Once the baskets are lowered back into the ocean, Vibrio levels gradually subside, but questions remain. “In our trials, we will look at how long after the oysters are resubmerged the Vibrio levels return to naturally occurring levels,” Dr. Walton says.
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To achieve the shell shape, meat quality, and cleanliness that high-end market demands, farmers routinely handle their oysters for grading, splitting, control of biofouling, and many other reasons. “Routine handling techniques include tumbling and desiccation to prevent biofouling from barnacles, mud worms, and other organisms that clog the mesh in the containers, thus reducing the flow of water, food, and oxygen, and also to eliminate predators such as oyster drills,” Dr. Walton explains.
In contrast to any elective handling techniques, farmers could just leave their oysters alone and let them grow, Dr. Walton mentions. “When they do, with minimal intervention/care/handling, the oysters tend to grow long, skinny, and flat, which diminishes their market value,” he relates.
Dr. Walton says the increased demand for farm-raised oysters, combined with improved handling techniques for farm-raised oysters, has raised concerns among public health agencies and scientists about how routine handling on the farm can elevate the risk to humans posed by Vibrio, which are concentrated within the oyster tissue.
“The routine handling exposes oysters to elevated air temperatures for extended periods of time, permitting increased growth of Vibrio within the oysters,” Dr. Walton points out. “Fortunately, re-submersing the oysters allows them to begin filtering again and reduce the Vibrio levels back down to the background levels, which can vary and depend on environmental conditions, including water temperature and salinity. Our prior work determined the time required for the reduction of the elevated risk to background levels is seven days when using suspended gear.”
Dr. Walton’s team will now determine the re-submersion period required for elevated levels of Vibrio to return to ambient levels in oysters grown in a different type of gear that is commonly used and using various handling treatments.
New Vibrio Test Kit
On Oct. 1, 2018, Bio-Rad Laboratories, Inc., Hercules, Calif., launched its iQ-Check Vibrio PCR (polymerase chain reaction) Detection Kit, which provides qualitative detection of the three main Vibrio species that cause vibriosis, V. cholerae, V. parahaemolyticus, and V. vulnificus, according to Frédéric Pastori, the company’s international product manager.
With its main targeted matrices being seafood, including fish and shellfish (mollusks and crustaceans), the kit is a multiplex test based on gene amplification and detection by real-time PCR, Pastori says, noting that one kit can be used for up to 94 samples.
“This kit uses Bio-Rad’s Vibrio Enrichment Broth that promotes the growth of Vibrio while inhibiting associated competitive flora,” he relates. “The broth shortens culture from 18 hours to eight hours incubation time, and the entire detection process can be completed within 11 hours.”
Pastori says the iQ-Check Vibrio Kit reduces false positive results that can sometimes occur with real-time PCR methods when Bio-Rad’s Free DNA Removal Solution is incorporated into the testing process.
“The solution is easy to use, non-toxic, and optimizes the extraction process by reducing free DNA in the sample that can lead to false positive results,” Pastori explains. “Additionally, in an effort to manage PCR inhibition, an internal amplification control is included in the reaction mix to validate negative results.”