Cryopreparation of live sporozoites and oocysts of the apicomplexan parasite Cryptosporidium parvum, followed by transmission electron microscopy, was undertaken to show the 3D arrangement of organelles, their number and distribution. Profiles of parasites obtained from energy-f iltering transmission electron microscopy of serial sections provided 3D reconstructions from which morphometric data and stereo images were derived. The results suggest that sporozoites have a single rhoptry containing an organized lamellar body, no mitochondria or conventional Golgi apparatus, and one or two crystalline bodies. Micronemes were shown to be spherical, numerous and apically located, and to account for 0 8 % of the total cell volume. Dense granules were less numerous, larger, accounted for 58% of the cell volume, and were located more posteriorly than micronemes. A structure juxtaposed to the nucleus with similarities to the plastid-like organelle reported for other members of the Apicomplexa was observed. The detailed analysis illustrates the advantages of cryopreparation in retaining ultrastructural fidelity of labile or difficult to preserve structures such as the sporozoite of Cryptosporidium.

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A new viability assay for Cryptosporidium and Eimeria sporozoites is described. It involves the use of both acridine orange and bis-benzimide and is more rapid, easier and less subjective than procedures used previously. The assay has been used to investigate the effects of respiratory inhibitors and pH on the sporozoites of C. parvum, C. muris and E. tenella. Neither cyanide nor azide reduced the viability of C. parvum or E. tenella, whereas they had some effect on C. muris. This latter organism, an intracellular parasite of stomach epithelial cells, also differed from the other two in being able to survive pH 2 for as long as 1 h.