Immunization of salmonids against furunculosis

Furunculosis is a severe, profit limiting disease affecting a wide variety of commercially reared fish species throughout the world. The etiologic agent, Aeromonas salmonicida, produces several potential virulence factors and has some interesting biochemical characteristics that make it a difficult organism to produce an effective vaccine against. A vaccine was prepared using an auto aggregating highly virulent strain of A. salmonicida. This strain was grown by continuous culture fermentation, centrifuged, washed and treated with 5 mM EDTA at an elevated pH. This preparation was used to vaccinate fish. Rainbow and brook trout and Atlantic salmon were immunized by injection or immersion with and without a hyperosmotic infiltration treatment. A dose related response was noted when the bacterin was injected. In juvenile brook trout, at a 1 : 5 dilution, 44% died due to A. salmonicida infection after challenge; at a 1 : 10 dilution, 67% died whereas 97% of the control, sham vaccinated fish died. Protection by immersion without a hyperosmotic infiltration treatment was variable with the greatest degree of protection occurring in fish immersed twice. In a typical experiment, mortalities among vaccinated fish ranged from 36.7% for fish immunized once at a 1 : 10 dilution to 0% for fish immunized twice at 1 : 10 dilutions two weeks apart. Control mortality was 70%. In fish immunized by immersion in a 1 : 5 diluted suspension of this bacterin and a concurrent 5% NaCl hyperosmotic treatment, substantial protection was apparent. In a typical experiment mortalities ranged from 7.4% to 22.2% for the vaccinates with control mortalities of 72.7%. It is apparent from this work that successful immersion immunization for the prevention of furunculosis is possible.