The researchers say the microbes, called Toxoplasma gondii, invade the human brain and appear to upset its chemistry — creating, in some people, the psychotic behaviors recognized as schizophrenia.
The cause of schizophrenia is unknown, but both genetic and environmental factors likely play a part, according to the National Institute of Mental Health. Yolken is among the researchers worldwide examining whether certain viral infections can increase the risk of developing the illness. Other studies have focused on flu and herpes viruses as possible triggers.
The notion that there are links between infections and schizophrenia is "intriguing," said Dr. Ken Duckworth, a clinical psychiatrist and medical director for the National Alliance on Mental Illness. "There's so much we don't know about schizophrenia that anybody paying attention would welcome any thoughtful inquiry. And the infectious disease corner of the field is a growth stock right now," he said. "If true, it would have a preventive implication which is profound."
In a study of 12- to 49-year-olds who were born in the U.S., about 9 percent were found to be infected with T. gondii in the early 2000s, according to the Centers for Disease Control and Prevention. That rate is down from 14 percent in the 1990s. However, Yolken believes infection rates are higher among older Americans.
Yolken, a cat owner who says he has tested positive for Toxoplasma antibodies, said the potential link between Toxoplasma infections and mental illness is no reason for cat owners to panic — they just need to keep some basic hygienic precautions in mind.
He was drawn into the field by some intriguing questions about schizophrenia.
"I couldn't understand why a disease like schizophrenia persists in humans," he said. Through much of our history, "people who have these diseases don't reproduce very well, either because they're sick, or they've been locked up, or because they were killed."
If the disorder were strictly genetic in origin, he added, those genes should have been culled from the gene pool long ago. But they weren't. That raises the question of an environmental, perhaps infectious origin — a germ that has evolved to benefit by infecting other species.
It was a colleague, Dr. E. Fuller Torrey of the Stanley Medical Research Institute in Chevy Chase, who sealed the deal for Yolken by unearthing a trove of studies from as far back as the 1950s, many of them not written in English, suggesting links between infections and schizophrenia.
Schizophrenia, a severely disabling brain disorder characterized by confusion, delusions and hallucinations, afflicts 1.1 percent of the U.S. population age 18 and older, according to the National Institute of Mental Health. Someone with a toxoplasma infection, called toxoplasmosis, is more than twice as likely to develop schizophrenia, according to Yolken.
First isolated in 1908, T. gondii is present in the bodies of a third of the world's population, on average. It is a lifelong infection.
"Humans can get infected either directly [from contact with cat feces], or from eating the [undercooked] meat of an animal that was itself infected by a cat," Yolken said. Some toxoplasmosis outbreaks have been traced to water supplies contaminated by cat feces.
When they're initially infected, most people have little more than flu-like symptoms, or none at all. But young children, people with weakened immune systems and women infected while pregnant can develop a brain inflammation called encephalitis, or suffer miscarriages, damage to the heart, liver, eyes or ears. Infection can be fatal, and fetuses that survive their mother's first infection may be born with retardation, deafness and other problems.
Life cycle of the parasite
Felines are typically infected when they eat an infected mouse or bird. They don't usually get sick. So far, there is no vaccine to prevent cats from becoming infected. So the microbe reproduces in the cat's gut. Its eggs, or oocysts, develop there, and are later shed with the cat's feces.
When another warm-blooded animal such as a mouse ingests the oocysts through contact with cat feces, the oocysts enter its gut. There they release cells that migrate mostly to muscles and the brain, forming cysts to protect themselves from the mouse's immune system.
But T. gondii must get back into a cat to reproduce again. To get there, it needs the mouse to be eaten by a cat. Scientists believe the parasite raises its odds of success by changing the mouse's behavior and reversing its natural fear of cats.