Genetic diversity of clinical and environmental Vibrio parahaemolyticus strains from the Pacific Northwest.

Since 1997, cases of Vibrio parahaemolyticus-related gastroenteritis from the consumption of raw oysters harvested in Washington State have been higher than historical levels. These cases have shown little or no correlation with concentrations of potentially pathogenic V. parahaemolyticus (positive for the thermostable direct hemolysin gene, tdh) in oysters, although significant concentrations of tdh(+) V. parahaemolyticus strains were isolated from shellfish-growing areas in the Pacific Northwest (PNW). We compared clinical and environmental strains isolated from the PNW to those from other geographic regions within the United States and Asia for the presence of virulence-associated genes, including the thermostable direct hemolysin (tdh), the thermostable-related hemolysin (trh), urease (ureR), the pandemic group specific markers orf8 and toxRS, and genes encoding both type 3 secretion systems (T3SS1 and T3SS2). The majority of clinical strains from the PNW were positive for tdh, trh, and ureR genes, while a significant proportion of environmental isolates were tdh(+) but trh negative. Hierarchical clustering grouped the majority of these clinical isolates into a cluster distinct from that including the pandemic strain RIMD2210633, clinical isolates from other geographical regions, and tdh(+), trh-negative environmental isolates from the PNW. We detected T3SS2-related genes (T3SS2ß) in environmental strains that were tdh and trh negative. The presence of significant concentrations of tdh(+), trh-negative environmental strains in the PNW that have not been responsible for illness and T3SS2ß in tdh- and trh-negative strains emphasizes the diversity in this species and the need to identify additional virulence markers for this bacterium to improve risk assessment tools for the detection of this pathogen.

Vibrio parahaemolyticus risk assessment in the Pacific Northwest: it's not what's in the water.

The Gram-negative bacterium Vibrio parahaemolyticus (Vp) is a major cause of illness associated with the consumption of raw or undercooked seafood, primarily oysters. This species is a natural member of the bacterial community in brackish waters and is bioaccumulated by oysters through filter feeding. Only a subset of strains is thought to be pathogenic. Currently known virulence markers include the gene for the thermostable direct hemolysin (tdh). In this work we analyzed water and oysters for total Vp and strains encoding tdh from 26 oyster-growing areas of the Puget Sound and Pacific coast of Washington state in 2007 and 2008. In addition, possible plankton-associated Vp were assessed from net tow samples. The density of both total and tdh+ Vp in the water column were considerably higher in 2008 than 2007. However, the concentrations of both total and tdh+ Vp in the oyster tissue was similar for both years. A high proportion of Vp strains in the water column was found to be tdh+ in both 2007 and 2008; however, tdh+ strains were detected at much lower levels in oysters. The data show that analysis of Vp density in the oysters is a better risk assessment tool than density in the overlying water column.

Immunosuppression in progeny of chinook salmon infected with Renibacterium salmoninarum: re-analysis of a brood stock segregation experiment.

Female spawner infection level and temperature variation through rearing are sufficient to explain in-hatchery mortality rates and infection levels and smolt to adult return ratios (SARs) of progeny of Renibacterium salmoninarum infected spring chinook salmon. Data from published reports and manuscripts regarding a 1988 brood stock segregation experiment that held progeny of highly infected female spring chinook salmon spawners separate from progeny of other spawners during 16 mo of hatchery rearing are analyzed to test the hypothesis that immunosuppression could account for differences in survival and infection levels between the 2 segregates. Immunosuppression, caused by the presence of the p57 antigen of R. salmoninarum in sufficient concentration within the salmon egg before spawning, can account for differences in infection levels, mortality rates, and SARs for each hatchery raceway in that study. This immunosuppression may be characterized by immunotolerance, or might only affect cell mediated immunity, which appears the most effective defense mechanism against R. salmoninarum infection, as antibody production can result in tissue damaging antibody-antigen complexes. Low-temperature mediated immunosuppression can account for the nearly identical trajectories of infection and mortality between the 2 segregates during the first 8 mo of hatchery rearing. There is no evidence of widespread vertical infection from spawner to progeny, nor is there evidence that brood stock segregation reduces overall mortality. Rather, the suppression of cell-mediated immune mechanisms may condemn progeny of highly infected female spawners to an almost certain eventual premature demise.

Relationship between antigen concentration and bacterial load in Pacific salmon with bacterial kidney disease.

Using data collected to test spawning female Pacific salmon (Oncorhynchus kisutch and O. tshawytscha for the presence and severity of bacterial kidney disease (BKD), a mathematical model of the relationship between bacterial load and antigen concentration in tissues and ovarian fluid is developed. Renibacterium salmoninarum, the causative agent of BKD, secretes large amounts of a 57 kDa protein ('p57'), its major soluble antigen, which eventually breaks down or is otherwise removed from free circulation. Bacterial load and soluble antigen concentration in tissues are strong indicators of fish health, while in ovarian fluid they are predictors of the success of offspring. Model results indicate either an exponentially increasing antigen removal rate or an exponentially decreasing per-bacterium antigen secretion rate with increasing antigen concentration. Possible mechanisms underlying the observed relationship include a nonlinear increasing autolytic rate of the 'p57' antigen and a bacterium-antigen interaction threshold which prevents bacterial antigen secretion.