Susceptibility and immunity to Vibrio anguillarum in post-hatching rainbow trout fry, Salmo gairdneri Richardson 1836.

Rainbow trout fry were tested for their susceptibility to experimental infection with Vibrio anguillarum, and their ability to mount an immune response against it, from the age of 2 weeks post-hatch onwards. Bath challenges were ineffective in inducing vibriosis until 6-8 weeks post-hatch, and then only low levels of specific mortality ensued, even at very high doses. However, at the earliest age tested, 7 weeks post-hatch, the fry were susceptible to infection by an intraperitoneal injection with live organisms. Protective immunity was evident in fry vaccinated by direct immersion as early as 2 weeks post-hatch (at an average weight of 0.14g), when tested by intraperitoneal challenge. By the time the fry reached 0.5g (10 weeks after hatching), protection levels had reached 50% for direct immersion vaccination and 100% for intraperitoneal vaccination. An oral vaccination, from first feeding onwards, proved ineffective in inducing immunity. The results are discussed in relation to the onset and maturation of immunological competence in rainbow trout.