Large-scale molecular serogroup analysis of Flavobacterium psychrophilum isolates recovered from captive, wild, and feral fishes in the USA

Flavobacterium psychrophilum , causative agent of bacterial coldwater disease (BCWD) and rainbow trout fry syndrome (RTFS), is one of the most problematic bacterial pathogens affecting captive-reared salmonids (Family Salmonidae ) worldwide. Considerable efforts have been undertaken to characterize the genetic diversity of F. psychrophilum using multilocus sequence typing (MLST) in the United States and elsewhere. In comparison, few studies have sought to characterize the serovariation of F. psychrophilum in the USA. To this end, 320 F . psychrophilum isolates, originating from 17 US states that were recovered from 12 fish species and 1 hybrid from 1981 to 2021, were analyzed using a multiplex polymerase chain reaction molecular serogrouping scheme. Results revealed substantial F. psychrophilum serovariation, with identification of all currently recognized molecular serogroups (i.e., Serogroups -0, -1, -2, -3, and -4). Significant associations between F. psychrophilum molecular serogroup and host fish species were identified: positive associations included rainbow/steelhead trout ( Oncorhynchus mykiss ) and Serogroup-2, and coho salmon ( O. kisutch ) and Serogroup-0; negative associations included rainbow/steelhead trout and Serogroup-0 as well as coho salmon and Serogroups-1 and -2. Some regional variation in the distribution of F. psychrophilum molecular serogroups was also apparent. Pairing MLST and molecular serogroup data uncovered further evidence of F. psychrophilum vertical transmission occurring in hatchery environments. Collectively, these findings provide a foundation for future F. psychrophilum vaccine research and further clarify the epidemiology of this problematic bacterium.