Influence of Perkinsus Marinus Infection and Oyster Health on Levels of Human-Pathogenic Vibrios in Oysters

The eastern oyster Crassostrea virginica is an ecologically and commercially important species whose natural populations have been devastated by overharvesting, habitat destruction, and disease, but the rapid growth of oyster aquaculture has shown potential to restore the economic significance of this species. A key threat to the growth and sustainability of oyster aquaculture is the association of human-pathogenic Vibrio bacteria with product marketed for raw consumption. Two Vibrio species, Vibrio vulnificus and Vibrio parahaemolyticus, are the causes of the highest rates of seafood consumption-related mortality and gastrointestinal illness, respectively. Identification of the factors influencing V. vulnificus and V. parahaemolyticus prevalence and intensity in oysters is fundamental to better risk management. Within the oyster, these bacterial species interact with the same tissues as the prevalent oyster parasite, Perkinsus marinus, yet little is known about the effect of P. marinus infection on bacterial levels. Answering the fundamental question of whether P. marinus correlates with V. vulnificus and V. parahaemolyticus levels in oysters was the focus of this research. Oysters were deployed in the York River, Gloucester Point, VA, where both Vibrio species and P. marinus are endemic, and were sampled at five time points when levels of both P. marinus and Vibrio spp. were expected to be high in oysters. Abundance of all three organisms and pathogenic strains of V. parahaemolyticus were determined in individual oysters using molecular methods to investigate potential correlations between parasite and bacterial abundance. Additionally, the levels of V. vulnificus and V. parahaemolyticus in relation to histopathology associated with P. marinus infection and other conditions were determined. The following year, manipulation of P. marinus disease progression, which is slowed by lower salinities and favored by higher salinities, was attempted by deploying oysters at two additional sites of different salinities to gain insight into whether the timing of P. marinus infection emergence directly influences Vibrio levels. No correlation was observed between total abundance of P. marinus and either V. vulnificus or V. parahaemolyticus. Manipulation of P. marinus disease progression produced no effect on P. marinus emergence, so this yielded no insight into P. marinus-Vibrio interactions. Histopathological analyses did not reveal any correlations between P. marinus ranking, distribution, or associated tissue damage and Vibrio spp. levels. Though few in number, oysters infected by Haplosporidium nelsoni were characterized by higher levels of V. vulnificus, and oysters of peak gametogenic development had significantly higher levels of pathogenic strains of V. parahaemolyticus. The results with regard to H. nelsoni and gametogenic state warrant further study. The primary conclusion of this study is that oyster health has little influence on levels of human-pathogenic Vibrio species in oysters, inter-host variability in Vibrio levels is likely explained by other factors

Influence of oyster genetic background on levels of human-pathogenic Vibrio spp.

Human-pathogenic Vibrio bacteria are common inhabitants of oyster tissues, but our understanding of factors driving the wide range of concentrations found in individual oysters is extremely limited. We examined the influence of oyster sex and parasitism in light of their profound effects on oyster tissues against a backdrop of eastern oysters, Crassostrea virginica, from two diploid and two triploid aquacultured lines. This allowed us to examine not only the effect of oyster ploidy but also of oyster genetics, a factor never investigated with regard to human-pathogenic Vibrio species. We measured levels of total Vibrio vulnificus (vvhA), and of total (tlh) and pathogenic (tdh+, trh+) V. parahaemolyticus, in each oyster, and analyzed the data through generalized linear mixed-effects models. A key outcome of these analyses was the consistent inclusion of oyster line as a predictor variable across Vibrio targets. A potential effect of Perkinsus marinus infections and/or oyster sex was also suggested, although the combination of variables varied with Vibrio target. This study suggests that the influence of oyster genetic background should be further investigated, and that the dynamics of human-pathogenic Vibrio spp. in oysters is likely driven by multiple, interacting factors, some of which may be under oyster host genetic control

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

Environmental Ecology And Life History Variation Of Perkinsus Marinus

Marine resources provide a growing world population with the opportunity to increase food security, particularly through aquaculture. Oysters are prominent aquacultured species, but marine diseases are a threat to the growth and sustainability of oyster culture. Perkinsus marinus is the most important oyster pathogen of the Atlantic and Gulf coasts of the U.S., where it causes substantial mortality. Despite being one of the most well-studied protozoan parasites of bivalve molluscs, much is still unknown about P. marinus. This research investigated multiple characteristics of P. marinus.In Chapter Two, the environmental ecology of P. marinus was explored through tidal sampling and year-long seasonal sampling of eDNA in both the water column and the benthos. Additionally, visualization of cell forms was undertaken. Perkinsus marinus was detectable across the tidal cycle, with levels generally highest at low tide. Notably, environmental abundance of P. marinus was highly variable on short time scales. Seasonally, P. marinus abundance generally increased with temperature, as expected, but it also exhibited marked variability. While P. marinus abundance fell below the level of detection in surface water in late winter, it was detectable year-round in the sediment, indicating that the benthos may be an underappreciated reservoir for P. marinus transmission in marine systems. Visualized life history stages from environmental samples were mainly P. marinus hypnospores. In Chapter Three, shifting baselines in P. marinus histological presentation were examined across a 21-year time period using environmental parameters and generalized mixed-effects models. The results indicated a phenotypic shift to smaller cell sizes had occurred during 1992 to 2012. The change was not fully explained by environmental influences in the models. This shift in size has also been associated with an increase in weighted prevalence in the late 1990s/early 2000s, hinting at a relationship between decreased cell size and increased severity. In Chapter Four, two unusual histopathological presentations of putative P. marinus infections, which could challenge diagnosticians, were characterized. Aberrant infections were analyzed in oyster histology samples from 2011 to 2021. One aberrant presentation was associated with the gonad while the other was within oocytes. Both types of infections were confirmed to be P. marinus through molecular work. Additionally, aberrant gonadal infections were found to be geographically widespread and mainly infecting males with cell characteristics that deviated from contemporary P. marinus. Oysters with infected oocytes were rarer but did span several years. Overall, the research accomplished here advanced understanding of multiple aspects of a significant marine pathogen. Insights into the seasonal environmental ecology of the parasite were uncovered, a past phenotypic change was investigated using statistical models, and unusual putative P. marinus infections were molecularly confirmed and described for this protozoan. These advances help support management of the oyster aquaculture industry and highlight the usefulness of monitoring programs and archival material