Characterization of Novel Flavobacterium spp. Involved in the Mortality of Coho Salmon (Oncorhynchuskisutch) in their Early Life Stages

Abstract

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.