A Parasite May Be Causing One-Fifth of Schizophrenia Cases

Cat feces, soil, and undercooked meat can transmit the Toxoplasma gondii parasite to humans. It has infected just over one-fifth of Americans, but the vast majority don’t know that they’ve contracted it. Although an estimated 60 million adults and children in the U.S. may have T. gondii infections, the Centers for Disease Control and Prevention reports that very few experience symptoms because their immune systems keep illness at bay typically.

Some populations are at risk, however. If a woman catches her first infection during pregnancy, her fetus may suffer serious developmental problems or die. People with HIV and other diseases that weaken their immune systems are susceptible to toxoplasmic encephalitis, a complication that can be deadly. Recent studies have found evidence of additional worrisome impacts including an association with schizophrenia because the parasite resides in the brain as well as the muscles. Field and laboratory studies in mice, rats, and people have discovered that T. gondii infection triggers changes in behavior and personality.

Establishing the T. Gondii Connection

Many factors, including family history, increase the risk of receiving a schizophrenia diagnosis. A by Gary Smith, professor of population biology and epidemiology at the University of Pennsylvania’s School of Veterinary Medicine, found that one-fifth of cases may be attributable to Toxoplasma gondii infection.

To investigate this connection further, Smith calculated the population attributable fraction (PAF), which is a metric epidemiologists use to determine how important a risk factor might be. In this case, Smith explained that the PAF is “the proportion of schizophrenia diagnoses that would not occur in a population if T. gondii infections were not present.” The usual method of calculating the PAF isn’t well suited to examining the link between schizophrenia and T. gondii because some of the variables are in flux constantly. For example, the proportion of people with T. gondii infections increases with age.

Using a standard epidemiological modeling format while taking all of the age-related changes in the relevant factors into account, Smith found the average PAF during an average lifetime to be 21.4 percent. He noted that the prevalence of T. gondii infection is much higher in some countries than in the U.S., and these locations also have greater schizophrenia rates. While the idea of a connection between this parasite and schizophrenia seems peculiar, Smith notes that finding more ways to reduce the number of infected people could improve multiple health outcomes including schizophrenia.

Contracting and Avoiding Infection

Felines acquire the T. gondii parasite when they eat infected prey such as mice or birds. They deposit about 1.2 million metric tons of feces into the U.S. environment every year, according to a . Dr. E. Fuller Torrey and Dr. Robert H. Yolken, scientists at Johns Hopkins University Medical Center, found that soil samples from diverse locales including California, China, Brazil, Panama, and Poland contained three to 434 oocysts, or T. gondii eggs, per square foot. About 1 percent of cats shed around 55 million oocysts per day. These parasitic eggs can survive up to 18 months or perhaps longer. Just a single egg may be able to cause an infection.

Cat ownership is on the rise. Between 1989 and 2006, the number of cats in U.S. homes increased about 50 percent from 54.6 to 81.7 million. During that time, dogs rose by 38 percent from 52.4 to 72.1 million. In 2012, Americans owned 86.4 million cats, compared with 78.2 million dogs, according to Humane Society statistics. The estimated number of feral cats in the U.S. is at least 25 million.

Responsible cat owners are likely at lower risk than others, notes Kimberly May, a veterinarian and spokeswoman for the American Veterinary Medical Association. They empty litter boxes frequently, wash their hands, and keep kids away from cat feces.

The greater adult risk for catching T. gondii comes from failing to follow these precautions and gardening without gloves. One estimate shows that the dirt under a gardener’s fingernails can harbor 100 oocysts. Children may ingest the eggs that are present in sandboxes and backyard dirt, Torrey warned.

1. gondii tissue cysts from raw beef also can lead to infection. The CDC reports that toxoplasmosis from eating infected meat is a leading cause of deaths from food-borne illnesses in the U.S. It creates flu-like symptoms and blurry vision. According to antibody levels, the incidence of toxoplasmosis has decreased steadily in the U.S. over the past two decades. The increased tendency to freeze meat may be a contributing factor.

Seeking Treatment

Research has shown that some antipsychotic drugs can stop T. gondii from reproducing. Geodon (Ziprasidone) is effective at treating the acute schizophrenia phase and reducing the risk of future psychotic episodes. During the acute stage, high doses may be necessary to help control psychotic symptoms. In the usually lifelong maintenance phase, doctors tend to reduce the dosage gradually to the minimum required to prevent further episodes and control inter-episode symptoms. If symptoms reappear or worsen on a lower dosage, an increase may be necessary to help avoid future relapses.

Although antipsychotics won’t cure your illness, they can reduce your symptoms greatly and allow you to function better, have a better quality of life, and enjoy an improved outlook. A well-trained physician who’s experienced in treating severe mental illness can individualize your treatment plan and modify it to keep your symptoms under control.