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A Common Parasite Reveals Its Strongest Asset: steaitn - ivew r orK I Imes ragc -t ut i Toxoplasma becomes a menace when it does not have a healthy immune system to control. Pregnant women infected for the first time by Toxoplasma may pass it to their unborn children. Without a strong immune system to keep the parasite in check, a fetus can suffer massive brain damage. Up to 4,000 children are estimated to staffer toxoplasmosis in the United States each year. Toxoplasma is also dangerous to adults with weakened immune systems. The cause may be AIDS or immune-suppressing drugs given to people who receive orggq transplants. A quiet Toxoplasma infection can suddenly explode. For decades, most scientists believed that people with healthy immune systems had no effects from Toxoplasma. But some studies in recent years have hinted that the parasite can exert surprising effects on behavior, at least in animals. In 2000, British scientists demonstrated that rats infected with Toxoplasma lost their fear of cats. They proposed that this strategy increased the parasite's chances of getting into its final host. Scientists at Stanford University recently followed up on these experiments, studying rats and mice. "They actually show a mild attraction to the cat odor," said Ajai Vyas, a Stanford neurobiologist. "It's not just the loss of an old behavior. A new behavior is being induced." Dr. Vyas and his colleagues found that Toxoplasma's effects were precisely aimed at cat odor. The rats were still afraid of dog odor but not of rabbit odor. They could also acquire new fearful responses. "Only the innate fear to the cat was different, which was very surprising," he said. "We don't really know how fear of a cat is hard-wired in the brain." How Toxoplasma incites this change is a mystery. It is possible that the parasite alters the production of certain neurotransmitters. "But I don't know how some global change could have such a specific effect," Dr. Vyas said. He reported his results in May at the annual meeting of the International Behavioral Neuroscience Society. Some scientists suspect Toxoplasma may influence the human brain. Several studies suggest a correlation between Toxoplasma and schizophrenia, but the claims for a connection are not widely accepted. Dr. Robert H. Yolken, the director of the Stanley Laboratory at Johns Hopkins University, and his colleagues reviewed military medical records. They found that soldiers who developed schizophrenia were twice as likely as other soldiers to show signs of Toxoplasma infection in http://www.nyf mes.com/2006/06/20/science/20toxo.hurl?ei=5070&en=7lbO423407cd4... 11/14/2006