The Apicomplexan Parasite Toxoplasma gondii

The Apicomplexan Parasite Toxoplasma gondii: History

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Toxoplasma gondii is a ubiquitous zoonotic parasite with an obligatory intracellular lifestyle. It relies on a specialized set of cytoskeletal and secretory organelles for host cell invasion. When infecting its felid definitive host, T. gondii undergoes sexual reproduction in the intestinal epithelium, producing oocysts that are excreted with the feces and sporulate in the environment. In other hosts and/or tissues, T. gondii multiplies by asexual reproduction. Rapidly dividing tachyzoites expand through multiple tissues, particularly nervous and muscular tissues, and eventually convert to slowly dividing bradyzoites which produce tissue cysts, structures that evade the immune system and remain infective within the host. Infection normally occurs through ingestion of sporulated oocysts or tissue cysts. While T. gondii is able to infect virtually all warm-blooded animals, most infections in humans are asymptomatic, with clinical disease occurring most often in immunocompromised hosts or fetuses carried by seronegative mothers that are infected during pregnancy.

Toxoplasma gondii
toxoplasmosis
parasite
tissue cyst
endodyogeny
lytic cycle
life cycle
oocyst
Apicomplexa

Toxoplasma gondii is frequently described as one of the most successful parasites, due to its ubiquitous distribution, the wide range of host species it is able to infect and its high prevalence rates around the world. The amenability of T. gondii to laboratory conservation and propagation both in vivo and in vitro as well as to genetic manipulation have made this parasite a widely used biological model for the study of conserved biological processes in closely related parasites more challenging to manipulate in a laboratory setting ^[1].

T. gondii was initially observed in the tissues of the rodent Ctenodactylus gundi by Nicolle and Manceaux (1908) ^[2] and in the tissues of a rabbit by Splendore (1908) ^[3]. At the time, the host Ctenodactylus gundi was erroneously identified as Ctenodactylus gondi, which resulted in the specific epithet of T. gondii. The name of the genus was given as a reference to the shape of the parasite, from the Greek tóxon meaning arc or bow ^[4]. T. gondii is an obligate intracellular parasite and is the causative agent of toxoplasmosis in humans and animals. T. gondii has a worldwide distribution and infects a wide variety of animals, although only a fraction develop disease ^[5]^[6]. Toxoplasmosis is usually acquired through ingestion of either sporulated oocysts that are shed by the felid definitive host and found in food and water supplies, or through consumption of contaminated meat products containing tissue cysts. Rapidly dividing tachyzoites readily invade and replicate in many tissues and cell types and, if not kept in check by the host’s immune system, extensive proliferation then causes acute disease and severe tissue pathology in multiple organs ^[7]. Thus, toxoplasmosis represents a health risk mainly for immunocompromised individuals and for fetuses upon primary infection during pregnancy ^[8]. Therapeutic and prophylactic options currently available are either insufficient, or cause severe adverse side effects in both humans and animals ^[9].

References

Jiménez-Ruiz, E.; Wong, E.H.; Pall, G.S.; Meissner, M. Advantages and disadvantages of conditional systems for characterization of essential genes in Toxoplasma gondii. Parasitology 2014, 141, 1390–1398.
Nicolle, C.; Manceaux, L. Sur Une Infection à Corps de Leishman (Ou Organismes Voisins) Du Gondi. Comptes Rendus Séances L’académie Sci. 1908, 147, 763–766.
Splendore, A. Un Nuovo Protozoa Parassita de’ Conigli. Rev. Soc. Sci. 1908, 3, 109–112.
Nicolle, C.; Manceaux, L. Sur Un Protozoaire Nouveau Du Gondi. Comptes Rendus Séances L’académie Sci. 1909, 148, 369–372.
McLeod, R.; Van Tubbergen, C.; Montoya, J.G.; Petersen, E. Human Toxoplasma Infection, 2nd ed.; Elsevier: Amsterdam, The Netherlands, 2014; ISBN 9780123964816.
Stelzer, S.; Basso, W.; Silván, J.B.; Ortega-Mora, L.-M.; Maksimov, P.; Gethmann, J.; Conraths, F.; Schares, G. Toxoplasma gondii infection and toxoplasmosis in farm animals: Risk factors and economic impact. Food Waterborne Parasitol. 2019, 15, e00037.
Black, M.W.; Boothroyd, J.C. Lytic Cycle of Toxoplasma Gondii. Microbiol. Mol. Biol. Rev. 2000, 64, 607–623.
Aguirre, A.A.; Longcore, T.; Barbieri, M.; Dabritz, H.; Hill, D.; Klein, P.N.; Lepczyk, C.; Lilly, E.L. The one health approach to toxoplasmosis: Epidemiology, Control, and Prevention Strategies. Ecohealth 2019, 16, 378–390.
Wang, J.-L.; Zhang, N.-Z.; Li, T.-T.; He, J.-J.; Elsheikha, H.M.; Zhu, X.-Q. Advances in the development of Anti-Toxoplasma Gondii Vaccines: Challenges, opportunities, and perspectives. Trends Parasitol. 2019, 35, 239–253.

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