The enteroepithelial cycle in the definitive host the cat

Five morphologically distinct asexual stages of T. gondii develop in entrocytes before gametogony begins (Dubey and Frenkel 1972). It is believed that merozoites develop into gamonts which occur throughout the small intestine, but are most prevalent in the ileum, where they are found 3-15 days after infection.

The microgamete (the male gamete) is biflagellate and fertilizes the macrogamete (the female gamete) within the enterocyte. Fertilization initiates oocyst wall formation. The oocyst is the developed zygote which is the product of the sexual reproduction through the fertilization of the macrogamete by the microgamete. The oocysts are discharged into the intestinal lumen by rupture of the epithelial cells, and thereafter excreted in cat faeces. The oocyst sporulates within 1-5 days after excretion, depending on aeration, humidity, and temperature, by dividing into two sporocysts. Each sporocyst contains four sporozoites. Thus, there are eight sporozoites in one oocyst. The sporulated oocyst can remain infectious in the environment for months, even years, in cold and dry climates (reviewed by Dubey 1977).

The prepatent period (interval between ingestion and shedding of oocysts) after the ingestion of tissue cysts is 3-10 days, with the peak oocyst production between five and eight days after a patent period varying from 7 to 20 days (Dubey and Frenkel 1972, 1976). Cats not previously infected with T. gondii shed oocysts after ingesting each of the infective stage of the parasite: the tachyzoite, the bradyzoite, and the sporozoite (Frenkel et al. 1970; Dubey and Frenkel 1976). The prepatent period varies according to which stage of T. gondii the cat is infected with, with a short (3-10 days) prepatent period when the oral inoculum contained bradyzoites and a long prepatent period (>21 days) when the inoculum contained tachyzoites or sporozoites (Freyre et al. 1989). Cats previously infected with T. gondii, and which produced oocysts during the previous infection, are generally immune against renewed oocyst shedding, but immunity is not life long (Frenkel and Smith 1982; Dubey 1995).

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