Development In The Intermediate Host

2.4.1 Tachyzoite development

When an intermediate host is infected by ingestion of tissue cysts or oocysts, the bradyzoites and sporozoites are released into the lumen of the small intestine. These invade the enterocyte or intra-epithelial lymphocytes of the small intestine, or pass through into the lamina propria and invade cells there. The process of invasion has not been observed, but is likely to be similar to that described previously for tachyzoites (see section 2.2.3). In either case, the parasite (bradyzoite or sporozoite) defaults to tachyzoite development with formation of the characteristic parasitophorous vacuole and undergoes multiplication by endodyogeny (see section 2.2.5). This process has been described in detail by Speer and Dubey (Dubey et al., 1998; Speer and Dubey, 1998). From in vitro studies it was originally proposed that the sporozoite entered a host cell and formed an enlarged PV, which it then left to form a second vacuole within which it underwent tachyzoite development (Speer et al., 1995). However, this was not observed in their in vivo studies, and could represent an in vitro artefact (Dubey et al., 1997). Thereafter, the tachyzoites disseminated systemically via the vascular system to all organs of the body. In the various organs they undergo proliferation by endodyogeny in many cell types, and are present initially in large numbers in the lungs and spleen, but by 6-10 days have invaded

FIGURE 2.19 A diagrammatic representation of (A) the cross-sectional and (B) the three-dimensional appearance of the sporocyst wall at the junction between the four plates which form the wall. The suture consists of a swelling of the adjacent plates forming the inner layer of the sporocyst wall (I) separated by a thin strip of intervening material (IS) which is connected to the plates by electron dense material toward the outer aspect (arrows). The suture is covered by the continuous thin outer layer (O) of the sporocyst wall. Reproduced from Ferguson et al., 1979b, with permission.

FIGURE 2.19 A diagrammatic representation of (A) the cross-sectional and (B) the three-dimensional appearance of the sporocyst wall at the junction between the four plates which form the wall. The suture consists of a swelling of the adjacent plates forming the inner layer of the sporocyst wall (I) separated by a thin strip of intervening material (IS) which is connected to the plates by electron dense material toward the outer aspect (arrows). The suture is covered by the continuous thin outer layer (O) of the sporocyst wall. Reproduced from Ferguson et al., 1979b, with permission.

all organs including the brain. However, in immunocompetent mice (genetically resistant to Toxoplasma) the number of lesions and tachyzoites peaked at about 12 days, and by 21 days it was difficult to identify tachyzoites in any organ, including the brain, even using immunocytochemistry.

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