Title: Studies of experimental infection of Arctic charr (Salvalinus alpinus L.) and Aeromonas salmonicida supsp. achromogenes

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A gradually increased mortality caused by atypical furunculosis induced by Aeromonas salmonicida subsp. achromogenes (Asa) in vaccinated Arctic char (Salvelinus alpinus, L.) farmed in Iceland has been reported. Therefore, resistance to atypical furunculosis has been included in the breeding aims for Arctic char in Iceland. Selection of genetic disease resistance and evaluation of vaccine efficacy requires reliable and applicable challenge methods. The aims of the present study were to develop an effective and reliable method for challenge of Arctic char with Asa, and to describe the gross pathology of atypical furunculosis in Arctic char. First a cohabitation challenge was performed where injection infected donor fish was cohabitated with vaccinated and unvaccinated fish. Different numbers (12, 18 & 20) of donor fish infected with Asa, strain Keldur265-87, were introduced into 4 tanks (2 replicates), once at water temperature 8°C and twice at 12oC. An immersion challenge was performed by bathing 200 unvaccinated char in a 120L of a suspension of Asa, strain F131-16 (106 CFU/mL) at 13oC. Moribund and dead fish were collected for pathological examinations. Disease signs included haemorrhages of fins, tail, skin, muscle and internal organs, pale skin and gill colour, loss of appetite and gaping. The cohabitation challenge induced very weak disease transmission under the conditions applied in this study and vaccinated fish were fully protected. The survival estimates were significantly elevated when the water temperature was increased (p= 0.015). Immersion challenge of char resulted in high and acute mortalities. The cumulative mortality reached 98.6 % (± 2.61) in 17 days with a mean day to death of 7.13 ± 0.23. Virulence of strain F131-16 to char was found to be superior to that of strain Keldur265-87. Strain F131-16 was recently isolated from vaccinated char with atypical furunculosis, but strain Keldur265-87, originating from diseased Atlantic salmon (Salmo salar L.), has been kept frozen for 3 decades, but occasionally passaged (injected into fish and re-isolated from the head kidney of dead fish) in fish. An interesting next step would be to try a cohabitation challenge of char with strain F131-16.

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