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Transmission
of Aeromonas salmonicida and Yersinia ruckeri In a Salmonid Recycle
System By Means Of Fluidized Sand Biofilters Graham L. Bullock1, B. Schill2, and E. Burchard1 1Freshwater
Institute, P.O. Box 1889, Shepherdstown, WV, 25443; 2National Fish Health Research Laboratory, U.S.
Geological Survey, 1700 Leetown Road,
Kearneysville, WV, 25430 Control
of infectious diseases in a recirculating fish culture system can be difficult
because the pathogen may become established in the biofilter and cause a
reoccurrence of the disease or infect newly stocked fish. We tested whether Aeromonas salmonicida, causative agent of furunculosis, or Yersinia ruckeri, causative agent of
enteric redmouth, disease could become established in fluidized sand biofilters
and then infect newly stocked salmonids.
Each pathogen was tested separately, and after fish were removed from
the recycle system, 48 hr broth cultures of the pathogen under study were
continuously pumped into the system for five days. Both pathogens could be cultured from the biofilters and tank
water after the five-day pumping of culture.
Immediately after addition of culture the entire system, except for the
biofilters was disinfected with 200 ppm chlorine for two hours and the system
was rinsed with spring water.
Biofilters were rinsed for 24 hr with spring water to attempt removal of
the pathogen. After disinfecting and
rinsing, the system was stocked with specific pathogen free rainbow trout (Oncorhynchus mykiss) to detect Y. ruckeri, or arctic char (Salvelinus alpinus) to detect A. salmonicida. Three weeks after addition of rainbow trout,Y. ruckeri was cultured from feces and
trout started dying from enteric redmouth disease a few days later. Two weeks after addition of char, clinical
furunculosis was diagnosed and A.
salmonicida was isolated from mucus of survivors. After furunculosis or redmouth occurred all trout or char were
removed, the entire system was disinfected with 10-ppm chloramine-T, and the
system was rinsed with spring water.
Specific pathogen free trout or char were stocked into the system and
monitored for redmouth or furunculosis for six weeks. No disease was seen in either salmonid species nor could either
pathogen be isolated from fish or biofilters.
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