Flavobacteriosis poses a serious threat to wild and propagated fish stocks alike, accounting for more fish mortality in Michigan and its associated state fish hatcheries than all other pathogens combined. Although this consortium of fish diseases has primarily been attributed to Flavobacterium psychrophilum, F. columnare, and F. branchiophilum, herein we describe a diverse assemblage of Flavobacterium and Chryseobacterium spp. isolates recovered from diseased as well as apparently healthy wild, feral, and farmed fish of Michigan. Among 254 fish-associated flavobacterial isolates recovered from 21 fish species during 2003-2010, 211 were identified as Flavobacterium spp., whereas 43 were identified as Chryseobacterium spp. according to ribosomal RNA partial gene sequencing and phylogenetic analysis. Although F. psychrophilum and F. columnare were indeed associated with multiple fish mortality events, many previously uncharacterized flavobacteria were recovered from systemically infected fish showing overt signs of disease, and in vitro protease assays demonstrated that these isolates were highly proteolytic to multiple substrates that comprise host tissues. Indeed, the majority of the isolates either (1) were most similar to recently described fish-associated Flavobacterium and Chryseobacterium spp. that have never before been reported in North America (e.g., F. oncorhynchi, F. araucananum, C. viscerum, C. piscicola, and C. chaponense) or (2) did not cluster with any described species and most likely represent novel flavobacterial taxa. This study highlights the extreme diversity of flavobacteria that are potentially associated with flavobacteriosis in Michigan.

Salmonid species in the Laurentian Great Lakes basin suffer from early mortalities that are often associated with low survival rates of swim-up fry. Coho salmon (Oncorhynchus kisutch) alevins exhibiting a spinning swimming behavior and convulsions were presented to the Aquatic Animal Health Laboratory of Michigan State University. Twelve yellow-pigmented bacterial isolates that were motile via gliding were recovered from the brains of dead and moribund fish and identified as Flavobacterium spp. (34)(35)(36)(37)(38)(40)(41)(42)(43). 16S rRNA sequence analysis indicated that 9 of the 12 sequences were grouped into a single clade (CS29,31,(34)(35)(36)(37)(38)40,41) and were essentially identical over the 676 aligned bases used in the phylogenetic analysis. This clade is closely related to the eel-pathogenic Flavobacterium johnsoniae-like YO60 recovered from South Africa. The remaining three isolates, CS30, CS42, CS43 were closest to Flavobacterium sp. Wuba46 (100%), Flavobacterium sp. EP125 (100%), and Flavobacterium sp. WB 4.3-15 (99.9%), all of which were reported from European waters. Experimental challenge of five month old coho salmon fry via intra-peritoneal injection with the CS36 isolate resulted in morbidity and mortality rates of approximately 10% in the two highest infection doses. Clinical signs included tachybranchia, hemorrhages, and fin erosion. Stained tissue sections from dead and moribund fish showed degeneration of kidney tubules, edema in the renal interstitial tissues, heterophilic cellulitis and myodegenerative changes within the caudal peduncle, and a proteinaceous exudate in the coelomic cavity. Based on this study, flavobacterial infections with these isolates, which were detected for the first time in North America, can potentially cause losses in yolk sac and swim-up coho salmon fry.