The life cycle of the coccidian parasite, Toxoplasma gondii, in the domestic cat

Abstract

Earlier work in which the coccidian nature of the protozoan parasite Toxoplasma gondii was revealed has been confirmed and extended in a series of experiments using only specific pathogen free (SPF) cats. All of these cats, prior to our experiments, were negative for both Coccidia and Toxoplasma. Experiments on 17 of these cats are described. 10 were fed with mouse brains containing tissue cysts of Toxoplasma. 8 of these shed Toxoplasma oöcysts in their faeces within 5 to 10 days of infection. 2 cats had shed no oöcysts before they were killed on the 3rd and 4th day after infection. 5 cats were fed with Toxoplasma oöcysts but gametogonic development resulting in the production of oöcysts was observed in only 1 individual; oöcysts appeared in the faeces 9 days after the infective meal. 2 cats were examined as controls; 1 received no addition to its diet, the other was fed “negative” brains. Neither showed any evidence of infection. Examination of the gut of infected cats showed that shizogonic and gametogonic stages develop usually in the ileum although the entire length of the small intestine can be affected. The parasites were observed in the epithelium of the tips of the villi, developing within vacuoles, and lying between the nucleus and brush border of the cell. All stages of schizogony and gametogony may be found together, occurring in no particular sequence along the epithelium of the villus; multiple infections of the host cells are common. Schizonts are characterized by granular, basophil cytoplasm and vesicular, karyosomic, Feulgen-negative nuclei which become dense and Feulgen-positive when merozoite formation takes place. The merozoites vary from 4 to 29 per schizont and measure 3·5-4·5μ by 1μ in sections; in smears their dimensions are 4·9 (SD = 1·0)μ by 1·5μ. They form clusters about a residual body and may appear to radiate fan-wise from it in sections. The microgametocyte is spherical, about 10μ in diameter and gives rise to 12–32 microgametes which form on its surface, are slender and crescentic and about 3μ in length, excluding the flagella. The macrogametocyte has, throughout development, a vesicular, karyosomic and Feulgen-negative nucleus. At the commencement of development, the cytoplasm contains many basophil and few PAS-positive granules; at its completion this condition is reversed. No granules were seen to be associated with the formation of the oöcyst wall. The oöcyst itself measures 12·7 (SD = 0·7)μ by 10·4 (SD = 0·8)μ. Sporulation, which does not occur within the gut of the cat, gives sporocysts measuring 7·8 (SD = 0·9)μ by 6 (SD = 0·5)μ. These coccidian stages of Toxoplasma are compared and contrasted with those of Isospora spp. of the cat. Despite its significant resemblances to Isospora, the retention of the existing nomenclature of Toxoplasma gondii is supported.