Impact of parasitism on levels of human-pathogenic Vibrio species in eastern oysters.

AIMS: To investigate the relationships between individual health status of oysters, particularly with regard to parasitic infection, and variability in abundance of human-pathogenic Vibrio species. METHODS AND RESULTS: Aquacultured eastern oysters, Crassostrea virginica, were analysed individually for infection by the protozoan parasite Perkinsus marinus through quantitative PCR, and total Vibrio vulnificus and total and pathogenic Vibrio parahaemolyticus abundance was assessed using a most probable number (MPN)-qPCR approach. Additionally, perspective on general oyster health and other parasitic infections was obtained through histopathology. Perkinsus marinus infection and human-pathogenic Vibrio species levels were not correlated, but through histology, analyses revealed that oysters infected by Haplosporidium nelsoni harboured more V. vulnificus. CONCLUSIONS: The highly prevalent parasite P. marinus had little influence on human-pathogenic Vibrio species levels in eastern oysters, but the less prevalent parasite, H. nelsoni, may influence V. vulnificus levels, highlighting the potential nuances of within-oyster dynamics of Vibrio species. SIGNIFICANCE AND IMPACT OF THE STUDY: Human-pathogenic bacteria continue to be a concern to the oyster industry and causes for individual oyster variation in bacterial levels remain unknown. The major oyster pathogen P. marinus does not appear to affect levels of these bacteria within oysters, suggesting that other factors may influence Vibrio spp. levels in oysters.

A rapid phenotype change in the pathogen Perkinsus marinus was associated with a historically significant marine disease emergence in the eastern oyster.

The protozoan parasite Perkinsus marinus, which causes dermo disease in Crassostrea virginica, is one of the most ecologically important and economically destructive marine pathogens. The rapid and persistent intensification of dermo in the USA in the 1980s has long been enigmatic. Attributed originally to the effects of multi-year drought, climatic factors fail to fully explain the geographic extent of dermo's intensification or the persistence of its intensified activity. Here we show that emergence of a unique, hypervirulent P. marinus phenotype was associated with the increase in prevalence and intensity of this disease and associated mortality. Retrospective histopathology of 8355 archival oysters from 1960 to 2018 spanning Chesapeake Bay, South Carolina, and New Jersey revealed that a new parasite phenotype emerged between 1983 and 1990, concurrent with major historical dermo disease outbreaks. Phenotypic changes included a shortening of the parasite's life cycle and a tropism shift from deeper connective tissues to digestive epithelia. The changes are likely adaptive with regard to the reduced oyster abundance and longevity faced by P. marinus after rapid establishment of exotic pathogen Haplosporidium nelsoni in 1959. Our findings, we hypothesize, illustrate a novel ecosystem response to a marine parasite invasion: an increase in virulence in a native parasite.