Experimental ovine sarcocystosis: sequential ultrastructural pathology in skeletal muscle

Six lambs inoculated with 70,000 Sarcocystis oocysts were killed at 14, 25, 33, 42, 60 and 81 days post-inoculation (pi). Tissue changes and developing cysts in one muscle (left M. semimembranosus) were studied by transmission electron microscopy. Perivascular macrophages contained intracytoplasmic merozoites at 25 and 33 days pi. Cysts with metrocytes were established at 42 days pi, at which time cyst wall zones were simple without surface projections. At 60 days pi, cysts contained metrocytes, some in division, and immature cystozoites. Cylindrical cyst surface projections were 1.55 microgram tall, 0.45 microgram wide and had specialized tip and base zones. Cysts at 81 days pi contained metrocytes and mature cystozoites. Cyst surface projections had a regular association with mitochondria of the parasitized myocyte. A mild or moderate myositis developed during both meront (25 to 33 days pi) and cyst-forming (42 to 81 days pi) stages. Initially histiocytic, the exudate eventually consisted of plasma cells and macrophages. A small minority of parasitized myocytes attracted a local inflammatory reaction. Following macrophage invasion of the sarcoplasm, the cyst wall zone degenerated and underwent phagocytosis and cystozoites became pyknotic. With advanced cyst degeneration there was local destruction of the host muscle fibre.