Transmission electron microscopic studies on the effects of toltrazuril on Glugea anomala, Moniez, 1887 (Microsporidia) infecting the three-spined stickleback Gasterosteus aculeatus.

A symmetric triazinone, toltrazuril, was tested in vivo against Glugea anomala parasitizing the connective tissue of sticklebacks (Gasterosteus aculeatus). Naturally infected sticklebacks were incubated in toltrazuril-containing water as intermittent therapy (3 x 2 micrograms/ml for 6 h at 3-day intervals or 3 x 2 micrograms/ml for 24 h at 2-day intervals). As seen at the ultrastructural level, the drug caused severe damage to all developmental stages of G. anomala. When treatment was carried out for 6 h, the xenoma wall exhibited a network appearance. The multinucleate meronts showed tapering at one end and were lysed at places; disintegration of the nuclei was also observed. The sporogonial plasmodia were partially fragmented, and the dumbbell-shaped nuclei of the sporoblast mother cells showed damaged spindle fibres and lysis of the chromosomal material and nuclear membranes. The shape of the mature spores was altered as well. When treatment was done for 24 h, the xenoma wall was completely destroyed. The uni- and multinucleate meronts were completely destroyed and lacked their nuclei, and the sporogonial plasmodia were frequently totally fragmented. The development of the sporophorous vesicle stopped in many cases. In the sporoblasts and their mother cells, vacuolization of the cytoplasma and lysis of the nuclei were observed. Destroyed spores with damaged polar tubes and polaroblasts were frequently detected. It seems likely that toltrazuril could be successfully applied against Microsporidia in fish when used as intermittent therapy at low doses and with prolonged exposure.