Can a parasite commonly transmitted by cat feces lead to schizophrenia?
One University of Pennsylvania researcher says the medical establishment shouldn’t be too quick to dismiss the possibility.
In a recent study, Gary Smith, professor of population biology and epidemiology at the Penn's School of Veterinary Medicine, attempted to estimate the proportion of schizophrenia cases that may be attributable to an infection with parasite Toxoplasma gondii.
The results, published in the Journal of Veterinary Medicine, suggest a sizable number – one fifth – of schizophrenia cases could be linked to the parasite, which is also transmitted by soil and undercooked meat.
"Infection with Toxoplasma is very common, so, even if only a small percentage of people suffer adverse consequences, we could be talking about problems that affect thousands and thousands of people," Smith said in a news release.
More than 60 million people in the country are thought to be infected with T. gondii, though the vast majority of them aren't aware of it, according to the U.S. Centers for Disease Control and Prevention.
Conventional medical wisdom has long dictated most healthy people suffer no adverse effects from a T. gondii infection, but more recent research suggests the parasite, which can enter the brain and muscles, might be more insidious, even when its sufferers are asymptomatic.
A number of field and laboratory studies in rodents and people have linked T. gondii infections to changes in behavior and personality, including schizophrenia.
T. gondii has been reported to remove mice's innate fear of cats, with some research suggesting the change in brain function persists months after infection and might be permanent.
Other works have linked the parasite to increased risks of self-harming behavior among women and traffic accidents, the latter possibly due to compromised reaction times.
Studies have also found that some antipsychotic drugs used to treat schizophrenia and bipolar disorder can stop T. gondii from replicating.
Smith sought to expand on the currently available knowledge by calculating the population attributable fraction, a metric used by epidemiologists to estimate the proportion of cases that might be attributable to a risk factor.
"In other words, we ask, if you could stop infections with this parasite, how many cases could you prevent?" Smith said. "Over a lifetime, we found that you could prevent one-fifth of all cases. That, to me, is significant."
He hopes his findings might help open the minds of researchers in the future.
"Instead of ridiculing the idea of a connection between T. gondii and schizophrenia because it seems so extraordinary, we can sit down and consider the evidence," Smith said. "Perhaps then we might be persuaded to look for more ways to reduce the number of people infected with Toxoplasma."