A New Furunculosis Challenge Method for Evaluation of Vaccine Efficacy in Rainbow Trout

Experimental infection of fish for vaccine efficacy studies is associated with severallimitations. Administration of live bacteria with the purpose of causing disease infish can be performed by co-habitation, immersion or injection. We have developeda new Aeromonas salmonicida challenge method for rainbow trout and have appliedit for evaluation of furunculosis vaccine efficacy. The method reveals development ofsystemic immunity in fish as live bacteria are introduced in the tail fin epidermisdistant from the vaccine injection site (peritoneal cavity). This method seeks tomimic natural infection in fish farms where tail biting and therefore bacterial exposureto tail fin ulcers is widespread. By use of a multi-needle device ten epidermalperforations were introduced in the dorsal part of the tail fin of anaesthetizedrainbow trout (vaccinated or naive). Subsequently 100 μL (3.4 × 108 colony-formingunits (CFU) mL-1) of a 48 hour culture of Aeromonas salmonicida subsp. salmonicida strain 090710-1/23 was placed at the perforation site for 60 sec whereafter fishwere allowed to regain consciousness in clean freshwater. Immunohistochemistryand scanning electron microscopy illustrated the spread of bacteria from theinjection site. Classical furunculosis symptoms associated with a high morbidity ratewere observed in control fish whereas vaccinated fish exhibited a significantly highersurvival.