Previous studies have also linked T. gondii infection to greater risk of mental disorders. In November 2014, for example, Medical News Today reported on a study claiming the parasite is responsible for around a fifth of schizophrenia cases . Now, new research provides further evidence of this association.

People with a healthy immune system often stave off T. gondii infection, so it does not present any symptoms. However, pregnant women and people with weakened immune systems are more susceptible to infection and may experience flu-like symptoms – such as muscle aches and pains and swollen lymph nodes – as a result, while more severe infection may cause blindness and even death.

T. gondii is the cause of a disease known as toxoplasmosis. According to the Centers for Disease Control and Prevention (CDC), more than 60 million people in the US are infected with the parasite, though the majority of people are not aware of it.

Two studies published in the journals Schizophrenia Research and Acta Psychiatrica Scandinavica attribute this association to Toxoplasma gondii – a parasite found in the intestines of cats. Humans can become infected with the parasite by accidentally swallowing it after coming into contact with the animal’s feces.

They are cute, fluffy and have that wide-eyed glare that few of us can resist; it is no wonder more than 95 million of us own a cat. But there may be a darker side to our four-legged friends. New research claims the animals could increase our risk of mental illnesses, including schizophrenia and bipolar disorder.

For one study, Dr. Robert H. Yolken, of the Stanley Laboratory of Developmental Neurovirology at Johns Hopkins University School of Medicine in Baltimore, MD, and colleagues assessed the results of two previous studies.

These studies had identified a link between cat ownership in childhood and development of later-life schizophrenia and other mental disorders, comparing them with the results of a 1982 National Alliance for the Mentally Ill (NAMI) questionnaire.

The NAMI questionnaire – conducted around a decade before any data was published on cat ownership and mental illness – revealed that around 50% of individuals who had a cat as a family pet during childhood were diagnosed with schizophrenia or other mental illnesses later in life, compared with 42% who did not have a cat during childhood.

The questionnaire, the researchers say, produced similar results to those of the two previous studies, suggesting that “cat ownership in childhood is significantly more common in families in which the child later becomes seriously mentally ill.”

“If true,” the authors add, “an explanatory mechanism may be T. gondii. We urge our colleagues to try and replicate these findings to clarify whether childhood cat ownership is truly a risk factor for later schizophrenia.”

In another study, A. L. Sutterland, of the Academic Medical Centre in Amsterdam, the Netherlands, and colleagues conducted a meta-analysis of more than 50 studies that established a link between T. gondii and increased risk of schizophrenia.

They found that people infected with T. gondii are at more than double the risk of developing schizophrenia than those not infected with the parasite.

The team also identified a link between T. gondii infection and greater risk of bipolar disorder, obsessive-compulsive disorder (OCD) and addiction.

“These findings suggest that T. gondii infection is associated with several psychiatric disorders and that in schizophrenia, reactivation of latent T. gondii infection may occur,” note the authors.

The CDC recommend changing a cat’s litter box every day to reduce the risk of T. gondii infection, noting that the parasite does not become infectious until 1-5 days after it has been shed in the animal’s feces.

They also recommend feeding cats only canned or dried commercial foods or well-cooked meats; feeding them raw or undercooked meats can increase the presence of T. gondii in a cat’s feces.

It is important to note that cat feces are not the only source of T. gondii infection. Humans can contract the parasite through consuming undercooked or contaminated meats and by drinking contaminated water.

In April, a Spotlight feature from MNT investigated toxoplasmosis and other health risks associated with pet ownership.