Toxoplasma gondii tachyzoites are disseminated throughout the body of the intermediate host in macrophages, lymphocytes, and are free in the plasma. Tachyzoites continue to divide within the host cell by endodyogeny (internal division into two) until the host cell is filled with parasites. At a given time when the dividing tachyzoites cannot be contained with the host cell which burst, the tachyzoites are released and seek new host cells to repeat the process. Depending on the strain of T. gondii and the host resistance, tachyzoites may be found for days, or even months, after acute infection. For example, tachyzoites persist in foetal membranes for weeks after infection of the mother or the dam, and are nearly always present in placentas of mothers at the time of parturition, if the foetus was infected in utero. Some time after infection the tachyzoites transform to bradyzoites in tissue cysts. The signals responsible for the transformation are not known, and it is still debated whether signals from the host immune system are needed. Bradyzoites also divide by endodyogeny.
Bradyzoites are enclosed in a thin cyst wall. Tissue cysts may be found as early as three days after infection but are usually not numerous until seven weeks after infection (Derouin and Garin 1991; Dubey and Frenkel 1976). Intact tissue cysts do not probably cause any inflammation and may persist for life. It has been suggested that tissue cysts may switch from
the bradyzoite stage to the tachyzoite stage throughout the life of the tissue cysts, producing new tachyzoites which may give rise to new tissue cysts thus ensuring a prolonged infective stage (Herion and Saavedra 1993). If the intermediate host is eaten by another warmblooded animal, tissue cysts are able to infect a new host.
Less than 50% of cats shed oocysts after ingesting tachyzoites or oocysts, whereas almost all cats shed oocysts after ingesting tissue cysts (Dubey and Frenkel 1976). Cats infected with oocysts and tachyzoites probably give rise to bradyzoites, which after a variable time may disseminate to the intestinal mucosa and start the enteroepithelial cycle with the resulting production of oocysts (Freyre et al. 1989).
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