New research led by a team at the Karolinska Institutet in Sweden reveals for the first time what the common “cat litter parasite” Toxoplasma gondii does once it gets into the brain. The study is important in the light of recent observations linking the parasite to risk-taking and other human behaviours, and associations with mental illness. The researchers write about their findings in a paper published online on 6 December in the journal PLoS Pathogens.

Toxoplasma Infection Infection by Toxoplasma gondii or Toxoplasma is called Toxoplasmosis. Estimates suggest between 30 and 50% of the global human population is infected. In Sweden the figure is nearer 20%. In the US, the Centers for Disease Control and Prevention (CDC), puts the number of infected men, women and children at 60 million. Animals can also become infected, especially domestic cats. People usually contract the parasite by eating poorly cooked meat: according to the CDC, toxoplasmosis is the leading cause of death from foodborne illness in the US. Another way people become infected is by touching cat feces, hence the expression “cat litter parasite” because one way of touching cat feces is handling the cat litter tray. The vast majority of people infected have few symptoms because their immune system usually stops the parasite from causing illness. In newly infected adults the parasite can cause mild flu-like symptoms, and then it usually enters a chronic dormant phase which was thought to be symptom free. However, when the parasite enters the brain of fetuses, and people with weak immune systems, it can be fatal. Because of this risk, uninfected pregnant women should not touch cat litter trays.

There is an emerging view that the toxoplasmosis parasite is active to some extent during what was previously regarded as a purely “dormant phase”. For example, rats infected with the parasite lose their fear of cats, and are even attracted by their scent, making them easy prey. Scientists have suggested this is how the parasite assures its own survival and propagation: the cats eat the infected rats, shed more parasite through their feces, and that in turn helps to infect more rats. Other studies have found schizophrenia, depression, anxiety, and other mental diseases are more common in people with toxoplasmosis, and there is also evidence to suggest infection by the parasite is linked to more extroverted, aggressive and risk-taking behavior. In a study published in the July 2012 issue of the Archives of Psychiatry, researchers from Denmark’s Statens Serum Institut and the University of Maryland in the US, found that women carrying IgG antibodies to Toxoplasma gondii when giving birth have a higher risk of self-harm or suicide later on. While such a description sounds alarming, study senior author Antonio Barragan, researcher at the Center for Infectious Medicine at Karolinska Institutet and the Swedish Institute for Communicable Disease Control, says: “At the same time, it’s important to emphasize that humans have lived with this parasite for many millennia, so today’s carriers of Toxoplasma need not be particularly worried.”