Identification of MH Class IIβ alleles conferring resistance/susceptibility to Flavobacterium psychrophilum in Oncorhynchus mykiss families

Flavobacterium psychrophilum is the causative agent of bacterial cold water disease (BCWD), which is a major concern for aquaculture and fish health, with mortality rates of 20-30%. Without a commercially approved vaccine, alternative methods of control are needed. One method involves selective breeding to produce fish with a higher natural level of resistance to BCWD. Major histocompatibility (MH) genes encode cellular receptors involved in antigen presentation and play an important role in adaptive immunity. MH genes are highly polymorphic and previous research has identified specific alleles of class I and class II that are associated with resistance/susceptibility to infectious salmon anemia virus (ISAv) as well as Aeromonas salmonicida in Atlantic salmon. This study examines the MH class IIβ genotype of six rainbow trout families infected with a pathogenic strain of F. psychrophilum and survival status to identify alleles associated with better or worse performance. In the survival analysis hazard ratios were calculated for each allele and genotype. None of the obtained alleles or genotypes demonstrated statistically significant hazard ratios. If MH class IIβ alleles did confer resistance/susceptibility to BCWD this would be valuable information for aquaculture businesses that want to use selective breeding to produce fish stocks with a reduced chance of succumbing to BCWD. However no statistically significant relationship between alleles or genotypes and resistance to BCWD was observed

Serum IgM, MH class IIβ genotype and respiratory burst activity do not differ between rainbow trout families displaying resistance or susceptibility to the coldwater pathogen, Flavobacterium psychrophilum

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.aquaculture.2017.10.020 © 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/Flavobacterium psychrophilum, the causative agent of bacterial coldwater disease (BCWD) is a significant threat to global aquaculture. True to its name, BCWD tends to occur at temperatures between 8 and 12°C and presents as a systemic disease with characteristic skin ulcerations. Juvenile rainbow trout are particularly susceptible and in these fish the condition is referred to as rainbow trout fry syndrome (RTFS). Resistance to F. psychrophilum is heritable and is not adversely correlated with the growth of fish, thus selective breeding appears to be an achievable approach to its control. The current study explores the connection between resistance to BCWD and several immunological markers. After determining resistance/susceptibility to F. psychrophilum following experimental infection in 40 full-sibling families of rainbow trout, selected families were experimentally infected with F. psychrophilum and differences in antibody production, major histocompatibility (MH) class IIβ genotype and respiratory burst activity (RBA) throughout infection were compared. Serum IgM production increased over time but significant differences between resistant and susceptible families were not observed at either 28days or 120days. Of the six families that were genotyped for MH class IIβ, there did not appear to be specific genotypes that conferred resistance or susceptibility to F. psychrophilum. Further, the RBA of both head kidney leukocytes and whole blood was not significantly different between the resistant and susceptible rainbow trout families. Although the selected immune markers did not differ based on resistance status, the RBA of head kidney leukocytes in all families studied dramatically decreased seven days after infection while total blood RBA remained constant. Day seven was also when severe symptoms and/or mortality due to BCWD was first observed, thus these results may reveal information regarding the pathogenesis of the organism. A better understanding of appropriate immune defenses could provide the basis for breeding programs to effectively combat this costly pathogen, but further study of functional immune markers particularly during the fry stage of development is required.Natural Sciences and Engineering Research Council’s Strategic Project Grant program [STPGP 430654-12]Canada Graduate Scholarship programOntario Graduate Scholarship progra