The morphology and life cycle of Nosema helminthorum Moniez, 1887

Abstract

The life cycle of Nosema helminthorum proceeds as follows (see Fig. 39).Emergence of the sporoplasm. The spores swallowed by the vertebrate host (sheep) reach the small intestine where the tapeworm (Moniezia) is already present. Here are extruded the filaments with the sporoplasms attached to their tips. These sporoplasms come in contact with the cuticle of the tapeworm at various places, round off and work their way through the cuticle into the tissues of the worm.Schizogony. They next proceed between the subcuticular cells (3) and start multiplying by binary fission or by multiple division (4, 5, 6, 7). Some of the so-called schizonts are spherical with three to four nuclei (6), while others are elongate plasmodia which give rise to chains of daughter-schizonts by division (7). All these schizonts give rise eventually to faintly staining schizonts in which the nuclei appear to be less compact. Some of these now enter the second phase of schizogony in which elongate fusiform or spindle-shaped cells are ultimately produced (10, 14). The nuclei of these elongate cells divide and move to the opposite poles, and the cytoplasm constricts between them. In this way a chain of fusiform schizonts may be formed (11). The final products of these divisions are fusiform or spindle-shaped cells (11, 15) which are really the precursors of sporonts, in which the nuclei are divided but not separated. These nuclei remain closely associated and eventually fuse either before the sporont stage is reached or later.Sporogony. The sporonts are ovoidal cells which are generally uninucleate after fusion of the double nuclei of the previous stage. The nucleus is at the centre. Soon a vacuole is formed at the posterior pole, and within it there appears a premetachromatic granule (16) which gives rise to the metachromatic body in the mature spore.Spore formation. A chitinous spore wall is then secreted (17) and the filament is probably formed at this stage in the central axis of the cytoplasm. The rest of the cytoplasm transforms into the sporoplasm, which surrounds the filament like a girdle (18). The spore now becomes mature and is infective to a new host. It is passed out within the gravid segments of the worm and, when it reaches the exterior, is liberated when the segments degenerate. These spores are quite resistant and probably remain dormant during the winter; they are swallowed by Moniezia-infected lambs in spring, when the cycle is resumed.It is unlikely that the oribatid mite, which is the vector of Moniezia, plays any role in the transmission of Nosema helminthorum. My experimental studies (to be published later) suggest that the only possible part played by the mite is to carry the spores, accidentally swallowed by it, protecting them till they reach the adult worm already present in the intestine of the vertebrate host.