Flavobacterium covae is the predominant species of columnaris‐causing bacteria impacting the Channel Catfish industry in the southeastern United States.

Objective: Columnaris disease is a leading cause of disease‐related losses in the catfish industry of the southeastern United States. 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 to (1) sample columnaris disease cases from the U.S. catfish industry and identify the species of CCB involved and (2) determine the virulence of the four CCB species in Channel Catfish Ictalurus punctatus in controlled laboratory challenges. Methods: Bacterial isolates or swabs of external lesions from catfish were collected from 259 columnaris disease cases in Mississippi and Alabama during 2015–2019. The DNA extracted from the samples was analyzed using a CCB‐specific multiplex polymerase chain reaction to identify the CCB present in each diagnostic case. Channel Catfish were challenged by immersion with isolates belonging to each CCB species to determine virulence at ~28°C and 20°C. Result: Flavobacterium covae was identified as the predominant CCB species impacting the U.S. catfish industry, as it was present in 94.2% (n = 244) of diagnostic case submissions. Challenge experiments demonstrated that F. covae and F. oreochromis were highly virulent to Channel Catfish, with most isolates resulting in near 100% mortality. In contrast, F. columnare and F. davisii were less virulent, with most isolates resulting in less than 40% mortality. Conclusion: Collectively, these results demonstrate that F. covae is the predominant CCB in the U.S. 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. Impact statementColumnaris disease is a leading cause of disease‐related losses in the southeastern U.S. catfish industry. Recently, four species of columnaris‐causing bacteria (CCB) have been described, but their prevalence in the industry was unknown. In this research, Flavobacterium covae was identified as the predominant species of CCB impacting the industry and was demonstrated to be highly virulent in Channel Catfish. Research aimed at developing new control and prevention strategies should target this bacterial species. [ABSTRACT FROM AUTHOR]