Yersiniosis

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

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