Challenge methods used to induce furunculosis in rainbow trout when testing the potency of vaccines may include exposure to Aeromonas salmonicida by intraperitoneal injection, cohabitation or bath immersion. Intraperitoneal injection is effective but will not reflect systemic immunity because inflammatory cells at the vaccine injection site may combat injected bacteria fast. The cohabitation and bath immersion methods both mimic the natural infection route but are less effective in inducing the disease.
We have tested a new challenge method mimicking that rainbow trout in fish farms might be infected with A. salmonicida through injured epidermis (fin biting) and our new method resembles closely the natural infection route where bacteria gain access to fish through the lesions. In our challenge procedure small skin lesions were made on the upper part of the caudal fin with a multipuncture device containing 10 needles puncturing the upper tail fin after the fish were anaesthetized. A volume of 100 µl of a 48 hour culture of A. salmonicida (3.4 × 108 CFU/ml) was layered at the puncture site for 60 seconds where after fish were placed in freshwater for recovery. This technique proved to be efficient in inducing a more natural disease progression in fish and a stable mortality. The method could differentiate efficacies of different vaccines with regard to adjuvant formulations and content of antigen.