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The epidemiology of ERM in European trout farms has been thoroughly investigated by many independent researchers. Their work has shown that a number of factors increase the prevalence and severity of Yersinia ruckeri infections and losses associated with them.

Seasonal Fluctuations
Yersinia ruckeri infection builds up in fish over a period of time. The level and intensity of this infection will dictate the onset of clinical disease. Investigations have shown that this occurs in both immersion vaccinated and unvaccinated control fish. The peak occurrence of the bacterium in fish coincides with those periods where temperature fluctuates between 7 and 15 C (spring and autumn).

Infection with Yersinia ruckeri will lead to clinical disease and subsequent mortality in farmed rainbow trout when bacterial loading rises sufficiently and coincides with environmental or production stresses. Mortality can be severe despite antibiotic treatments. Infected populations contain carrier fish which will be a source of subsequent outbreaks of disease. In fact unvaccinated infected populations often require repeated antibiotic treatments during the production cycle.Infection and repeated disease and treatment cycles result in poor growth, a wide size variation in batches and poor quality fish as well as an increased risk of antibiotic residues or antibiotic resistance. Vaccination by immersion with AquaVac™ ERM vaccine followed by booster vaccination with AquaVac™ ERM Oral vaccine can eliminate the need for antibiotic treatments by significantly reducing infection levels in the vaccinated population and preventing clinical ERM outbreaks.

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