Piscine Orthoreovirus, is a segmented virus, non-enveloped RNA, 60 – 80 µm, with icosahedral shape, belonging to the family Reoviridae, subfamily Spinareovirinae and is the aetiological agent of Heart and Skeletal Muscle Inflammation (HSMI).
The HSMI disease is diagnosed by the occurrence of histopathologic lesions in the heart and skeletal muscle; moderate to severe panmyocarditis (inflammation in the compact and spongy layers of the myocardium); and myocardial degeneration and necrosis.
The skeletal muscle also shows moderate to severe myodegeneration and necrosis, especially of the red muscle fibres, as well as inflammation.
Lesions in the skeletal muscle tend to occur mostly during the peak of the outbreak and, to a lesser extent, in the recovery phase. Histopathological lesions in other organs include liver necrosis and congestion, haemorrhages in liver, kidney, spleen and gills.
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