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

The 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). Naive smolts were exposed to RSW for either 38 days (440 degree-days, DD) or 82 days (950 DD) after which they were maintained in UVSW. Control fish were maintained on UVSW only. Microscopic examination at day 176 (1985 DD) 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 (4275 DD), 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, naive smolts were exposed to RSW at weights of 101 g, 180 g, 210 g or 332 g 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 101 g fish were not different from those in control fish (495 g) 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 Relevance: We 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. Crown Copyright (C) 2015 Published by Elsevier B.V. All rights reserved.