Efficacy of orally-delivered virulent Aeromonas hydrophila bacterin vaccines in channel catfish (Ictalurus punctatus)

Virulent Aeromonas hydrophila (vAh) is one of the top three bacterial pathogens affecting farm-raised catfish in the U.S., capable of causing over 50 % mortality within a week, resulting in significant economic losses. Due to the acuteness of the disease an effective prevention is urgently needed, and vaccination is a promising solution. While injectable and immersion vaccines have been tested, their feasibility remains uncertain. Oral bacterin vaccines with adjuvants offer a practical alternative due to ease of administration and reduced stress on fish, allowing for vaccination at various production stages. This study evaluated the efficacy of orally delivered killed vAh vaccines, with and without adjuvants, in juvenile channel catfish. Fish were orally vaccinated and challenged 21 days later. All vaccine formulations provided significant protection compared to placebo controls except one preparation. Remaining fish were then booster vaccinated at nine weeks, followed by a second challenge at 12 weeks with both vAh isolates. All formulations improved survival rates, regardless of adjuvant inclusion. Hazard analysis confirmed significantly higher survival odds (P < 0.001) for all vaccinated groups compared to controls. Serum antibody levels measured at 21 days, 9 weeks, and 12 weeks post-vaccination showed no notable differences from controls. However, survivor antibody levels at 5 days post-challenge were significantly elevated, suggesting the vaccine effectively primed the immune system for rapid response upon pathogen exposure. These results support oral vaccination with a booster dose as a practical and effective strategy to protect farm-raised catfish against vAh, offering notable economic and regulatory advantages due to its formulation as an orally delivered killed vaccine.