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On the Trail of a Stealthy Parasite Biologist Shows Why Some Strains of Toxoplasma Are More Dangerous Than Others

About one-third of the human population is infected with a parasite called Toxoplasma gondii, but most of them don't know it. Though Toxoplasma causes no symptoms in most people, it can be harmful to individuals with suppressed immune systems, and to fetuses whose mothers become infected during pregnancy. Toxoplasma spores are found in dirt and easily infect farm animals such as cows, sheep, pigs and chickens. Humans can be infected by eating undercooked meat or unwashed vegetables.

Jeroen Saeij, an assistant professor of biology at MIT is investigating a key question: why certain strains of the Toxoplasma parasite (there are at least a dozen) are more dangerous to humans than others. He and his colleagues have focused their attention on the type II strain, which is the most common in the United States and Europe, and is also the most likely to produce symptoms. In a paper appearing in the Jan. 3 online edition of the Journal of Experimental Medicine, the researchers report the discovery of a new Toxoplasma protein that may help explain why type II is more virulent than others.

Toxoplasma infection rates vary around the world. In the United States, it's about 10 to 15 percent, while rates in Europe and Brazil are much higher, around 50 to 80 percent. However, these are only estimates -- it is difficult to calculate precise rates because most infected people don't have any symptoms.

After an infection is established, the parasite forms cysts, which contain many slowly reproducing parasites, in muscle tissue and the brain. If the cysts rupture, immune cells called T cells will usually kill the parasites before they spread further. However, people with suppressed immune systems, such as AIDS patients or people undergoing chemotherapy, can't mount an effective defense.

"In AIDS patients, T cells are essentially gone, so once a cyst ruptures, it can infect more brain cells, which eventually causes real damage to the brain," says Saeij.

The infection can also cause birth defects, if the mother is infected for the first time while pregnant. (If she is already infected before becoming pregnant, there is usually no danger to the fetus.)
 
 

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