*Life Cycle and Transmission

There are three infectious stages of T. gondii:  rapidly dividing tachyzoites (in pseudocysts),  slow growing bradyzoites (in tissue cysts), and sporozoites (in oocysts).

http://www.biosci.ohio-state.edu/~parasite/toxoplasma.html

 

If cats are fed T. gondii (or what was originally  thought to be Isospora bigemina var. cati) oocysts, a typical coccidian life cycle is observed. Unsporulated oocysts are shed  with the feces, and sporulation  takes place  outside the cat within 1-5 days, depending on environmental conditions. Excystation occurs in the intestine, the sporozoites invade cells in the intestine, where  we presume they multiply, eventually  differentiating into  gametocytes, fertilization  then occurs,the zygote forms a sporoblast, and unsporulated oocysts are found in the feces in ~20 days.  

 

(from Schmidt and Roberts, Foundations of Parasitology, W.C. Brown, 1996).

 Life cycle of T. gondii

 

Cats may also be infected by ingestion of meat containing tissue cysts with bradyzoites. In the cat stomach and intestine  the cyst wall is dissolved by the action of proteolytic enzymes, and the released bradyzoites invade the cells of the intestinal epithelium and divide  by a process called endodyogeny. Endodyogeny  is the development of 2 daughter parasites within a mother cell by a process of intracellular binary fission.

 

 Tachyzoites in endodyogeny by TEM

MC, mother cell; PV, parasitophorous vacuole; HC, host cell; RO, rhoptry; M, mitochondrion; MC, micronemes; DG, dense granule; IT, intravacuolar tubules; MI, micropore

 

The inner membrane of the mother cell disappears and the inner membrane of the daughter cell joins the other membrane of the mother cell. If held together at the posterior end with repeated cycles of endodyogeny a rosette of crescent-shaped parasites is formed. After several  cycles  of asexual reproduction gamogony begins: the intracellular stages   do not undergo endodyogeny, but instead differentiate into gametocytes, and fertilization occurs.Two walls are laid down around the zygote to form the oocyst, and  unsporulated oocysts are found in the feces in less than a week. Sexual stages are produced only in the definitive feline host!

 

 

Unsporulated oocysts in cat feces.  F. Isospora felis and T. Toxoplasma gondii

 

Sex is not determined genetically in that a single tachyzoite or sporozoite can go on to form either male or female gametocytes. A single tachyzoite or sporozoite infecting a cat gives rise to normal oocysts in the cat. Therefore, a physiologic trigger for gametocytogenesis must exist and it must involve the cat intestinal epithelium since gametocytes never form in vitro. The cue for the microgamete finding the macrogamete is unknown.)

 

 Simultaneously, the bradyzoites penetrate the lamina propia of the cat intestine where they multiply as  crescent-shaped tachyzoites. Aggregates of tachyzoites  are enclosed within a pseudocyst of parasite and host origin. Tachyzoites can enter almost any nucleated cell, and multiply until the host cell is filled with the next generation of tachyzoites. In the process the host cell is killed. This cycle of  parasite replication and host cell death may result in microscopic centers of  necrosis.

 

 

 

Within hours  after infection the tachyzoites  are engulfed by phagocytic cells and move  via the lymph and blood to extra-intestinal sites  where endodyogeny  continues. The host usually overcomes this acute phase of infection, and then the chronic phase begins:  the parasites  (now called bradyzoites), within a cyst,  divide slowly by endodyogeny, and persist in the tissues for months if not for the life of the cat. Thus, if a cat is fed bradyzoites  development of Toxoplasma occurs in both the intestine and the tissues. The tissue cysts usually cause no host reaction.  If a cat is fed bradyzoites  in meat there will be oocysts in the feces in <10 days, whereas if fed tissue containing tachyzoites oocysts are shed within 2 weeks, and if fed oocysts then oocysts are found in the feces in >18days.

 

Fewer than 50% of cats shed oocysts if fed tachyzoites or oocysts, but 100% shed oocysts if fed bradyzoites. Clearly, Toxoplasma is a coccidian parasite that has evolved mechanisms for infecting its definitive host, the cat, and it utilizes its prey  (rodents and birds), as well as oocysts, to accomplish its dispersal from one host to another.

 

In humans, after ingestion of oocysts, tachyzoites are found during the first 2 weeks,  and constitute the acute phase of the infection.   

 

 

 The life cycle of T. gondii

 

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*Formation of Cysts with Brayzoites

 

In the mouse development may be more rapid than in humans but it is clear that tachyzoites are disseminated throughout the body tissues via the blood and lymph and this takes 4 days. Five days after oocyst ingestion there are tissue cysts in the intestine, and by 8 days tissue cysts are found in the brain. The tissue cyst is pepsin-resistant and develops from the parasitophorous vacuolar membrane. A cyst may contain up to 3000 bradyzoites, and most cysts occur in the muscle and brain and can persist for years. This is the chronic phase of infection. If humans or other carnivores (not cats!) eat meat containing cysts with bradyzoites then these stages emerge and invade cells (other than intestinal cells) and they divide as tachyzoites first leading to an acute disease, then to a chronic disease.  No oocysts are formed!

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 Cyst with bradyzoites