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==Description==
==Description==
''Toxoplasma gondii'' is an intracellular protozoan parasite of warm-blooded mammals and birds, including man.
Toxoplasmosis is the disease caused by ''Toxoplasma gondii'', an intracellular protozoan parasite of warm-blooded mammals and birds. The cat (and other felids) is the definitive host of ''Toxoplasma gondii'', and all other species, including man, are intermediate hosts.
''T. gondii'' has three infectious stages: 1) sporozoites; 2) actively reproducing 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. In naive cats, ''Toxoplasma gondii'' undergoes an enteroepithelial life cycle. Cats ingests intermediate hosts containing tissue cysts, which release bradyzoites in the gastrointestinal tract. The bradyzoites penetrate the small intestinal epithelium and sexual reproduction ensues, eventually resulting the production of oocysts. Oocysts are passed in the cat's faeces and sporulate to become infectious once in the environment. These can then be ingested by other mammals, including humans.
T. gondii is ubiquitous in birds and mammals. This obligate intracellular parasite invades and multiplies asexually as tachyzoites within the cytoplasm of any nucleated cell (see Fig. 2: Extraintestinal Protozoa: Toxoplasma gondii life cycle.). When host immunity develops, multiplication of tachyzoites ceases and tissue cysts form; cysts persist in a dormant state for years, especially in brain and muscle. The dormant Toxoplasma forms within the cysts are called bradyzoites. Sexual reproduction of T. gondii occurs only in the intestinal tract of cats; the resultant oocysts passed in the feces remain infectious in moist soil for months.
When man or another animal ingests oocysts or tissue cysts, ''T. gondii'' intiates extraintestinal replication. This process is the same for all intermediate hosts, although the form ingested depends on diet. Sporozoites (from oocysts) or bradyzoites (from tissue cysts) are released in the intestine to infect the intestinal epithelium where they replicate. This produces tachyzoites, which reproduce asexually within the infected cell. When the infected cell ruptures, tachyzoites are released and disseminate via blood and lymph to infect other tissues. Tachyzoites then replicate intracellularly again, and the process continues until the host becomes immune or dies. If the infected cell does not burst, tachyzoites eventually encyst as bradyzoites and persist for the life of the host, most commonly in the brain or skeletal muscle.
Human exposure to toxoplasmosis is common wherever cats are found; 20 to 40% of healthy adults in the US are seropositive. The risk of developing disease is very low except for a fetus infected in utero and people who are or become immunocompromised
Ingestion of oocysts in food or water contaminated with cat feces is the most common mode of oral infection. Infection can also occur by eating raw or undercooked meat containing tissue cysts, most commonly lamb, pork, or rarely beef. After ingestion of oocysts or tissue cysts, tachyzoites are released and spread throughout the body. This acute infection is followed by the development of protective immune responses and the formation of tissue cysts in many organs. These cysts can reactivate, primarily in immunocompromised patients. Toxoplasmosis reactivates in 30 to 40% of AIDS patients who are not taking antibiotic prophylaxis, but the widespread use of trimethoprim/sulfamethoxazole Some Trade Names
Ingestion of oocysts in food or water contaminated with cat feces is the most common mode of oral infection. Infection can also occur by eating raw or undercooked meat containing tissue cysts, most commonly lamb, pork, or rarely beef. After ingestion of oocysts or tissue cysts, tachyzoites are released and spread throughout the body. This acute infection is followed by the development of protective immune responses and the formation of tissue cysts in many organs. These cysts can reactivate, primarily in immunocompromised patients. Toxoplasmosis reactivates in 30 to 40% of AIDS patients who are not taking antibiotic prophylaxis, but the widespread use of trimethoprim/sulfamethoxazole
for Pneumocystis prophylaxis has dramatically reduced the incidence.
for Pneumocystis prophylaxis has dramatically reduced the incidence.