Relative to the 61 percent mortality, Dr. Audemard believes the oysters were either stressed by the temperature abuse they underwent before relay to increase the Vibrio levels in these oysters, or the relay gear was not optimal, or some other factors. “We actually do not think high salinity relay in itself caused the mortality based on this study and previous ones by us and others,” she mentions.
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Indicator for Pathogenic Vibrios
In 2005, under the leadership of Gary Richards, PhD, a USDA Agricultural Research Service laboratory team in Dover, Del., developed and published a simple and rapid procedure called the colony overlay procedure for peptidases (COPP) assay to quantify total vibrios (TV) in oysters and seawater.
Salina Parveen, PhD, a professor in the Food Science and Technology Program at the University of Maryland Eastern Shore, and several collaborators sought to validate the use of the COPP assay. “Regulatory agencies and industry currently use comparable testing of fecal coliform bacteria as an indicator of fecal pollution,” Dr. Parveen relates. “We thought the use of the COPP assay might serve a similar role as an indicator of pathogenic vibrios.”
Dr. Parveen and her colleagues collected oyster and seawater samples from the Delaware Inland Bays and the Maryland Chesapeake Bay and analyzed for TV, as well as pathogenic strains of V. vulnificus and V. parahaemolyticus.
They compared the COPP assay with direct plating and a molecular method that detects TV and pathogenic vibrios.“The results of the study indicate that the COPP assay is a viable alternative to other, more complicated methods, such as the most probable number (MPN) method, for the detection of V. vulnificus in oysters and seawater and it is currently under further evaluation for its ability to serve as an indicator for V. parahaemolyticus,” Dr. Parveen explains.
Dr. Parveen points out that the COPP assay reduces analysis time from three days (using the standard FDA-approved MPN-based method) to 24 hours, and it provides a practical method for monitoring pathogenic Vibrio species in shellfish and the aquatic environment.
Farm-Raised Oyster Project
Just underway is a three-year study to determine whether an oyster farm’s geographic location, handling practices, and choice of equipment affect Vibrio levels in farm-raised oysters.
William Walton, PhD, an associate professor in Auburn University’s School of Fisheries, Aquaculture and Aquatic Sciences, landed a $456,646 USDA NIFA grant in August 2018 to fund the project.
Dr. Walton and his collaborators, FDA microbiologist Jessica Jones, PhD, and Auburn doctoral student Victoria Pruente, are focusing on a management system called off-bottom oyster farming, where oysters are maintained in floating cages or suspended baskets above the ocean floor in food-rich coastal waters.
“The off-bottom industry has boomed over the last two decades and continues to expand,” Dr. Walton relates. “Oysters grown this way are typically hatchery-reared single set oysters instead of clumps of oysters normally found in the wild. When properly operated, the containers provide protection from predators and eliminate burial in the sediment, allowing oysters to be cultured in areas where they would not survive on the bottom. Adverse bottom environments include high salinity areas with high predation rates or areas where the substrate is too soft.”
Once a week, off-bottom farmers raise the baskets out of the water and allow the oysters to air-dry. “This practice prevents barnacles, seaweed, and other undesirable organisms from attaching to and marring the oysters,” Dr. Walton notes.
“Though the air-drying is process is crucial to product quality, it is not risk-free,” Pruente interjects. “The frequent handling exposes the oysters to elevated air temperatures and also interrupts filter feeding, and those conditions cause Vibrio levels to rise.”