Channel catfish (Ictalurus punctatus) are the top food fish species produced in the U.S., with most fish cultured in Mississippi, Alabama, Arkansas, and Texas. In 2009, an outbreak of hypervirulent Aeromonas hydrophila (vAh) occurred in the west Alabama catfish industry. Infections with vAh cause motile Aeromonas septicemia (MAS), which results in significant production losses. This opportunistic pathogen can cause large-scale fish kills within 48-72 h. Preventing vAh outbreaks is critical to pathogen control, and bacterin vaccines and probiotics have been developed to mitigate vAh infections. Given the success of these approaches, an experiment was designed to assess the potential synergy of combining a probiotic and an orally-delivered vaccine. It was hypothesized that the probiotic + vaccine treatment group will have the highest survival and growth than the control, probiotic, and vaccine treatment groups.
For the experiment, 40 individual tanks, within a recirculating aquaculture system (RAS), were each stocked with twenty-three fingerling channel catfish per tank (18.4 ± 0.4 g). Four treatments (control, probiotic, vaccine, and probiotic + vaccine) were administered over five weeks. At trial end, no differences in weight gain (P = 0.371) or feed conversion ratio (FCR; P = 0.162) were observed. At days 0, 14, and 28 post-vaccination, kidney, spleen, fecal, and blood samples were collected for gene expression, gut microbiome, and ELISAs. After the five-week feeding trial, a 1h immersion challenge was initiated. There were differences in cumulative percent mortality (CPM) during the one-week challenge (P = <0.001; Figure A). The control and vaccine treatment had a significant difference (P = <0.001) and control and probiotic + vaccine had a significant difference (P = <0.001). The control and probiotic treatment groups were not significantly different (P = 0.369), and the vaccine and probiotic + vaccine treatment groups were not significantly different (P = 0.870). For ELISA analysis there were no significant differences in antibody titers during vaccine day 0, 14, and 28 (P = 0.630, P = 0.932, P = 0.083; Figure B). In conclusion, oral vaccines have been shown to decrease mortality rates against vAh in channel catfish culture.