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===Life Cycle===

===Life Cycle===

[[Image:Toxoplasmosis Life Cycle.jpg|Thumb|right|300px| Life cycle of ''Toxoplasma gondii''. Source: Wikimedia Commons; Author: LadyofHats (2010)]]

[[Image:Toxoplasmosis Life Cycle.jpg|thumb|right|300px|Life cycle of ''Toxoplasma gondii''. Source: Wikimedia Commons; Author: LadyofHats (2010)]]

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 sporocysts can remain viable in the environment for up to 18 months even if exposed to high or freezing temperatures and low humidity. As cats generally develop immunity to ''T. gondii'' after the initial infection, they will only shed oocysts once in their lifetime.

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 sporocysts can remain viable in the environment for up to 18 months even if exposed to high or freezing temperatures and low humidity. As cats generally develop immunity to ''T. gondii'' after the initial infection, they will only shed oocysts once in their lifetime.

Although any of the three life stages described above can infect warm-blooded vertebrates, most infections are acquired following the ingestion of sporozoites or bradyzoites, as tachyzoites are easily inactivated in the gastric environment. As cats rarely practice coprophagy, infection is usually acquired through the ingestion of infected intermediate hosts such as rodents. Dogs tend to consume food or water contaminated with oocysts from cat faeces.

Although any of the three life stages described above can infect warm-blooded vertebrates, most infections are acquired following the ingestion of sporozoites or bradyzoites, as tachyzoites are easily inactivated in the gastric environment. As cats rarely practice coprophagy, infection is usually acquired through the ingestion of infected intermediate hosts such as rodents. Dogs tend to consume food or water contaminated with oocysts from cat faeces.

 

If a pregnant queen  is naive to ''Toxoplasma gondii'' at the time of ingestion, transplacental infection can occur. The outcome of this depends on the stage of gestation. Infection during the first trimester usually has severe consequences, such as stillbirth or abortion; infection during the second or third trimesters are more likely to give rise to an infected foetus. Kittens infected neonatally commonly show interstitial pneumonia, necrotising hepatitis, myocardidits, non-suppurative encephalits and uveitis on post-mortem examination<Sup>1</sup>.

If a pregnant queen  is naive to ''Toxoplasma gondii'' at the time of ingestion, transplacental infection can occur. The outcome of this depends on the stage of gestation. Infection during the first trimester usually has severe consequences, such as stillbirth or abortion; infection during the second or third trimesters are more likely to give rise to an infected foetus. Kittens infected neonatally commonly show interstitial pneumonia, necrotising hepatitis, myocardidits, non-suppurative encephalits and uveitis on post-mortem examination<Sup>1</sup>.

===Pathogenesis===

===Pathogenesis===

 

The outcome of primary infection depends on the immune status of the host, as well as the location of and degree of injury caused by tissue cysts. Primary infection normally results in chornic disease, where tissue cysts form but clinical signs are not normally apparent. In immunodeficient animals, or in animals with concurrent illness, chronic infections may become symptomatic as the organism is allowed to proliferate. Acute primary infection in these animals can, rarely, prove fatal.  

The outcome of primary infection depends on the immune status of the host, as well as the location of and degree of injury caused by tissue cysts. Primary infection normally results in chornic disease, where tissue cysts form but clinical signs are not normally apparent. In immunodeficient animals, or in animals with concurrent illness, chronic infections may become symptomatic as the organism is allowed to proliferate. Acute primary infection in these animals can, rarely, prove fatal.