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is a banana-shaped
parasite
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| Although the true color is not yellow, like a banana, the parasite has two differently shaped ends. The apical end, the right side in the picture above, contains a number of specialized structures. This basic body-plan is conserved in many different other parasites which gives this group the name Apicomplexa. Among these are the causitive agent of malaria (Plasmodium species) and several other human pathogens (Cryptosporidium, Cyclospora) as well as a number of veterinary important parasites (Theileria, Babesia, Eimeria, Sarcocystis). | |||
| The parasite consists of only a single eukaryotic cell and replicates inside cells from the host. Virtually all birds and mammals can serve as intermediate hosts wherein the parasite replicates asexually, but the cat is the final host wherein sexual reproduction takes place. In humans, infection can originate from contact with cat-litter (soil or water) or through consumption of undercooked, infected meat. Upon ingestion, the parasites penetrate the gut-wall and infect cells of the host wherein they start to replicate (see Figure below). After several division rounds parasites emerge from the infected cell, destroying it upon egress, and quickly invade neighboring cells to repeat the cycle. Due to popular demand, click here for a black/white version of the lytic cycle (feel free to use). | |||
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| In healthy individuals the replicative, acute stage is controlled by the immune system, forcing differentiation of the tachyzoite stage into the dormant, life-long maintained bradyzoite stage. The bradyzoites are encysted and reside in muscle and central nervous tissue and replicate at a very slow pace. | |||
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Organization of
T. gondii
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T. gondii has a nucleus (grey), endoplasmic reticulum, mitochondria (red), and Golgi (light blue) like all other eukaryotic cells. The parasite has several unique organelles, mostly located at the apical end (top). Three different secretory organelles are involved in invasion; micronemes (yellow), rhoptries (orange) and dense granules (dark blue) Another unique compartment is the apicoplast (green), a remnant chloroplast. This organelle is surrounded by four membranes and was acquired through engulfing an algae (secondary endosymbiosis). The cytoskeleton consist of several elements; again at the apical end, there is a basket-shaped tubulin structure called the conoid (thick black lines) and a polar ring (not shown). Microtubuli further run along the plasma membrane to about two-thirds of the length of the parasite. In addition there is another element, the inner membrane complex (IMC, thin black lines under surface) which are flattened vesicles in combination with protein filaments that give the parasite its characteristic shape. To see fluoresencent markers targeted to many of these organelles see the Striepen GFP page. |
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Human disease
caused by T. gondii
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The disease caused by T. gondii is called toxoplasmosis. There are various different forms, depending on the development or contraction of the disease. Primary infection in most people passes with no or mild symptoms. People at risk are those who are immunocompromised (AIDS patients, organ-transplant recipients) and carry a dormant infection. Upon lowered immune system function, the bradyzoites differentiate back into the quickly replicating tachyzoite stage, resulting in tissue lesions. Most notably this occurs in the brain causing often life-threatening encephalitis. Other organs can be affected to, for example the heart resulting in mostly fatal myocarditis. |
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| Another risk group is women who contract a primary infection during pregnancy. The parasite can traverse the placenta and infect the unborn child. Depending on a number of factors this can lead to a wide array of developmental defects, mostly neurological. However, disease may manifest only later in childhood as ocular toxoplasmosis as the parasite resides in the eye, and may eventually cause blindness. | |||
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More info
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Visit the following websites for more detail
on the parasite and disease:
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For up to date new developments, see Bill Sullivan's Toxoplasma blog: The Anti-Toxo |
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And for a lighter note: see the "Killer Bananazz" website |
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| Top banana-image with courtesy from David Morrison; Subcellular-schematic with courtesy from Boris Striepen | |||
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