New attenuated vaccine against columnaris disease in fish: choosing the right parental strain is critical for vaccine efficacy.

Flavobacterium columnare, the causative agent of columnaris disease, is a highly diverse species comprised by three genomovars. Genomovar II strains are more virulent toward catfishes than genomovar I isolates. The objective of this study was to compare the vaccine efficacy of avirulent mutants derived from genomovars I and II using a rifampicin-resistance strategy. First, we compared the efficacy of 13 genomovar II mutants in channel catfish (Ictalurus punctatus) fingerlings and identified mutant 17-23 as the best vaccine candidate based on their relative percent survival (RPS) against a highly virulent genomovar II strain (BGFS-27). In the second experiment, we vaccinated zebrafish (Danio rerio) with two genomovar II mutants (17-23 and 16-534) and FCRR (genomovar I mutant) followed by exposure to BGFS-27 strain. RPS values were 28.4, 20.3 and 8.1% for 17-23, 16-534, and FCRR, respectively. For experiments 3 and 4, we tested both 17-23 and FCRR in channel catfish fry and Nile tilapia (Oreochromis niloticus). In both experiments, vaccinated fish were divided in two groups and each challenged with either a genomovar I (ARS-1) or a II (BGFS-27) strain. Channel catfish fry vaccinated with 17-23 and FCRR followed by challenge with BGFS-27 resulted in RPS values of 37.0% and 4.4%. When fish were challenged with ARS-1, RPS values were 90.9% and 72.7% for fish vaccinated with 17-23 and FCRR, respectively. Nile tilapia vaccinated with 17-23 and FCRR followed by challenged with BGFS-27 had RPS values of 82.1% and 16.1%, respectively. When fish were challenged with strain ARS-1, RPS values were 86.9% and 75.5%. Overall, our results demonstrated that vaccination with genomovar II mutant 17-23 confers better protection in channel catfish and Nile tilapia than FCRR against columnaris disease caused by genomovar II. Both mutants were equally protective against columnaris caused by genomovar I showing that 17-23 mutant cross-protected against both genomovars.