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A Common Parasite Reveals Its Strongest Asset: Stealth -New York rimes rage s or into a cell by latching onto its surface and pulling the membrane over itself. "You can think of it like sticking your finger into a balloon," said Vernon B. Carruthers of the University of Michigan. David Sibley, an associate professor of molecular microbiology at Washington University in St. Louis, said, "it just sits there, and the host doesn't recognize it as a foreign body it should destroy." If Toxoplasma simply spread from cell to cell, it could cause serious harm. But killing its host is not in the parasite's best interests: its goal is to get into its final host, cats, the only creature in which Toxoplasma can reproduce by making oocysts that are shed in feces. And cats do not like eating dead animals. Instead, Toxoplasma has evolved to be extremely contagious, but not very harmful. 'Ms is an organism that has very cleverly worked out a way to go from one host to another," said Alan Sher, chief of the laboratory of Parasitic Diseases at the National Institutes of Health. Dr. Sher studies how Toxoplasma manipulates its host's immune system. In the early stages of infection, the parasite sets off the production of signaling molecules called cytokines. They cause the immune system to attack Toxoplasma, killing off free-floating parasites. The parasites that happen to be inside cells during the attack somehow recognize what is happening and enter a kind of hibernation. Their host cell turns into a cyst in which they can hide from the immune system. The strong response Toxoplasma provokes from the immune system carries a risk of its own. "These cytokines just get out of control and cause tissue damage," Dr. Sher said. In experiments with mice, this response can be fatal, he said. Toxoplasma steers its hosts away from this danger, Dr. Sher has found. It causes its host to make molecules that rein in the immune system. An infection with Toxoplasma may feel like nothing more than a mild case of the flu, and the symptoms pass once the parasite has snuggled itself away in its cysts. In later years, cysts occasionally break open, but the immune system quickly destroys most of the free parasites. The few survivors invade new cells. "That`s the key of this infection," Dr. Barragan said. "I think this is why this parasite is so tremendously successful worldwide." http://www.nytimes.com/2006/06/20/science/20toxo.html?ei=5070&en=71 bO423407cd4... 11/14/2006