Studies with experimental transmission of heart and skeletal muscle inflammation in Atlantic salmon, Salmo salar L.

Heart and skeletal muscle inflammation (HSMI) is a transmissible disease causing mortality in farmed Atlantic salmon, Salmo salar L. It is characterized by epi-, endo- and myocarditis and myocardial necrosis, as well as myositis and necrosis of red skeletal muscle. The present paper describes two infection experiments, with the aim of further exploring the infectivity and pathogenesis of HSMI. In both experiments, Atlantic salmon were intraperitonally injected with putatively infective material. The first experiment was carried out in fresh water, using cardiac tissue, blood plasma and cell cultured material as inoculates. In the second experiment, various tissues sampled from fish in the mid-outbreak phase were used to inoculate experimental fish in sea water. Also, cardiac tissue sampled before and after the outbreak phase was used. Finally, cardiac tissue pre-treated with chloroform was tested. In both experiments, all inoculates resulted in cardiac inflammation during the study period of 8 weeks. Early cardiac changes included perivasculitis and endocarditis, which were observed from 1-3 weeks post-challenge (p.c.). Focal myocarditis first appeared 3 weeks p.c., and the number of fish showing myocardial changes at 8 weeks p.c. was high in all groups. A possible mechanism for the development of HSMI is discussed.