SUMMARY Infections by the protozoan parasite Toxoplasma gondii are widely prevalent worldwide in animals and humans. This paper reviews the life cycle; the structure of tachyzoites, bradyzoites, oocysts, sporocysts, sporozoites and enteroepithelial stages of T. gondii; and the mode of penetration of T. gondii. The review provides a detailed account of the biology of tissue cysts and bradyzoites including in vivo and in vitro development, methods of separation from host tissue, tissue cyst rupture, and relapse. The mechanism of in vivo and in vitro stage conversion from sporozoites to tachyzoites to bradyzoites and from bradyzoites to tachyzoites to bradyzoites is also discussed.

Neosporosis is a disease caused by the Toxoplasmosis gondii-like parasite, Neospora caninum. Until 1988, N caninum was confused with the structurally similar coccidian T gondii. 1 The disease was recognized and the features of the clinical disease it causes in congenitally infected dogs was reported in dogs from Norway in 1984. 2 This report prompted a retrospective study that provided a scientific description of the parasite and the name N caninum. 3 The parasite was isolated in cell cultures from congenitally infected Labrador retriever pups. 4 This process provided a source of antigen for immunologic 4 and immunohistochemical 5 studies. Researchers used these tools and within a few years it became apparent that neosporosis was an important cause

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