Macrophages and giant cells associated with a microsporidian parasite causing liquefaction of the skeletal muscle of the Norway pout, Trisopterus esmarkii (Nilsson)

. A mature microsporidian infection of the skeletal muscles of Norway pout, Trisopterus esmarkii Nilsson, is described. Infected fish had noticeable liquefaction of the muscle. The white foci of infection contained packed microsporidian spores, and the surrounding infected muscle fibres were degenerate. The host response involved invasion by phagocytic cells and encapsulation by fibroblasts, forming large granulomas. Macrophages, 17–20 μm in diameter, were actively ingesting the spores and typically had a foamy cytoplasm. Some had formed Langhans‐type giant cells, 35 μm in diameter, with peripherally arranged nuclei. Sporulation was complete and appeared to have occurred within sporo‐ phorous vacuoles in which numerous spores were formed. Individual spores measured 2·8 × 1·5 μm and were characterized by irregular electron dense areas, a short polar filament with four coils and a large posterior vacuole with three inclusions.