Piscirickettsia salmonis is the aetiological agent of piscirickettsiosis, a severe salmon disease that causes necrosis of all lymphoid tissues and damages the liver, kidney and spleen resulting in high mortality rates in different salmonid species. Prevention of piscirickettsiosis includes the use of a widely diverse but traditional set of strategies including vaccines, immunostimulants, probiotics and dietary supplements. Selection of resistant fish in a non-vaccinated environment has been proposed and implemented as a complementary strategy to vaccination to prevent mortality associated to Piscirickettsiosis. However, this strategy may not be the most appropriate in a context in which all fish are vaccinated against the bacteria before being transferred to sea cages. Natural resistance against P. salmonis in Atlantic salmon seems to be polygenic and partially related to an iron deprivation mechanism that robs bacteria of nutrients, which is different from the mechanisms of antibody-mediated protection that are intended to be obtained with vaccination.
Here, we report that resistance against P. salmonis showed additive genetic variation of medium magnitude in vaccinated fish (0.36 > h2 > 0.38) in two different populations of Atlantic salmon (F10 and L20). These estimations are of similar magnitude when we compared with heritability estimates in unvaccinated fish. Notably, additive genetic variation of resistance was higher (0.50 > h2 > 0.65) in coinfected and vaccinated fish in comparison with single infection of either vaccinated or unvaccinated families, and also in comparison with coinfected unvaccinated fish. The genetic correlation between resistance of vaccinated fish and unvaccinated fish although positive was of medium magnitude indicating a moderate genetic association between natural resistance and resistance mediated by vaccines. Interestingly, three out of four estimates of heritability and all genetic correlations were higher in populations F10 than in population L20, revealing that, although no phenotypic differences were observed in the response to vaccination at the population level, there are genetic differences between both populations.
Our findings suggest that: 1) Since response to vaccination is heritable, it could be used in selective breeding to increase the effectiveness of vaccination and survival, as has been proposed for terrestrial animals and successfully applied in livestock. 2) We do not know if the fish that were "high level responders" to vaccination in this study would maintain their status when adults, presenting the future challenge of selecting families that are "high level responders" and also "long level responders".