Abstract
This paper is the first description of a spontaneous glycogen‐storage disease in a lower vertebrate, as previous descriptions deal with humans and other mammals, or fish where the condition has been experimentally induced. Affected farmed rainbow trout experienced increased mortality from 60 days post‐startfeeding and displayed clinical signs of heart failure with abnormal behaviour, exophthalmia, distended abdomen and ventral skin petechiation. Necropsy revealed alterations in cardiac shape with distended atria and rounded ventricles. Microscopically, the compact wall of the ventricle was absent, uneven or thinner than normal. The cardiac myocytes contained extensive amounts of glycogen in cytoplasmic vacuoles as demonstrated by periodic acid‐Schiff staining that was abolished by saliva‐diastase pretreatment on serial sections. Associated lesions included conspicuous subepicardial and myocardial vascularization, epicardial thickening and necrosis of the ventricular compactum/spongiosum interphase. The lesions in cardiac myocytes had a striking resemblance to glycogenosis type II (Pompe disease), a rare autosomal recessive lysosomal storage disease in humans. This condition was more severe and mortality was higher in a replicate/parallel fish group treated perorally with 17 α ‐methyltestosterone to produce all‐female progeny, indicating that the hormone treatment aggravated the condition resulting in earlier and more severe manifestation of the disease in this group.