Flavobacterium covae is the predominant species of columnaris causing bacteria impacting the channel catfish (Ictalurus punctatus) industry in the southeastern USA.

Columnaris disease is a leading cause of disease related losses in the catfish industry of the Southeastern USA. The term 'columnaris causing bacteria' (CCB) has been coined in reference to the four described species that cause columnaris disease, Flavobacterium columnare, F. covae, F. davisii, and F. oreochromis. Historically, F. columnare, F. covae, and F. davisii have been isolated from columnaris disease cases in the catfish industry; however, there is a lack of knowledge of which CCB species are most prevalent in farm-raised catfish. The current research objectives were (1) sample columnaris disease cases from the US catfish industry and identify species of CCB involved, and (2) determine the virulence of the four CCB species in channel catfish in controlled laboratory challenges. Bacterial isolates or swabs of external lesions from catfish were collected from 259 columnaris disease cases in Mississippi and Alabama from 2015-0027. DNA extracted from the samples were analyzed using a CCB-specific multiplex PCR to identify the CCB present in each diagnostic case. Results demonstrated that F. covae is the predominant species of CCB impacting the US catfish industry, present in 94.2% (n = 244) of diagnostic case submissions. Challenge experiments demonstrated F. covae and F. oreochromis were highly virulent to channel catfish resulting in near 100% mortality for most isolates. In contrast, F. columnare and F. davisii were less virulent resulting in less than 40% mortality for most isolates. Collectively, these results demonstrate F. covae is the predominant CCB in the US catfish industry and research aimed at developing new control and prevention strategies should target this bacterial species. The methods described herein can be used to continue monitoring the prevalence of CCB in the catfish industry and can be easily applied to other industries to identify which Flavobacterium species have the greatest impact.