Investigating the Geno- and Serotypic Diversity of Flavobacterium psychrophilum Infecting Captive-Reared Salmonids of the North Central Region of the USA

Nisha Shrestha1,2, Myron Kebus3, Sean Lennox1,2, Megan Shavalier1,2, Christopher Knupp1,2, Fabiana Pilarski1,3, Matthew Smith4, Nicholas Phelps5, Thomas Loch1,2,3,6

 1Michigan State University - Aquatic Animal Health Laboratory, Aquatic Animal Disease Ecology Program​; 2Department of Fisheries and Wildlife, College of Agriculture and Natural Resources, Michigan State University; 3Dept. of Pathobiology & Diagnostic Investigation, College of Veterinary Medicine, Michigan State University; 4Analytics, Xylem/YSI​; 5Aquatic Invasive Species Research Center, University of Minnesota; 6Department of Comparative and Integrative Biology, Michigan State University​

ABSTRACT

Flavobacterium psychrophilum, causative agent of bacterial coldwater disease (BCWD) and rainbow trout fry syndrome, is a top contributor to disease-associated losses in salmonid (Family Salmonidae) aquaculture around the world. In the USA, BCWD outbreaks are a perennial problem, including within aquaculture facilities and hatcheries of the North Central Region (NCR). Although a range of BCWD prevention and control measures are available, their efficacy is often inconsistent at best. One possible factor contributing to these inconsistencies is the intraspecific geno- and serodiversity of F. psychrophilum that has become increasingly apparent in some regions of the USA and abroad. Unfortunately, most of the specific F. psychrophilum geno- and sero-variants responsible for losses in NCR trout and salmon farms have not been identified, a matter with likely implications for vaccine development and efficacy. To this end and as part of a larger study aiming to enhance the health of US farmed fishes, efforts to isolate, identify, and characterize the predominating F. psychrophilum variants in NCR trout and salmon facilities are underway. Thus far, moribund salmonids from six facilities in six NCR states (e.g., Michigan, Ohio, Iowa, Wisconsin, Minnesota, and South Dakota) have been collected, clinically examined, and bacteriologically analyzed. Examined fish (n=117) presented with a range of gross disease signs consistent with BCWD, including fin erosion, external ulceration, exophthalmia, visceral hemorrhage and/or pallor, and splenic swelling and enlargement. Cultures for flavobacterial isolation (n=486) were prepared on FPM-A medium and inoculated with tissues from the gills, brain, kidney, spleen, and representative external lesions. As a result, 171 yellow-pigmented bacterial isolates were recovered, 118 of which were identified as F. psychrophilum via F. psychrophilum-specific endpoint PCR analyses. Overall, F. psychrophilum was detected in ~43% of the examined fish and from five of the six facilities. As additional facilities in the NCR are being sampled, genotyping via multi locus sequence typing and molecular serotyping of the recovered F. psychrophilum isolates are underway. After identifying the predominating F. psychrophilum variants in the sampled NCR facilities, lab and field-based experiments will test the protective efficacy of various autogenous bacterin preparations, with the aim of producing efficacious, site-specific bacterins capable of enhancing fish health and productivity throughout the region. The findings from the study are expected to be of interest not only to fish health specialists but also to stakeholders in the aquaculture sector seeking effective strategies to mitigate the impacts of BCWD in the NCR.