Acquired resistance to Kudoa thyrsites in Atlantic salmon Salmo salar following recovery from a primary infection with the parasite

AbstractThe influence of prior infection with Kudoa thyrsites or host size on the susceptibility of Atlantic salmon post-smolts to infection with the parasite was investigated. Exposure to infective K. thyrsites in raw seawater (RSW) was regulated by the use of ultraviolet irradiation (UVSW). Naïve smolts were exposed to RSW for either 38days (440degree-days, DD) or 82days (950DD) after which they were maintained in UVSW. Control fish were maintained on UVSW only. Microscopic examination at day 176 (1985DD) revealed K. thyrsites infection in nearly 90% of exposed fish but not in controls. Prevalence and severity of the infection decreased in later samples. Following a second exposure of all fish at day 415 (4275DD), prevalence and severity were elevated in the UVSW controls compared to previously exposed fish groups, suggesting the acquisition of protective immunity. In a second experiment, naïve smolts were exposed to RSW at weights of 101g, 180g, 210g or 332g and the prevalence and severity of K. thyrsites in the smallest fish group were higher than in any other group. However, infection parameters in the 101g fish were not different from those in control fish (495g) exposed in the first experiment. Concurrently, RSW was screened for K. thyrsites ribosomal DNA (rDNA) and the number of rDNA copies L−1 seawater did not agree well with prevalence or severity of infections. Possible reasons for this poor agreement are discussed. Increased resistance to K. thyrsites was evident in Atlantic salmon post-smolts that have recovered from the infection, suggesting strategies for the mitigation of soft-flesh in market-size salmon.Statement of RelevanceWe showed that Atlantic salmon which had recovered from infection with Kudoa thyrsites were resistant to reinfection with the parasite, suggesting an acquired immunity and the possibility of vaccination as a management strategy. Quantitative detection of parasite DNA in seawater provides a tool for predicting the risk of infection with this economically important parasite