Aquaculture America 2024

February 18 - 21, 2024

San Antonio, Texas

INVESTIGATING THE GENO- AND SEROTYPIC DIVERSITY OF Flavobacterium psychrophilum INFECTING CAPTIVE REARED SALMONIDS OF THE NORTH CENTRAL REGION OF THE USA

Nisha Shrestha*,  Myron Kebus , Sean Lennox,  Megan Shavalier , Christopher Knupp, Fabiana Pilarski, Matthew Smith, Nicholas Phelps , Thomas Loch

 

Michigan State University - Aquatic Animal Health Laboratory

1129 Farm Ln, East Lansing, MI 48824

Shrest60@msu.edu

 



 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 sero- diversity  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 s tudy 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 seven  facilities  in  seven  NCR states  e.g., Michigan (MI), Ohio (OH), Iowa (IA), Wisconsin (WI), Minnesota (MN) , Missouri (MO) , and South Dakota (SD) have been collected, clinically examined, and bacteriologically analyzed. Examined fish (n=161 )  presented with a range of gross disease signs consistent with BCWD, including  fin erosion,  external ulceration, exophthalmia, v isceral hemorrhage and/or pallor ,  and splenic swelling and enlargement . C ultures  for flavobacterial isolation (n=651 )  were prepared  on FPM- A medium  and inoculated with tissues f rom  the  gills, brain, kidney, spleen,  and  representative external lesions . As a result, 183 yellow- pigmented bacterial isolates were recovered, 121 of which were  identified  as  F. psychrophilum via  F. psychrophilum-specific endpoint PCR analyses . Overall ,  F. psychrophilum was detected in ~33% of the ex amined fish and  from six of the seven facilities.  As additional  facilities in the NCR are being sampled, genotyping  via multi locus sequence typing  is  underway.  Likewise, molecular  serotyping  has been completed for a subset of the recovered  F. psychrophilum  isolates,  revealing some interesting trends, including host species to serotype associations and  that some facilities  were affected by a single molecular serotype and others by two or more serotypes.  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 preparation s, 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.