The largest comparison study to date of blood samples from healthy people and patients with schizophrenia suggests infection with the parasite Toxoplasma gondii may increase risk for developing the disease, according to an article in the January issue of the American Journal of Psychiatry. In the U.S., infection is common in domestic cats and farm animals.

The research was conducted among U.S. military personnel by researchers from Walter Reed Army Institute of Research and Johns Hopkins Children's Center.

Researchers found that of the 180 study subjects diagnosed with schizophrenia, 7 percent had been infected with toxoplasma prior to their diagnosis compared with 5 percent among 532 healthy recruits, a 24-percent increase in risk of developing schizophrenia.

The difference, while seemingly small, is important, researchers say, because the ability to explain even a small portion of the 2 million cases of schizophrenia in the United States may offer clues to the disease and some possible treatments.

For example, the investigators say they plan to study whether aggressive treatment of toxoplasma infection with antiparasitic drugs in patients with schizophrenia could halt the progression of the mental disorder, characterized by paranoia, delusions and hallucinations.

Most infections with toxoplasma occur early in life following exposure to the parasite in cat feces or undercooked beef or pork. Infections rarely cause symptoms, but the parasite remains in the body and can reactivate after lying dormant for years.

"Our findings reveal the strongest association we've seen yet between infection with this very common parasite and the subsequent development of schizophrenia," said Robert Yolken, M. D., a neurovirologist at Hopkins Children's who was among those conducting the analysis.

Previous studies have reported on the link between schizophrenia and the presence of toxoplasma antibodies, which are evidence of past infection, but this is the first study to show that infection with the parasite can precede the initial onset of symptoms and subsequent diagnosis with schizophrenia, Yolken said.

Because the U.S. military routinely tests its active personnel for toxoplasma, among other infectious agents, and stores blood samples in a central repository, researchers were able to determine the time line between infection and a diagnosis of schizophrenia.

"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," Yolken added. "With our current study, we were able to show that infection came first."

While most people infected with toxoplasma never develop schizophrenia, the parasite may be a trigger in those genetically predisposed to the disorder, a classic example of how genes and environment come together in the development of disease, Yolken concluded.