Aquaculture America 2020

February 9 - 12, 2020

Honolulu, Hawaii

DETERMINATION OF CLINICAL RELEVANCE OF ANTIBIOTIC RESISTANCE IN Flavobacterium psychrophilum STRAINS

Jie Ma*, Timothy J. Bruce, Christopher Knupp, Thomas P. Loch, and Kenneth D. Cain
 
Department of Fish and Wildlife Sciences
875 Perimeter Dr. MS1136
University of Idaho
Moscow, ID, 83844-1136
jiema@uidaho.edu
 

Bacterial coldwater disease (BCWD), caused by Flavobacterium psychrophilum (Fp ), remains one of the most significant bacterial diseases of salmonids worldwide, and losses sustained in farmed salmonids are particularly substantial. Currently there are  two antibiotics, Terramycin 200 (oxytetracycline dihydrate; OTC ) and Aquaflor (florfenicol; FFC)  approved to treat Fp infections in food fish by the U.S. FDA . Treatment efficacy with these two antibiotics can vary ,  and  antimicrobial resistance  (especially to OTC) has been documented  in some Fp strains. Therefore, research investigating the relationships betwe en antibiotic resistance in Fp , and in vivo virulence is imperative. Our lab tested t wo Fp  strains (US57 and US87)  showing  varied  in vitro  antibiotic sensitivity against OTC and FFC. To evaluate the clinical relevance of antibiotic resistance in th e Fp strains, experimental challenges of rainbow trout fry were conducted by intramuscular injection (IM) Fp US57 or Fp US87 str ain with 105 CFU/fis h. Five days post-challenge, fish were fed at 1% body weight (BW)/d for 10 days on the oxytetracycline-medicated feed at high rate (3.75g/100lb fish) or low rate (2.0g/100lb fish), or florfenicol-medicated feed at high rate (15 mg/kg BW/day) or low (10 mg/kg BW/day), respectively. All treatment groups except  for  the mock-challenged fish had mortalities with symptoms of BCWD, but the cumulative percent mortality (CPM) varied significantly among treatments. FFC treatments significantly (p<0.01) reduced  the mortality of rainbow trout infected w ith US87 strain, but  only  had  a moderate inhibition effect on US57-challenged fish . Both US57 and US87 strains  showed resistance to OTC as CPM was not different from untreated controls at either high or low treatment rates .  Results from this study illustrate the importance of antibiotic sensitivity testing on Fp  strain and point to the need to utilize antibiotic treatment judiciously for control of BCWD.