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Cats Again Get a Bad Rap in Toxoplasmosis Coverage

A study linking the disease to a psychiatric disorder marked by aggression didn't include cats -- but you'd never know it from the headlines.

s.e. smith  |  Apr 20th 2016


Toxoplasmosis is back. A new study led by researchers from the University of Chicago links the disease with Intermittent Explosive Disorder, in which patients experience outbursts of extreme anger. Headlines such as “Could germ from cat poop trigger rage disorder in people?” and “Cats Might Be the Reason Some People Are So Terrible” are circulating, but this is not in fact a study about cats. It’s a study about toxoplasmosis and the parasite that causes it, Toxoplasma gondii.

The study ran ran in the Journal of Clinical Psychiatry, which has a rigorous peer-review process and has been published for more than 75 years. In other words, it carries some serious weight, and the researchers knew what they were doing. Put in layperson’s terms, the effort was building on existing studies (like this one from 2013 and another from 2015) suggesting that there was a correlation between T. gondii infection and psychiatric conditions. Specifically, researchers were curious to see whether the infection was associated with more aggressive behavior.

This study was published in 2016, drawing upon years of data, including blood samples that indicated whether patients have been infected with T. gondii. The extended collection period would have allowed researchers to get a big-picture finding and control for other factors that might influence personality and behavior. The study involved 358 patients including a control group, people diagnosed with Intermittent Explosive Disorder, and those with other mental health conditions. These are all sound research practices, and the recent publication date indicates that the study took advantage of the latest developments in psychiatry and clinical research.

Those involved in the study have noted a consistent link between suicidal behavior and impulsive aggression over the course of their research, and they were also aware of studies like the ones linked above suggesting that T. gondii is connected to aggression. They hypothesized that in patients with Intermittent Explosive Disorder, there would be a higher statistical probability of T. gondii infection. They also suspected that people with higher rates of aggression who don’t have IED might also be more likely to have T. gondii than the rest of the population.

While recruiting their subjects, the researchers also assessed physical health with the goal of controlling for other factors that might contribute to aggression. For example, a patient with a brain tumor might experience personality changes, or a patient recovering from a major accident might be processing emotional trauma. All of the psychiatric evaluations were conducted by clinical psychiatrists with a minimum of a master’s degree, and they were not told about the hypothesis of the research. The clinicians were also provided with training to standardize their patient evaluations.

A microscopy image of Toxoplasma gondii

Here’s T. gondii up close. (Photo: Michael Wunderli)

The researchers used standardized tests to evaluate all of their subjects. They also discussed each patient’s history of suicidal attempts and self-harming behaviors and collected all of their blood samples in the same way for further consistency.

They found that 15.9 percent of the study subjects tested positive, a little bit more than the national average (14.1 percent), and they also found that T. gondii infection was a strong predictor of aggressive behavior in patients with IED — possibly because the parasite wants to perpetuate itself, and one way to do it is by making hosts unafraid of predators. Along the way, they also noted that T. gondii tended to be more common in people with mental health conditions in general, especially personality disorders like Borderline Personality Disorder.

Generally, toxoplasmosis seems to indicate a higher risk of psychiatric disorders. However, the researchers also pointed out that their sample size was small, especially when it came to groups like people with a history of suicidal behavior — they could not, for example, make any definitive comments about whether T. gondii was linked with suicidal ideation because they didn’t have enough data. They suggested that the parasite may trigger immune responses that act on the brain to change the patient’s neurochemistry, and this could explain behavioral changes.

The study pointedly did not look at a history of cat guardianship in study subjects, which highlights the misleading nature of the media coverage. Researchers were interested solely in the history of parasitic infection, and left the question of cats to other studies. Moreover, while the introduction noted that patients can be infected via handling cat feces, the researchers went on to state that there are numerous other mechanisms of transmission, including contaminated water, exposure to undercooked meat, and congenital infection. The CDC also stresses these facts in its factsheet on toxoplasmosis, adding that it’s pretty easy to avoid infection by means of washing your hands after handling things that may have been contaminated.

Heather Fritz, a veterinary parasitology researcher at the University of Washington, has conducted a whole lot of research on the organism, which is indeed only capable of reproducing in feline stools, including larger members of the cat family. T. gondii can remain dormant in the environment for as long as 18 months, infecting groundwater, lying around in the soil, and, yes, being eaten by cats, but also livestock and other animals.

Indoor cats are unlikely to come into contact with it unless their guardians are infected, or they eat raw meat. Keeping cats indoors and using best practices with meat makes it highly unlikely that you will contract the infection, assuming you don’t already have it.

Bottom line: Don’t believe the media hype. While this study definitely showed a correlation between T. gondii and mental health conditions, and suggested some possible avenues of causation, it didn’t say anything about cats, beyond a perfunctory mention of the fact that they’re known carriers of the parasite, but, surprise, so is almost every animal on Earth! For now, you can return to your regularly scheduled programming of reminding anti-cat people that their claims about toxoplasmosis should go back in the litterbox where they belong.

Top Photo: Zhao!/Flickr