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IN VIVO STAGE CONVERSION AND FORMATION OF BRADYZOITES AND TISSUE CYSTS

The period needed for stage conversion of T. gondiivaries with the inoculum, route of inoculation, and method of examination (Table 5). In these studies, tissue cyst formation was sought in the brains of mice. To circumvent the process by which the parasite reaches the brain tissue from the site of inoculation and is then converted to bradyzoites, Dubey and Frenkel (48) inoculated tachyzoites directly into the brains of mice and found that some tachyzoites converted to bradyzoites between 2 and 3 days p.i. At daily intervals, T. gondii-injected mice were fed to cats and the feces of the cats were examined for oocyst shedding. Cats fed mice infected 1 or 2 days previously never shed oocysts with a short prepatent period (<10 days). The results were not related to the number ofT. gondii organisms inoculated, because the cats were fed homogenates of many mice inoculated simultaneously by 4 routes (i.p., s.c., intramuscular, and intracerebral). Dubey and Frenkel (48) also demonstrated tissue cysts by conventional histology with special stains (PAS and silver impregnation method).

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TABLE 5.

Summary of in vivo formation of bradyzoites and tissue cysts of T. gondii

· ↵a i.m., intramuscular; i.c., intracerebral.

· ↵b IHC, Immunohistochemical staining with bradyzoite-specific antibody.

Recently, tissue cyst formation in mice inoculated orally with tissue cysts or bradyzoites or with oocysts or sporozoites was examined (42, 52). Tissues of mice fed tissue cysts or oocysts were bioassayed in cats, in mice after pepsin digestion, and by immunohistochemical staining with a bradyzoite-specific antibody (BAG-5).

After the mice were fed tissue cysts or bradyzoites, bradyzoites penetrated through enterocytes, entered various cell types in the lamina propria, and divided into tachyzoites by 18 h p.i. The infection disseminated to extraintestinal organs, with parasitemia occurring at 24 to 48 h p.i. Bioassay and histologic examination showed that tissue cysts formed as early as 6 days p.i. in the brain and other tissues. In general, the detection of BAG-5 antigen paralleled that in the cat bioassay. The acid-pepsin digestion procedure was unreliable because it gave inconsistent results with tissue cysts produced during acute infection. Evidence for direct conversion of bradyzoites to bradyzoites was not found.

After the mice were fed oocysts, sporozoites penetrated enterocytes but developed only in lamina propria cells during the first 12 h p.i. The conversion of sporozoites to tachyzoites to bradyzoites required 7 days. Thus, there was a delay of 1 day in bradyzoite formation after oocyst feeding with respect to that after tissue cyst feeding.

In both bradyzoite-induced and sporozoite-induced oral infections,T. gondiiorganisms were not seen in histologic sections of the brain until 6 days p.i., and at that time individual BAG-5-positive organisms were seen mixed with BAG-5-negative organisms (Fig.31). Whether individual BAG-5-positive organisms seen in the brain had migrated from other tissues or were released from tissue cysts could not be determined. The stage of the tachyzoite-to-bradyzoite conversion at which the tissue cyst wall is formed in acute infection is uncertain. In chronically infected mice, a tissue cyst wall is clearly visible around two bradyzoites, both of which also have a terminal nucleus (Fig. 11). Recent studies by Sahm et al. (141) suggest that bradyzoites invading the host cell secrete the ground substance of the cyst wall and that during the invasion process the parasite determines whether it will form tissue cysts or tachyzoites.

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FIG. 31.

Section of the brain of a mouse 10 days after it was fed T. gondii oocysts. There are individual (arrowheads) and a large group of (large arrow) BAG-5 positive organisms with different intensities of reactivity. Numerous BAG-5-negative organisms (small arrows) are also present. Immunohistochemical staining with anti-BAG-5 antibody.

Date: 2016-01-03; view: 663