Jan. 25 (HealthDay News) -- Infection with the common Toxoplasma gondii parasite -- carried by cats and farm animals -- may increase a person's risk of schizophrenia, a U.S. study suggests.

Publishing in the January issue of The American Journal of Psychiatry, researchers from the Walter Reed Army Institute of Research and Johns Hopkins Children's Center found that 7 percent of the 180 schizophrenia patients in the study had been infected with toxoplasma before their diagnosis, compared to 5 percent of 532 people without schizophrenia.

That means that those exposed to toxoplasma had a 24 percent greater risk of developing schizophrenia. While this represents a small increase in risk, it's important because it may offer new clues about how the disease occurs in some of the 2 million cases of schizophrenia in the United States, the study authors said. That may help lead to new treatments.

"Our findings reveal the strongest association we've seen yet between infection with this very common parasite and the subsequent development of schizophrenia," researcher Dr. Robert Yolken, a neurovirologist at Johns Hopkins Children's Center, said in a prepared statement.

Previous studies had found an association between schizophrenia and the presence of toxoplasma antibodies, which are evidence of past infection. This new study is the first to show that infection with the parasite can precede the initial onset of schizophrenia symptoms and subsequent diagnosis, Yolken said.

"Until now, the only thing we could say is that some people with schizophrenia also had been infected with toxoplasma at some point, but we couldn't tease out which came first," he said. "With our current study, we were able to show that infection came first."

Yolken and his colleagues plan to study whether aggressive use of antiparasitic drugs to treat toxoplasma infection in schizophrenia patients can halt progression of the mental illness.

Most toxoplasma infections occur early in life through exposure to cat feces or undercooked beef or pork. Infections rarely cause symptoms, but the parasite remains in the body and can become active after being dormant for many years, according to background information in a news release about the study.

Most people infected with toxoplasma never develop schizophrenia, but the parasite may trigger the mental illness in people who are genetically predisposed to it, explained Yolken.

SOURCE: Johns Hopkins Children's Center, news release, Jan. 18, 2008