Ultrastructural Comparison of Cysts and Zoites of Toxoplasma gondii, Sarcocystis muris, and Hammondia hammondi in Skeletal Muscle of Mice

Abstract

Cysts of Hammondia hammondi and Toxoplasma gondii in skeletal muscle of mice were compared at the ultrastructural level with cysts of Sarcocystis muris. Cysts of H. hammondi and T. gondii were similar, except for the size of the cystozoite, which was about 4 to 5 /im in H. hammondi, but about 7 to 8 ,tm in T. gondii. Both cysts were limited by a relatively smooth, but intensely folded, primary cyst wall, under which a zone of granular ground substance was always present. No secondary cyst walls were observed around the parasitized fiber. Both cysts differed from Sarcocystis cysts, which possessed a thick primary cyst wall, septa, and metrocytes that were found to persist even in old cysts. Septa were not found in cysts of H. hammondi or T. gondii, probably because of the uniformly close packing of the parasites inside the cysts and the absence of metrocytes. All these findings indicate that one can distinguish Toxoplasma and Hammondia from Sarcocystis on a morphological basis, but that there are no significant morphologic differences between the cysts of Hammondia and Toxoplasma. The generic distinction is based on the distribution of zoites and cysts in organs in intermediate and definitive hosts, and by the distinctive cycles. Hammondia hammondi, described by Frenkel and Dubey (1975), is an intestinal coccidium of cats. The coccidium forms cysts, similar to those of Sarcosporidia and Toxoplasma gondii, in striated muscles of intermediate hosts such as mice. Isolations have been reported from the central United States, Hawaii (Wallace, 1975), and Germany (Rommel and Seyerl, 1976). Hammondia hammondi forms cysts principally in skeletal muscle, and its life cycle is obligatorily heteroxenous. Thus, cats can be infected only with tissue cysts of H. hammondi from the intermediate hosts, and not with oocysts from its own feces; mice can be infected only with sporozoites, and not with zoites from the tissue cysts of mice. Toxoplasma gondii, on the other hand, forms cysts with about equal frequency in nervous tissue, skeletal muscle, heart muscle, and commonly, in smooth muscle and various other tissues, and its life cycle may be homoxenous or heteroxenous. Received for publication 8 December 1978. * Institut fur Zoologie II der Universitat Dusseldorf, D-4000 Dusseldorf, Federal Republic of Germany. Research supported by the Deutsche Forschungsgemeinschaft. t Department of Pathology and Oncology, The University of Kansas Medical Center, Kansas City, Kansas 66103. Research supported by a Humboldt Foundation Senior Scientist Award and Research Grant AI-07489 from the National Institute of Allergy and Infectious Diseases. The cycles of Hammondia and the Sarcosporidia are superficially similar. However, young H. hammondi cysts are infectious, whereas sarcocysts must mature before they infect the final host. Hammondia hammondi is distinguished by the occurrence of asexual multiplication in the gut of cats, the final host, preceding gamogony, whereas in Sarcocystis species, the cystozoites develop directly into gametocytes. Furthermore, in the genus Hamm ndia, oocysts are shed unsporulated, like those of T. gondii, whereas sarcosporidian oocysts always develop sporocysts with sporozoites prior to being shed. Apart from these important biological differences, all three genera form intracellular cysts in striated muscles of mice, and these cysts look similar by light microscopy. Therefore, we wanted to study and compare skeletal muscle cysts of H. hammondi by means of electron microscopy, and to compare them with cysts of a Sarcocystis species and of T. gondii (Senaud and Mehlhor, 1974; Mehlhorn et al., 1976; Sheffield et al., 1977). MATERIALS AND METHODS