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  • Family : Enterobactericeae
  • Gram-negative
  • Motile
  • rod-shaped
  • Catalase positive
  • oxidase negative
  • Both immersion and injection vaccines are available and provide good protection against clinical outbreaks



  • Widespread
  • important pathogen of salmonids
  • Rainbow trout are especially susceptible, but steelhead, lake, cutthroat, brown and brook trout, and coho, sockeye, chinook, and Atlantic salmon are also affected
  • The bacterium has also been less frequently isolated from diseased channel catfish, European sea bass, emerald shiners, fathead minnows, cisco, baeri sturgeon, turbot, peled, whitefish, and muskum whitefish
  • It has also been isolated from asymptomatic goldfish, common carp, European eel, burbot, coalfish, and arctic char
  • Aquatic invertebrates (crayfish) and even mammals (muskrats) can harbor large numbers of bacteria


  • ERM outbreaks usually begin with chronic, low mortality, which generally escalates
  • Severity of ERM outbreaks depend mainly on strain virulence and degree of environmental stress
  • There are six serovars of Y. ruckeri. Type I (Hagerman) is the most common, widely distributed, and pathogenic
  • Rainbow trout are most commonly affected at 7.5 cm with more chronic infections occurring in larger (12.5 cm) fish
  • Peak disease severity is at 15 – 18 ° C
  • There is lower morbidity/mortality at low ( < 10 ° C) temperatures
  • Mortalities may occur for up to 60 days
  • A high percentage ( > 75%) of recovered fish may become carriers
  • Subclinical carriers cyclically shed bacteria from the lower intestine
  • the periodicity of the shedding cycle probably varies with environmental conditions
  • Cyclic shedding helps to explain fluctuation in pathogen prevalence in fish populations
  • High (15 – 18 °) temperature can cause carriers to begin shedding, leading to clinical disease
  • Clinical signs can develop within several days of the stress
  • There may be up to 70% mortality initially
  • Transmission occurs horizontally
  •  Kidney is the best organ for isolation during epidemics. Lower intestine appears to be better for isolating the bacterium from asymptomatic carriers


Clinical signs

  • Darkening
  • Anorexia
  • Lethargy
  • reddening in the mouth
  • abdominal distension
  • unilateral or bilateral exophthalmos
  • hyphema (blood spot)
  • reddened skin erosions found mainly on the head or mouth



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