28th ANNUAL EASTERN FISH HEALTH WORKSHOP April 21-25, 2003 Establishment Of Aeromonas salmonicida And Yersinia ruckeri In Fluidized Sand Biofilters Of A Recycle Culture System And Subsequent Transmission Of These Pathogens To Newly Stocked Salmonids Graham L. Bullock1 and W. Bane Schill2 1The Conservation Fund’s Freshwater Institute, P.O. Box 1889, Shepherdstown, WV, 25443;  2U.S. Geological Survey, National Fish Health Research Laboratory, 11700 Leetown Road, Kearneysville WV 25430 Infectious diseases are a major cause of mortality in flow through intensive aquaculture systems and can be a larger problem in recycle systems because of the possibility of pathogens becoming established in biofilters and infecting newly stocked fish.  The present research was undertaken to determine if Aeromonas salmonicida or Yersinia ruckeri added to a recycle system would colonize fluidized sand biofilters in the system and infect newly stocked fish. Experiments were conducted in two identical recycle systems. Broth cultures of A. salmonicida or Y. ruckeri were pumped for five consecutive days into the pump inlet of each system (without fish).  Both bacterial species could be isolated from biofilters after addition of pathogens.  Washing biofilters and chlorine disinfection of the rest of the system did not prevent transmission of either pathogen to newly stocked fish.  Aeromonas salmonicida or Y. ruckeri were again pumped into the system for five days followed by disinfection of the entire recycle system with 12 ppm chloramine-T.  Arctic char or rainbow trout were stocked into the recycle system and three infectivity trials were carried out in chloramine –T disinfected recycle systems with each pathogen. Yersinia ruckeri was cultured from two of six biofilters in one trial but none of the rainbow trout in the three trials became infected within six weeks after chloramine-T disinfection.  In two of the A. salmonicida trials, the pathogen could not be isolated from biofilters, but char in one trial became infected.  In the third trial, A. salmonicida was isolated from four of six biofilters, but char did not become infected within six weeks.  We also found that A. salmonicida from infected Atlantic salmon stocked into both recycle systems containing rainbow trout colonized biofilters and mucus of trout within two months. Disinfection of one system with 12 ppm chloramine-T prevented transmission of A.salmonicida to stocked char, but char became infected when they were stocked in the system that was only drained and refilled. Return to 28th Annual Eastern Fish Health Workshop Return to Leetown Science Center Home Page