Quantifying the presence of Vibrio harveyi and Vibrio campbellii pathogens in Georgia’s Crassostrea virginica oysters and Mercenaria mercenaria clams: presence, distribution, and concentrations*

Global warming and ocean acidification have allowed for Vibrio pathogens to geographically expand, as well as rapidly evolve. V. harveyi and V. campbellii were reclassified as two separate species just this year. Natively found in Asia and South America waters, these species are well-established pathogens that infect fish, shrimp, and mollusks. Neither strain is pathogenic to humans, however have been found to cause significant impact to the aquaculture industry. Due to climate change South Eastern United States now has a hospital environment for these pathogens, however this has never been previously studied. It is crucial for Georgia waters to be monitored regularly because if present in high concentrations the established clam farms and wild oysters could face danger. Using reference strains of V. harveyi ATCC 14126 and V. campbellii ATCC BAA-1116 / BB120, seven previously described primers for species-specific detection of V. harveyi virulence genes (toxR, luxR, vhh, vhh(a), srp, vhp, and rpoA) were tested with both strains. All the seven genes were reliable detected in both reference strains. These signature genes were used for qPCR detection and quantitation of V. harveyi and V. campbellii in wild oysters and farmed clams collected at random from three sites around Townsend, GA. In summer 2022 monitoring, vhh(a) and vhp genes were consistently detected in both oysters and clams in the range of 10^2 to 10^10 copy numbers per gram of tissue, and no vhh gene was ever detected. Distribution and concentrations of the other genes varied between three sites. These results evidenced high abundance of V. harveyi and V. campbellii in wild and farmed mollusks. They also evidenced that not all previously published target genes are reliable for detection of these pathogens, and suggested vhh(a) and vhp genes as best targets. Data obtained in a complimentary study evidenced their presence in water and sediments