Having a pet cat as a child may increase the risks for developing schizophrenia later in life, because of a brain-damaging parasite that can pass from felines to their owners.

That’s the upshot of new research by Johns Hopkins University scientists who have spent three decades studying cat-to-human transmission of the parasite, called Toxoplasma gondii (T. gondii).

The cat-carried parasite can infect any warm-blooded species, and the Centers for Disease Control and Prevention estimates more than 60 million people in the U.S. may have it — and most have no symptoms, CBS News reports.

Although the parasite causes no harm to most people, those with weaker immune systems can suffer an illness called toxoplasmosis, which can result in miscarriages, fetal development disorders, weeks of flu-like illness, blindness, mental disorders, and even death.

The latest research compared two previous studies that found a link between childhood cat ownership and the development of schizophrenia later in life with an unpublished survey on mental health from 1982, 10 years before any data on cat ownership and mental illness had been published.

Results of the analysis — led y E. Fuller Torrey of the Stanley Medical Research Institute and Robert H. Yolken, M.D., of Stanley Laboratory of Developmental Neurovirology at the Johns Hopkins University School of Medicine — indicated that cat exposure in childhood may be a risk factor for developing mental disorders.

"Cat ownership in childhood has now been reported in three studies to be significantly more common in families in which the child is later diagnosed with schizophrenia or another serious mental illness," the authors reported in a press release issued with the study, The findings were published in the journal Schizophrenia Research.

Other research, by A.L. Sutterland from the Department of Psychiatry at the Academic Medical Centre in Amsterdam, analyzed the findings of 50 published studies to confirm that T. gondii infection is associated with mental disorders. The results, published in the journal Acta Psychiatrica Scandinavica, showed that an individual infected with T. gondii was almost twice as likely to develop schizophrenia.

The Humane Society estimates there are 75 to 80 million pet cats and another 30 to 40 million stray or feral cats roaming the United States. Outdoor cats have an increased likelihood of infection with T. gondii. Cats excrete millions of infected eggs, or oocysts, in their feces, which can remain in soil or water for years.