An Overview of Vaccine Development Strategies for Columnaris-Causing Bacteria in Cultured Fish

Columnaris disease, caused by columnaris-causing bacteria (CCB), affects cultured and wild freshwater fish worldwide. Belonging to the phylum Bacteroidota and the family Flavobacteriaceae, CCB are gram-negative, yellow-pigmented bacteria that are presumed ubiquitous in freshwater habitats. Since the first report of the disease in 1922, growth and survival, pathogenesis, and transmission of these pathogens have been intensely researched, aiding in a greater understanding of host-pathogen interactions. Recent classification of Flavobacterium columnare into four distinct species based on genetic similarity, including F. columnare, F. covae, F. davisii, and F. oreochromis, along with their host associations is critical to understanding disease dynamics and prevention strategies. Outbreaks of the disease typically follow stressful events where CCB colonise external tissues, such as skin and gill. Vaccination against columnaris disease was practiced as early as the 1970's. Since then, many different vaccine types have been under development, including killed, live-attenuated, recombinant, and micro- and nano-particle vaccines. Among the different CCB vaccine types, immune responses and level of protection have been variable. However, some vaccines have been reported to offer moderate-significant protection in some fish species. This review synthesises the current knowledge of CCB taxonomy, genetic diversity, virulence, and immunity in various aquaculture species, with the goal of identifying knowledge gaps to be addressed for future CCB vaccine development. The development of an efficacious columnaris vaccine remains a challenge, with promising results in the laboratory that have yet to be reflected under production settings. This highlights the need for improved strategies tailored to the species of CCB most appropriate for the target host species.