Infectious hematopoietic necrosis virus in juvenile Chinook salmon: Mx-1 and VIG-8 gene transcription in infected fish and synergistic effects of concurrent esfenvalerate exposure

Exposure to pesticides may compromise the immune function of fish resulting in greater mortality than exposure to microbial pathogens alone. Groups of juvenile Chinook salmon (Oncorhynchus tshawytscha) exposed to infectious hematopoietic necrosis virus (IHNV) and to sublethal concentrations of esfenvalerate resulted in significant mortality by 3 d post-virus exposure, an event not seen in fish exposed to either agent alone. Assays of dead fish indicate that disruption of physiologic or immunologic mechanisms resulted in a lethal synergistic effect. Mx-1 and VIG-8, two genes important in the early, non-specific, anti-viral immune response, were detected in Chinook salmon to investigate the role of these genes in resistance to IHNV, and to determine if concurrent esfenvalerate exposure affected their transcription. Quantitative reverse transcriptase PCR revealed significantly more Mx-1 and VIG-8 transcripts in Chinook salmon exposed to IHNV as compared to control fish. Significantly more Mx-1 and VIG-8 transcripts were also detected in Chinook salmon demonstrating age-related resistance to IHNV as compared to younger, more susceptible fish. Same-aged fish from two strains of Chinook salmon demonstrating different susceptibility were used to investigate the role of these genes in strain-related resistance to IHNV. Significantly higher constitutive Mx-1 transcription was detected in the more IHNV-resistant strain. Lastly, Chinook salmon exposed to both IHNV and to esfenvalerate had significantly less Mx-1 and VIG-8 transcripts than fish exposed to the virus alone. Analysis of blood plasma suggested that osmolality was not a contributing factor to the observed lethal synergism. Likewise, stained histological sections of gill and kidney sections did not reveal pathologic microscopic changes in fish exposed to IHNV and to esfenvalerate. These studies suggest that juvenile Chinook salmon concurrently exposed to IHNV and to sublethal levels of esfenvalerate suffer mortality due to a synergistic effect potentially related to inhibited transcription of early, non-specific, anti-viral cytokines which may be indicative of disrupted upstream immunological mechanisms. These studies also suggest that increased transcription of Mx-1 and VIG-8 is a first-line defense against IHNV in juvenile Chinook salmon but is only one part of a more complex immune response that determines survivorship or susceptibility.