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Toxoplasmosis - Cat and Dog (view source)
Revision as of 19:07, 12 August 2010
, 19:07, 12 August 2010→Description
''Toxoplasma gondii'' is an obligate, intracellular protozoa that is capable of infecting most mammals. Cats and other felids are the definitive host for ''T. gondii'', and all other mammals, including dogs, are intermediate hosts.
''Toxoplasma gondii'' is an obligate, intracellular protozoa that is capable of infecting most mammals. Cats and other felids are the definitive host for ''T. gondii'', and all other mammals, including dogs, are intermediate hosts.
Felids are the only definitive hosts of T gondii ; both wild and domestic cats therefore serve as the main reservoir of infection. There are 3 infectious stages of T gondii ; tachyzoites (rapidly multiplying form), bradyzoites (tissue cyst form), and sporozoites (in oocysts).
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==