Toxoplasma gondii is an obligate intracellular parasite that is the causative agent of toxoplasmosis. This member of the phylum Apicomplexa may infect any warm-blooded animals and humans due to its facultatively heteroxenous life cycle. Most infections are asymptomatic but at times the parasite may...
The life cycle and transmission cycle of Toxoplasma gondii
Toxoplasma gondii is an obligate intracellular parasite that is the causative agent of
toxoplasmosis (Hökelek, 2019). This member of the phylum Apicomplexa may infect any warm-
blooded animals and humans due to its facultatively heteroxenous life cycle (Tenter, Heckeroth
& Weiss, 2000). Most infections are asymptomatic but at times the parasite may cause retinal
lesions. T. gondii can also lead to life-threatening infections including the central nervous system
when contracted congenitally or in immunocompromised individuals (Lourido, 2019). Members
of the family Felidae including the domestic cat, are the definitive hosts, in which the parasite
develops sexually. Asexual cycle occurs in several mammals and birds, which act as the
intermediate hosts, acquiring the infection by ingesting oocysts or tissues from chronically
infected animals as shown in Figure 1 (Dubey, 1996). The purpose of this essay is to explain the
life cycle and transmission cycle of Toxoplasma gondii.
Figure 1. Transmission pathways of Toxoplasma gondii. a) The definitive feline host. b) Cat
feces with unsporulated oocysts. c) Food contaminated with oocysts that have been speculated.
d) Intermediate host can consume oocysts through raw vegetables or water. e) Intermediate hosts
including cattle, poultry, sheep and swine. f) Tissue cysts ingestion through uncooked meat. g)
Humans (intermediate hosts). h) Transmission of tachyzoites to the fetus via placenta. i)
Transmission via transfusion of blood. j) Transmission via transplantation of organ. (Attias et al.,
2020).
Image source: https://doi.org/10.1186/s13071-020-04445-z
The definitive host may become infected with T. gondii by ingestion of any of three infectious
stages: tachyzoites that multiply rapidly, bradyzoites that inhabit cysts in infected tissue, and
, oocysts that contain sporozoites excreted by another feline (Dubey, 1996). Following the
ingestion of prey which contains tissue cysts, the disruption of the wall of the cysts occurs in the
stomach by proteolytic enzymes releasing bradyzoites or sporozoites. In both cases, bradyzoites
or sporozoites firstly invade the epithelial cells which turn into schizonts in the stage of asexual
reproduction as shown in Figure 2 (Attias et al., 2020). Bradyzoites divided by schizogony in the
enterocytes give rise to merozoites after several rounds of nuclear divisions (Dubey, Lindsay &
Speer, 1998). The schizonts and merozoites are mostly present in the ileum portion of the
intestine three to fifteen days after initial feline infection and some begin to develop into
gametes. Oval shaped macrogametes are produced from a single merozoite, whereas elongated
microgametes are produced by a schizogonic division (Attias et al., 2020). Macrogametocytes
and microgametocytes fuse to form zygotes which then become encapsulated within a rigid wall
(Hökelek, 2019). After fertilization, oocysts produced within enterocytes are liberated by
disruption of the cell and excreted with the feces of the cat (Gangeux & Dardé, 2012). Oocysts
undergo sporulation in the environment to become infectious sporulated oocysts. This process
takes around 7 days at 20-25 degrees Celsius and results in the development of two sporocysts,
each with four sporozoites (Freppela et al., 2019).
Figure 2. Life cycle of Toxoplasma gondii in a definitive host (domestic cat). a) Ingestion of
prey that contains tissue cysts by a definitive host. b) Digestion of the cyst wall in the stomach
and intestine, releasing bradyzoites. c) Invasion of the intestine epithelial cells by bradyzoites. d)
Bradyzoites division in the enterocytes by schizogony producing merozoites. e) Differentiation
of the merozoites into macrogametes or microgametes (f). g) Fertilization producing
unsporulated oocysts shed with feces of the domestic cat (h). i) Sporulation and production of
two sporocysts with four sporozoites each (j) (Attias et al., 2020).
Image source: https://doi.org/10.1186/s13071-020-04445-z
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