Toxoplasmosis: The Truth About Cats and Germs

I should start this article with a disclaimer: I am unashamedly and whole-heartedly a dog person. Being a dog person typically excludes you from also being a cat person. Imagine my delight then, when a story recently popped up across multiple online news and science sites entitled ‘Cat owners more likely to experience road rage’ (or some variation thereon). The dog-loving part of my brain wanted to take this news and run with it. Those haughty, fluffy jerks! But the scientist in me insisted on investigating further. Of course, I found that the original study findings had been somewhat over-blown. I’ll explain why, but first, some background. Cats carry in their intestines a protozoan (literally ‘first animal’) parasite called Toxoplasma gondii, or T. gondii to its friends. Cats spread the parasite through ‘oocysts’, egg-like cysts in their droppings. One cat infected with T. gondii sheds around 100 million oocysts in their lifetime, and these can survive in soil or water for over a year. T. gondii infects humans through contact with cat faeces, the consumption of un-cooked meat or contaminated water, and then hijacks cells in the intestine in order to travel through the human body. The parasite actually encourages these hijacked cells to move faster than usual in order to spread quickly and take up final residence in the brain. Here T. gondii hides inside nerve cells and their surroundings in the form of cysts; which are hidden from the body’s immune system. Sneaky parasitic bastard. Infection with T. gondii is called toxoplasmosis. For unborn babies, infants, or anyone with a weakened immune system, toxoplasmosis can be fatal. In the case of healthy humans, toxoplasmosis is typically latent. There may be initial flu-like symptoms, but nothing immediately major or even noticeable. So stealthy is toxoplasmosis, that over one-third of the world’s population are estimated to be infected. This is determined by measuring Toxoplasma-specific antibody levels present in blood plasma. Certain antibodies (IgG) indicate past infection, whereas others (IgM) indicate recent infection. In the study that launched a thousand headlines, researchers from the University of Chicago measured the levels of IgG antibodies in a sample of 358 psychiatric patients. One third of this group had been diagnosed with Intermittent Explosive Disorder (IED), which is characterised by aggressive outbursts, such as road rage and temper tantrums. Twenty-two percent of the IED group had been exposed to toxoplasmosis, and were more than twice as likely to have been exposed than participants with no psychiatric disorder. 16 percent of a comparison group made up of psychiatric patients with a diagnosis other than IED had also been exposed to toxoplasmosis. Most interestingly, those who had been previously infected with toxoplasmosis had substantially higher levels of aggression. The authors conclude that, “latent infection with the toxoplasma gondii parasite may change brain chemistry in a fashion that increases the risk of aggressive behaviour”. This study isn’t the first to find a relationship between T. gondii infection and mental disorder. Indeed, toxoplasmosis has previously been associated with bipolar disorder, personality disorder, schizophrenia and, most notably, suicide. One study found T. gondii infection to be associated with suicide in postmenopausal women. It may be pejorative but ‘Crazy cat lady stereotype confirmed’ seems a much more sensational headline suited for pseudo-science websites, no? One question these studies have been unable to answer is, ‘which comes first?’ Not everyone infected with T. gondii also has signs of mental disorder, and not everyone with mental disorder has been infected with the parasite. In fact, existing studies have only demonstrated a correlation between these variables, and anyone who has ever taken an Intro to Statistics class knows that correlation does not necessarily equal causation. That is, there is no evidence that T. gondii actually causes mental disorder, only that the two tend to co-occur. Perhaps a vulnerability to mental disorder leads to increased exposure to cats? Or a preference for under-cooked meat? Perhaps toxoplasmosis only manifests as mental disorder in those pre-disposed to negative emotional states, or those exposed to significant life stress? Most likely, this is the first stage of the diabolical plan for world-domination that we have long suspected cats are plotting. First step: human mind control. With toxoplasmosis predicted to rise in the coming years, in large part due to the environmental effects of climate change, understanding the relationship between the T. gondii parasite and human behaviour will be critical to protect future generations from its insidious effects. Or at least from cats. Dr Jessica L Paterson, Senior Research Fellow, CQUniversity, Appleton Institute