INFLUENCE OF Perkinsus marinus INFECTION ON LEVELS OF HUMAN-PATHOGENIC VIBRIOS IN THE OYSTER Crassostrea virginica

Vibrio vulnificus and Vibrio parahaemolyticus are naturally occurring marine and estuarine bacteria that can accumulate in oysters and other shellfish filter-feeding in these systems. While harmless to their hosts, these bacteria are the causes of the highest rates of gastrointestinal illness and mortality related to seafood consumption in humans. In the eastern oyster Crassostrea virginica, Vibrio spp. and the important and prevalent pathogen Perkinsus marinus co-occur, potentially even within the same tissue spaces (gut, hemocytes), yet little is known about the potential interaction between the bacteria and this oyster parasite. While it is conceivable that the physiological stress associated with intense P. marinus parasitism could produce elevated Vibrio levels and result in a positive correlation between the two, it may also be possible that activities or secretions of the parasite could inhibit Vibrio spp., producing a negative correlation.

To address this question, we sought to determine the relationship between P. marinus infection intensity (determined using quantitative PCR, qPCR) and Vibrio levels (determined using qPCR on alkaline peptone water most-probable number (MPN) series) in individual oysters sampled from natural waters of Chesapeake Bay. Initial results with 30 oysters from both the Piankatank and York Rivers suggested a negative relationship between P. marinus and both V. vulnificus (Fig. 1) and V. parahaemolyticus. This study is presently being repeated with a larger sample size (individual analyses on > 400 oysters) and a broader focus not just on P. marinus infection intensity but on disease severity and the presence of other pathogens like Haplosporidium nelsoni, with the objective being to gain a fuller appreciation for the possible influences on Vibrio levels in oysters.

These results will inform management practices for Vibrio spp. in populations of aquacultured oysters. In a practical sense, a negative relationship between P. marinus and Vibrio spp., if borne out by work in progress, would suggest that aquaculture breeding specifically for tolerance to P. marinus, moderate infection levels with minimal physiological costs, may have merit if oysters with such infections are likely to carry lower Vibrio burdens.