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A Common Parasite Reveals Its Strongest Asset: Stealth -New Yorx times rage z or Cats play a major role in the parasite's success. They can carry it in their intestines, where they can produce egglike cysts called oocysts. A single infected cat can shed ioo million oocysts in its droppings. The oocysts can survive in the soil for over a year and can contaminate drinking water. Oocysts can infect humans, as well as other mammals and birds. Undercooked pork, chicken and other meat is another route Toxoplasma can take into our bodies. Once Toxoplasma enters a host, it spreads quickly. Within hours it can be detected in the heart and other organs. It is even able to infect the brain, which is protected from most pathogens by a tight barrier. Antonio Barragan and his colleagues at the Karolinska Institute in Stockholm were puzzled at first about how Toxoplasma managed this swift journey. "When we looked for parasites in the blood, we found very few that were just swimming around," said Dr. Barragan, an associate professor. But the scientists observed many of the parasites inside immune cells known as dendritic cells. Dr. Barragan was intrigued. Dendritic cells, common in the gut, often come into contact with pathogens. They respond by crawling to the lymph nodes or the spleen, where they communicate with other immune cells. 'Mat led us to think, what if this parasite is directing these cells to move and to disseminate through the body?" Dr. Barragan said. He and his colleagues put dendritic cells in a dish and injected them with Toxoplasma. They noticed that the parasites triggered a peculiar change: the dendritic cells became hyperactive, crawling for an entire day. Next, the scientists observed how Toxoplasma spread through a living animal. They added a firefly gene to the parasites so that they produced a glow. When they injected the parasites into mice, a little of the light escaped from the animals. By putting the mice in a darkened box, Dr. Barragan and his colleagues could track the parasites as they spread. Injecting dendritic cells carrying Toxoplasma spread the parasites to the brain and other organs far faster than injecting Toxoplasma alone. The researchers concluded that Toxoplasma was taking charge of the dendritic cells and riding along with them. Their results are published online in the journal Cellular Microbiology. As Toxoplasma spreads through the body, it invades cells. Unlike other pathogens, Toxoplasma can enter almost every type of cell in the bodies of thousands of host species. The parasite slips http://www. nytimes.com/2006/06/20/science/20toxo.html?ei=5070&en=7lb0423407cd4... 11/14/2006