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Nosema algerae and N. eurytremae were successfully cultured in embryonic rat brain, Xenopus XTC-6 and Chang liver cells, and in embryonic rat brain and XTC-6 cells respectively. No parasites grew in cultures incubated at 38 °C but development took place in cells incubated at 34 °C and 27 °C. Increased levels of infections of the cultured cells were achieved by centrifugation of the spores on to the cells. The level of infection was also related to the type of medium used for hatching the spores, modifies NMRI giving better results than Leibowitz L15, and to the interval between adding the spores to the cell cultures and centrifugation.

SUMMARYA microsporidium, isolated from echinostome and strigeid larval trematodes in Lymnaea peregra, is described as a new species Unikaryon slaptonleyi sp.nov. The nuclei isolated at all stages of development, the disporoblastic sporogony and development in contact with host cell cytoplasm are used to assign the species to the genus Unikaryon. The absence of a vacuolar membrane to isolate the meronts and stages of sporulation from the host cell cytoplasm differentiates this genus from Encephalitozoon. Spores are uninucleate, have 17–21 turns of the polar filament coil and measure 5·0 × 2·8/μm fresh. U. slaptonleyi was isolated from rediae and metacercariae of Echinoparyphium recurvatum and sporo-cysts and cercariae of an unidentified strigeid trematode in L. peregra. It was transmitted in the laboratory to unidentified echinostomes in L. peregra and to unidentified strigeids in Planorbis planorbis by feeding the spores to field-collected snails from which cercariae were already emerging. In these natural and experimental hyperinfections the snail tissues were lightly infected but, in the helminths, much of the parenchyma and germinal tissue was destroyed, so that few cercariae were released and most of those were distorted. Similar heavy infections were produced in Fasciola hepatica in Lymnaea truncatula, when spores were fed to the snails 14 days after miracidial penetration, but even high doses (106 spores/snail) produced only light infections in Schistosoma mansoni in Biomphalaria glabrata, in only 2 out of 9 snails. No infections were obtained in larvae producing xiphidiocercariae in P. planorbis although echinostomes became infected under the same conditions. Of a number of aquatic and terrestrial arthropods tested for susceptibility by feeding or by inoculation of spores into the haemocoele, only Pieris brassicae became infected. In a small proportion of pupae surviving from larvae which had been inoculated with spores at 3rd or 4th instar, there was clear evidence of spore replication.

The transmission mechanisms of microsporidia between generations of Operophtera brumata, a geometrid moth with an univoltine life-cycle, have been investigated. The parasites, Orthosoma operophterae, Pleistophora operophterae and Nosema wistmansi were present in moths collected from Wistman's Wood on Dartmoor. An infection rate of 100% was found in eggs laid by moderately or heavily infected female moths. Thus, 100% infection was found in eggs laid by 78 out of 90 infected females. Lower prevalences were found in eggs laid by the remaining 12 and these moths were subsequently shown to have very light infections. Male moths do not pass on this infection to progeny. The development of O. operophterae was followed during embryonation. The microsporidia do not invade the cells of the developing larva but are restricted to the yolk, where spores accumulate in abundance after earlier merogonies. When the larva is mature and ready to hatch, spores are ingested with the remains of the yolk, as the larva eats its way through the egg-shell. Spores are then found in the meconium in the lumen of the gut. These spores are infective and, once the gut of the larva has become functional, are responsible for infecting the tissues of the larvae which harbour them, as well as healthy larvae reared with them. Spores of O. operophterae held in the natural environment for 7 months from winter until mid-summer still retained infectivity and those contaminating the environment from the bodies of one generation must contribute to transmission between generations.