Toxoplasma gondii
Jennifer Luebbering

Toxoplasma gondii is a crescent-shaped parasite that is responsible for the disease Toxoplasmosis, a common disorder that generally only causes symptoms in immunocompromised people, fetuses, and newborns, with pregnant woman also being at risk because of the harm the parasite can do to their unborn child.  It has been found nearly everywhere in the world except for Antarctica.  T. gondii has two life cycles in which it requires two hosts to complete both the asexual and sexual portions of its life.  Generally, cats are the definitive host, in which T. gondii completes its sexual life cycle; the intermediate hosts are mammals, including humans, and birds in which the parasite carries out the asexual portion of its life.

The sexual portion of the life cycle begins after cats have ingested tissue cysts that can be found in undercooked meat, intermediate hosts such as rodents or birds, or soil, or oocysts from cat feces.  The cyst becomes activated in the acidic and basic environments of the stomach and gastrointestinal tract and releases bradyzoites that infiltrate the walls of the small intestine and produces generations of the asexual from (tachyzoites) and also release oocysts.  The tachyzoites are then released in the lymphatic system and can invade nearly any cell in the body.  This form can enter the cell and multiply until the host cell is overrun with tachyzoites and dies, forming foci of necrosis.  This process is repeated until an immune response occurs after two to three weeks and the tachyzoites disappear from the body. 

The asexual portion of the life cycles that occurs in intermediate hosts is very similar to the cycle that occurs in cats, except that the sexual portion does not happen.  The patient (or other intermediate host) is left with cysts that can contain around 60,000 bradyzoites and are generally found in the brain, liver, and muscles.  Generally the cysts do not cause an immune response and can remain in the patient indefinitely.  The cysts have been known to rupture occasionally and can cause the release of bradyzoites and subsequent necrosis of the tissue.  In healthy adults, this generally does not cause re-infection as an immune response kills the bradyzoites.
In women who are pregnant, the risk to their fetus is high, especially since the mother generally has no symptoms.  Mild symptoms seen in the fetus can include slight vision loss, while severe symptoms can include hydrocephalus, convulsions, retinochoroiditis, and ocular disease.  In immunocompromised patients, including those with AIDS, the most common symptom is encephalitis, beginning with severe headaches that can lead to coma.  Sulfonamides can be given to immunocompromised patients curb the growth of tissue cysts.

Since toxoplasmosis generally causes no symptoms, the rate of infection can be hard to determine.  It is estimated, though, that between 3-70% of adults in America have been infected by T. gondii.  It causes around 750 deaths in America each year, making it the third most common cause of lethal food-borne pathogen.

References

“Toxoplasmosis.”  United States Department of Agriculture – Agricultural Research Service. November 2005. <http://www.ars.usda.gov/main/docs.htm?docid=11013>.

Hokelek, Murat.  “Toxoplasmosis.”  eMedicine.  January 2009. <http://emedicine.medscape.com/article/229969-overview>.

Dubey, J.P.  “Toxoplasma gondii.”  < http://gsbs.utmb.edu/microbook/ch084.htm>.
*Disclaimer - This report was written by a student participaring in a microbiology course at the Missouri University of Science and Technology. The accuracy of the contents of this report is not guaranteed and it is recommended that you seek additional sources of information to verify the contents.

 

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