| Summary: | Significant variations in susceptibility and immunological response against live (pathogenic and vaccine strains) and killed 'Cryptobia salmositica ' were found between full-sib families of 'Salmo salar' from the Atlantic Salmon Broodstock Development Program. One family (B) had significantly lower parasitemias, and this was associated with an earlier humoral immune response (antibodies detected at 3 weeks post-infection; w.p.i.). Susceptible families (e.g. Family C) had significantly higher parasitemias and a delayed humoral response (e.g. antibodies detected at 5 w.p.i.). The live 'C. salmositica' vaccine protected all families from a rise in parasitemia. Humoral immune response to 'Aeromonas salmonicida ' was similar to that against 'C. salmositica '. Activation of respiratory burst activity in circulating phagocytes (cell-mediated immunity) was demonstrated in fish given the live ' C. salmositica' vaccine. This activity corresponded with changes in parasitemia, leukocyte composition, and acquisition of protective immunity. ' C. salmositica' lysate provided partial protection from high parasitemias, but the immunological mechanism(s) behind this protection was not clearly elucidated.
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