Abstract
Two vaccination strategies were developed to prevent furunculosis in rainbow trout caused by the bacterial pathogen (A. salmonicida). Chapter 2 evaluated heterologous primary/booster schedules combining bath (BA) or oral (OV) vaccines with an intraperitoneal (IP) injection. By 8 weeks post-vaccination (wpv), groups receiving an IP vaccine as either primary or booster had significantly higher pathogen-specific IgM titers (average log ≥ 4) than bath/oral-only groups (average log ≥ 2, p < 0.05). Treatment groups that had higher IgM production pre-challenge also had higher survival post-challenge. As inclusion of an IP vaccine likely improved protection. A bath primary with IP booster achieved the lowest cumulative percent mortality (CPM) at 16.7%, compared to 38.3% for BA-OV. Order of heterologous routes had no significant effect (OV–IP 25.0% vs. IP–OV 23.3% CPM). BA–OV elevated IgT expression (p < 0.05 vs. BA–IP and IP–OV), while IP-inclusive schedules expressed higher IgM than BA–OV (p < 0.05), indicating that elevated IgM expression may be a factor that corresponded to better protection in the challenge. These results show that IP vaccines remain most effective, but bath and oral delivery are valuable for developing practical, scalable vaccination options and can be used interchangeably for primary or booster applications without sacrificing protection. Chapter 3 attempted to address efficacy gaps in mucosal vaccines by developing a live-partially attenuated bath-deliverable vaccine against furunculosis. An IP primary and booster vaccination of rainbow trout with an antibiotic and thermally mutated stain of A. salmonicida (15-021.5) at dose of 4.0 × 106 CFU/fish offered 90.5% RPS against their virulent parent strain at 8 wpv. Immersion vaccination of rainbow trout with the mutant strain at a dose of 3.6-5.5 × 107 cfu/ml offered 47.4% RPS against their virulent parent strain at 8 wpv. Primary and booster vaccination with 15-021.5 elicited A. salmonicida specific IgM production that was significant from control treatments (p < 0.05). Slight differences in banding patterns of outer membrane proteins between the 15-021.5 strain and parent strain indicate that 15-021.5 maintained key virulence factors necessary for adaptive immune development in response to vaccination, while still weakening the certain virulence factors of 15-021. Virulence from 15-021.5 detected during vaccination indicates that this strain is only partially attenuated and requires further attenuation. Although 15-021.5 produced a viable immunogenic reaction from fish and created protection in both bath and injection delivery forms, further optimization of the immersion vaccine will be required to improve its efficacy and justify it as a practical vaccine option against furunculosis.