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''Toxoplasma gondii'' is an obligate, intracellular coccidian parasite that is capable of infecting most mammals including man. Cats and other Felidae are the definitive host for ''T. gondii'', and all other mammals are intermediate hosts. ''Toxoplasma gondii'' has three infectious stages: 1) sporozoites; 2) an actively reproducing stage called tachyzoites; and 3) slowly multiplying bradyzoites. Tachyzoites and bradyzoites are found in tissue cysts, whereas sporozoites are containted within oocysts, which are excreted in the faeces. This means that the protozoa can be transmitted by ingestion of oocyst-contaminated food or water, or by consumption of infected tissue. Transplacental infection is also possible.

''Toxoplasma gondii'' is an obligate, intracellular coccidian parasite that is capable of infecting most mammals including man. Cats and other Felidae are the definitive host for ''T. gondii'', and all other mammals are intermediate hosts. ''Toxoplasma gondii'' has three infectious stages: 1) sporozoites; 2) an actively reproducing stage called tachyzoites; and 3) slowly multiplying bradyzoites. Tachyzoites and bradyzoites are found in tissue cysts, whereas sporozoites are containted within oocysts, which are excreted in the faeces. This means that the protozoa can be transmitted by ingestion of oocyst-contaminated food or water, or by consumption of infected tissue. Transplacental infection is also possible.

In the definitive host, ''Toxoplasma gondii'' undergoes an enteroepithelial life cycle. Cats become infected by ingesting intermediate hosts containing tissue cysts, which release their bradyzoites in the gastrointestinal tract when the wall is digested. Bradyzoites then penetrate the small intestinal epithelium and produce five types of schizonts, which then give rise to merozoites. Male and female gamonts are formed from merozoites, which fertilise to form a macrogamont. A wall forms aroung the macrogamont to produce an oocyst.  

In the naive definitive host, ''Toxoplasma gondii'' undergoes an enteroepithelial life cycle. Cats become infected by ingesting intermediate hosts containing tissue cysts, which release their bradyzoites in the gastrointestinal tract when the wall is digested. Bradyzoites then penetrate the small intestinal epithelium and produce five types of schizonts, which then give rise to merozoites. Male and female gamonts are formed from merozoites, which fertilise to form a macrogamont. A wall forms aroung the macrogamont to produce an oocyst, which is passed in the faeces approximately three days after ingestion of the tissue cyst. Initially, these oocysts are unsporulated and are therefore not infectious, but after 1 to 5 days sporulation occurs to produce two sporocysts, each with four infectious sporozoites. This sporulation is dependent on temperature and aeration, and sporocyts can remain viable in the environment for several months. As cats generally develop immunity to ''T. gondii'' after the initial infection, they will only shed oocysts once in their lifetime.

When other, non-feline, carnivores (such as dogs) consume tissue cysts or oocysts from cat faeces, ''Toxoplasma gondii'' initiates extraintestinal replication. This process is the same for all hosts, and does not vary with the form of the parasite ingested. Bradyzoites and sporozoites, from cysts and oocysts respectively, are released in the intestine and infect the intestinal epithelium where they replicate. This produces tachyzoites, which are lunate in shape, about 6 microns in diameter and possess the ability to multiply in almost any cell type. The infected cell ruptures to release tachyzoites which then disseminate via blood and lymph to infect other tissues. Tachyzoites then replicate intracellularly and, if the cell does not burst, they eventually encyst and persist for the life of the host.

passed in feces. After exposure to air and moisture for 1 to 5 days, oocysts sporulate and contain

two sporocysts, each with four sporozoites.

The entire enteroepithelial (coccidian) cycle of T. gondii can be completed within 3 to 10 days

after ingestion of tissue cysts and occurs in up to 97% of naive cats. However, after ingestion of

oocysts or tachyzoites, the formation of oocysts is delayed until 18 days or more, and only 20% of

cats fed oocysts will develop patency.

T gondii is transmitted by consumption of infectious oocysts in cat feces, consumption of tissue cysts in infected meat, and by transplacental transfer of tachyzoites from mother to fetus. T gondii initiates enteroepithelial replication in unexposed cats after ingestion of uncooked meat containing tissue cysts. Bradyzoites are released from tissue cysts by digestion in the stomach and small intestine, invade intestinal epithelium, and undergo sexual replication, culminating in the release of oocysts (10 µm diameter) in the feces. Oocysts are first seen in the feces at 3 days after infection and may be released for up to 20 days. Oocysts sporulate (become infectious) outside the cat within 1-5 days, depending on aeration and temperature, and remain viable in the environment for several months. Cats generally develop immunity to T gondii after the initial infection and therefore shed oocysts only once in their lifetime.

Following consumption of uncooked meat containing tissue cysts (carnivores) or feed or drink contaminated with cat feces containing oocysts (all warm-blooded animals), T gondii initiates extraintestinal replication. Bradyzoites and sporozoites, respectively, are released and infect intestinal epithelium. After several rounds of epithelial replication, tachyzoites emerge and disseminate via the bloodstream and lymph. Tachyzoites infect tissues throughout the body and replicate intracellularly until the cells burst, causing tissue necrosis. Tachyzoites measure 4-6 × 2-4 µm in diameter and stain with Giemsa. Young and immunocompromised animals may succumb to generalized toxoplasmosis at this stage. Older animals mount a powerful cell-mediated immune response to the tachyzoites (mediated by cytokines) and control infection, driving the tachyzoites into the tissue cyst or bradyzoite stage. Tissue cysts are usually seen in neurons but also occur in other tissues. Individual cysts are microscopic, up to 70 m in diameter, and may enclose hundreds of bradyzoites in a thin, resilient cyst wall. Tissue cysts in the host remain viable for many years, and possibly for the life of the host.

==Signalment==

==Signalment==