Parasite Morphology And Life Cycle

The life cycle of Toxoplasma gondii is illustrated in Figure 1.1.

1.3.1 Tachyzoites

The tachyzoite (Frenkel, 1973) is lunate (Figure 1.2A), and is the stage that Nicolle and Manceaux (1909) found in the gundi. This stage has also been called trophozoite, the proliferative form, the feeding form, and endozoite. It can infect virtually any cell in the body. It divides by a specialized process called endodyogeny, first described by Goldman et al. (1958). Gustafson et al. (1954) first studied the ultrastructure of the tachyzoite. Sheffield and Melton (1968) provided a complete description of endodyogeny when they fully described its ultrastructure.

1.3.2 Bradyzoite and tissue cysts

The term 'bradyzoite' (Gr. brady = slow) was proposed by Frenkel (1973) to describe the stage encysted in tissues. Bradyzoites are also called cysto-zoites. Dubey and Beattie (1988) proposed that cysts should be called tissue cysts (Figures 1.2B, 1.2C) to avoid confusion with oocysts. It is difficult to determine from the early literature who first identified the encysted stage of the parasite (Lainson, 1958). Levaditi et al. (1928) apparently were the first to report that T. gondii may persist in tissues for many months as 'cysts'; however, considerable confusion between the terms 'pseudocysts' (group of tachyzoites) and 'tissue cysts' existed for many years. Frenkel and Friedlander (1951) and Frenkel (1956) characterized cysts cytologically as containing organisms with a subterminal nucleus and periodic acid Schiff (PAS)-positive granules (Figure 1.2C) surrounded by an argyrophilic cyst

Tachyzoites transmitted through placenta

Definitive host (cat)

Tachyzoites transmitted through placenta

Contaminated food and water

Definitive host (cat)

Unsporulated oocysts passed in feces

Tissue cysts ingested by cat ^

Unsporulated oocysts passed in feces

Tissue cysts ingested by cat ^

Tissue cysts in intermediate hosts

Tissue cysts ingested in infected uncooked meat

Contaminated food and water

Tissue cysts in intermediate hosts

FIGURE 1.1 Life cycle of T. gondii.

Intermediate hosts oocysts in feed, water, or soil ingested by intermediate host oocysts in feed, water, or soil ingested by intermediate host

Sporulated oocysts

Sporulated oocysts

FIGURE 1.1 Life cycle of T. gondii.

FIGURE 1.2 Life-cycle stages of T. gondii.

(A) Tachyzoites (arrowhead) in smear. Giemsa stain. Note nucleus dividing into two nuclei (arrow).

(B) A small tissue cyst in smear stained with Giemsa and a silver stain. Note the silver-positive tissue cyst wall (arrowhead) enclosing bradyzoites that have a terminal nucleus (arrow).

(C) Tissue cyst in section, PAS. Note PAS-positive bradyzoites (arrow) enclosed in a thin PAS-negative cyst wall (arrowhead).

(D) Unsporulated oocysts in cat feces, unstained.

This figure is reproduced in color in the color plate section.

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