Life cycle of Toxoplasma gondii, the cause of toxoplasmosis in humans

Life cycle of Toxoplasma gondii, the cause of  toxoplasmosis in humans
Photographer: Centers for Disease Control/Division of Parasitic Diseases and MalariaRights holder: Centers for Disease Control/Division of Parasitic Diseases and Malaria

The only known definitive hosts for the apicomplexan protozoan parasite Toxoplasma gondii are members of family Felidae (domestic cats and their relatives).  Unsporulated oocysts are shed in the cat’s feces (1).  Although oocysts are usually only shed for 1-2 weeks, large numbers may be shed.  Oocysts take 1-5 days to sporulate in the environment and become infective.  Intermediate hosts in nature (including birds and rodents) become infected after ingesting soil, water or plant material contaminated with oocysts (2). Oocysts transform into tachyzoites shortly after ingestion.  These tachyzoites localize in neural and muscle tissue and develop into tissue cyst bradyzoites (3).  Cats become infected after consuming intermediate hosts harboring tissue cysts (4).  Cats may also become infected directly by ingestion of sporulated oocysts.  Animals bred for human consumption and wild game may also become infected with tissue cysts after ingestion of sporulated oocysts in the environment (5).  Humans can become infected by any of several routes:

  • eating undercooked meat of animals harboring tissue cysts (6).
  • consuming food or water contaminated with cat feces or by contaminated environmental samples (such as fecal-contaminated soil or changing the litter box of a pet cat) (7).
  • blood transfusion or organ transplantation (8).
  • transplacentally from mother to fetus (9).

In the human host, the parasites form tissue cysts, most commonly in skeletal muscle, myocardium, brain, and eyes; these cysts may remain throughout the life of the host.  Diagnosis is usually achieved by serology, although tissue cysts may be observed in stained biopsy specimens (10).  Diagnosis of congenital infections can be achieved by detecting T. gondii DNA in amniotic fluid using molecular methods such as PCR (11).

From Centers for Disease Control Parasites and Health website