based methods of disease detection. They require less time and training and are also more sensitive and specific.

But keeping an eye out for emerging threats may mean looking through a microscope. Traditional microscope-based techniques allow evaluators to see a wider array of pathogens and identify diseases the DNA-based techniques don’t.

The report also points out there’s a need to catalogue diseases that are already present in an area. A lot of uncertainty could be removed from shellfish disease management if states knew what pathogens were in their waters.

Profiling all the pathogens in Chesapeake Bay, for example, could prevent unnecessary bans on transport of shellfish. If Maryland and Virginia know they have similar levels of the same pathogen in their waters, for example, there’s no need for these states to stop the transfer of shellfish between them. A ban could force growers to bypass the approval process, if, for example, they needed to buy small shellfish, called seed, to plant their next crop and their home state had already sold out. Managers worry that such unregulated imports could bring new diseases to the Bay region.

“Profiling pathogens by zone would allow managers to streamline regulation and approvals for seed transfers within zones where commerce is safe,” Carnegie says. “We could spend less time counting Dermo cells that we know are there and focus instead on emerging disease threats.”

The report is the result of Virginia Sea Grant-funded workshops that brought together researchers, regulatory agency members, and shellfish industry members to discuss shellfish management and concerns for emerging diseases.

Abstract for the paper “Managing marine mollusc diseases in the context of regional and international commerce: policy issues and emerging concerns” by Carnegie, Isabelle Arzul (Laboratoire de Génétique et Pathologie des Mollusques Marins), and David Bushek (Rutgers University) can be found at the Royal Society Publishing website: