Fig. 1. Life cycle and transmission pathways of Toxoplasma gondii. The typical coccidian life cycle proceeds in the intestinal epithelium of felids (final host) which are infected by oral uptake of sporulated oocysts (2), ingestion of “pseudocysts” (4.1; 8) or tissue cysts (6.1; 11) with meat of various intermediate hosts (of two types). 1 Unsporulated oocysts are excreted with feces. 2 Sporulation (i.e. formation of sporocysts and sporozoites) occurs outside the final host. These stages may become spread by transport hosts such as flies and cockroaches. 3 After ingestion of oocysts by intermediate hosts of type 1, the sporozoites are set free inside its intestine and penetrate numerous types of extraintestinal cells (i.e. cells of the RES). 4 Inside the host cell the parasites reproduce by a typical binary fission (endodyogeny) leading to “pseudocysts” which are filled with merozoites (i.e. tachyzoites). 4.1 After ingestion of such pseudocysts, cats may become infected. 5 Free merozoite (tachyzoite) in blood or lymph fluid after bursting of a pseudocyst. 5.1 When the first infection is in pregnant women (or animals), these merozoites may pass into the placenta and infect the fetus, leading to severe damage. 6 Formation of tissue cyst, mainly inside brain and muscle cells. After several endodyogenies these cysts (waiting stages) contain numerous cyst merozoites (bradyzoites, cystozoites) which are infectious for cats (6.1). 7–10 When carnivorous animals or man (intermediate hosts of type 2) ingest such tissue cysts with raw or insufficiently cooked meat, reproduction (see 3–6) via pseudocysts is repeated, leading to the same tissue cysts (10) as in intermediate hosts of type 1. Diaplacental transmission (9.1) may also occur (see 5.1), leading to congenital toxoplasmosis. 11 Cats may also become infected by ingestion of tissue cysts from type 2 intermediate hosts. Then they pass oocysts after 3–5 days, whereas this prepatent period is longer after inoculation of pseudocysts (9–11 days) or oocysts (21–24 days). EN, division by endodyogeny; HC, host cell; N, nucleus; NH, nucleus of host cell; OC, oocyst; PC, primary cyst wall; PV, parasitophorous vacuole; RB, residual body; SP, sporozoite; SPC, sporocyst (for related species see Coccidia/Table 5).