Sexual replication of Toxoplasma gondii occurs in the gut of the cat during the enteroepithelial stage of the life cycle, which takes about 3-10 days. Sexual replication leads to the production of oocysts. The oocysts are passed in the cat's feces and sporulate in the soil to become infectious. After ingestion ingested by intermediate hosts (rodents, birds, sheep, pigs, [humans]), they cause a systemic infection (asexual replication, initially as rapidly dividing tachyzoites, followed by encystation as bradyzoites). The entire life cycle is completed when cats ingest tissues of the intermediate hosts containing Toxoplasma cysts containing bradyzoites. Cats can also be infected by ingestion of oocysts shed from other cats, but this is a much less efficient method of infection. Only ~20% of cats exposed to oocysts become productively infected, whereas virtually 100% of naïve cats that ingest tissues cysts will become infected and shed oocysts. When dogs consume cat feces, oocysts may pass through the GI tract into feces, with subsequent mechanical dispersal of the organism, but dogs do not support actual replication of Toxoplasma in their gut.

• Note: research suggests that recombination between different genotypes of T. gondii (there are currently 3 major genotypes/lineages recognized) during sexual replication can lead to selection of strains of greater virulence.

Click here to see a graphic depiction of the life cycle of Toxoplasma gondii.

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