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What Is It?

Toxoplasmosis is a parasitic infection that infects a large proportion of the world's population, but rarely causes disease. Certain people, however, are at high risk of severe or life-threatening disease from this parasite. They include infants who are infected at birth, people with AIDS, people with cancer, and people who have had bone marrow or organ transplantation.

Toxoplasmosis is an infection caused by Toxoplasma gondii, a single-celled parasite that spends most of its life cycle inside cats. Because an infected cat can pass millions of Toxoplasma parasites daily in its feces, toxoplasmosis can spread easily to almost any other animal that shares the environment with cats. In humans, Toxoplasma parasites usually enter the body by being swallowed. This can happen when people touch their mouth with soiled hands, especially after changing cat litter, or if they eat pork, lamb or venison that has not been cooked thoroughly.

The Toxoplasma parasites multiply within cells that line the human digestive tract. Toxoplasma parasites can spread to almost any organ in the body, including the brain, skeletal muscles, heart muscle, eyes, lungs and lymph nodes. In healthy people, the body's immune system eventually stops the spread of Toxoplasma parasites, although some remaining parasites can lie dormant indefinitely in the brain or retina.

In people whose immune defenses are weakened because of AIDS, cancer or immunosuppressant medication, a new toxoplasmosis infection may spread out of control and become deadly, or dormant Toxoplasma parasites from an old toxoplasmosis infection may suddenly become active again and cause severe illness. This situation is especially dangerous for people with AIDS. In these people, dormant toxoplasmosis can reactivate and cause a severe brain infection (encephalitis), which can lead to seizures and other neurological problems. If left untreated, the death rate from encephalitis is very high. In addition to being swallowed, Toxoplasma parasites can enter the body through contaminated blood transfusions or through organ transplants taken from infected donors. Also, if a toxoplasmosis infection develops in a pregnant woman, the parasites can cross the placenta and cause toxoplasmosis in the infant. This is called congenital toxoplasmosis. These newborns are at high risk of toxoplasmosis-related eye problems and developmental disabilities.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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