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Principles and Practice of Clinical Parasitology. Edited by S. Gillespie & Richard D. Pearson Copyright © 2001 John Wiley & Sons Ltd Print ISBN 0-471-97729-2 Online ISBN 0-470-84250-4
Joseph D. Schwartzman
Dartmouth-Hitchcock Medical Center, Lebanon, NH, USA
Toxoplasma gondii is a complex eukaryotic parasite that appears to have all the cellular machinery required for independent life but that has adopted an obligate intracellular existence. It can infect and grow within many cell types of a wide range of animal species. T. gondii causes a broad spectrum of disease in the various species it infects, including humans; however, most infections are asymptomatic. The parasite actively penetrates host cells, sets up a privileged compartment in which it replicates and finally kills the cell. A coordinated host cell-mediated immune response is required to control the acute infection and maintain suppression of the long-lived cysts, which may persist for the life of the host. The ability of T. gondii to infect many animal species and its worldwide distribution set it apart from other members of the phylum Apicomplexa, which typically are strictly host-specific and geographically constrained. T. gondii is found on all continents, with only a few isolated islands apparently free of this successful parasite. The ability to infect birds, domestic animals and people by several possible routes, and the wide distribution of the parasite, are the likely causes of a high prevalence of infection in humans. Up to one-third of the world’s population has evidence of exposure to and chronic infection with T. gondii. Symptomatic disease is unusual except in those with severe immunocompromise and in infants with congenital infection.
In the agricultural sector, toxoplasmosis causes an economically important incidence of fetal loss among domesticated animals such as goats, sheep and pigs.
Investigators on two continents first described T. gondii in 1908. Nicolle and Manceaux, at the Institut Pasteur in Tunis, identified and named the parasite in a cricitine rodent, the North African gondi (Ctenodactylus gundi), native to the mountains of southern Tunisia, and maintained in their laboratory (Nicolle and Manceaux, 1908, 1909). Splendore, in Brazil, noted identical forms in a laboratory rabbit (Splendore, 1908, 1909). Appreciation of the spectrum of disease that the parasite can cause came slowly. Wolf and Cowen (1937) at Columbia University identified the parasite in central nervous system lesions in infants that had been diagnosed with meningoencephalitis. Understanding of the role of chronic infection came with the identification by Wilder (1952) of Toxoplasma in necrotic lesions of the retina of eyes previously thought to have been involved with tuberculosis or syphilis. The high prevalence of the infection in various populations was first shown by the serological test developed by Sabin and Feldman (1948), which relied on the ability of human serum to induce leakage of
Principles and Practice of Clinical Parasitology
Edited by Stephen Gillespie and Richard D. Pearson © 2001 John Wiley & Sons Ltd
PRINCIPLES AND PRACTICE OF CLINICAL PARASITOLOGY
extracellular dye into live tachyzoites in the presence of complement. The recognition of congenital toxoplasmosis in infants came before either generalized disease in adults or the lymphadenitis of primary Toxoplasma infections in adults was appreciated (Wolf and Cowen, 1937). The role of reactivation of latent infections in the production of disease in immunosup-pressed adults was recognized at the outset of solid organ transplantation (Ruskin and Remington, 1976). In the early 1980s, central nervous system reactivation with multifocal encephalitis became a major presentation of disease in patients with AIDS (Luft et al., 1983a).
DESCRIPTION OF THE ORGANISM Classification
The parasite is a member of the phylum Apicomplexa, class Sporozoa, subclass Coccidia, order Eucoccidia and suborder Eimeria (Levine et al., 1980). It is therefore related to malaria and a large number of coccidians that generally infect birds and mammals. The parasite was recognized as a coccidian only in 1969, when four laboratories independently established the sexual cycle (Frenkel, 1970; Frenkel et al., 1970). Traditional classification schemes have relied on morphological comparisons of the various life stages, most importantly the sexual stages. By these criteria, T. gondii closely resembles Isospora spp. and Sarcocystis spp. and, although it has been argued that the organism name should be changed, it continues to be validly named as T. gondii. More recently, molecular genetic techniques have shown that T. gondii is a single species related to Isospora, Sarcocystis, Frenkelia and Hammon-dia but most closely related to Neospora caninum (Guo and Johnson, 1995).