In what researchers describe as the largest study of its kind, scientists have found new evidence of a link between infection with the protozoan parasite, Toxoplasma gondii, and schizophrenia.
T. gondii, a brain-dwelling parasite estimated to be hosted by at least 2 billion people around the world, doesn't create symptoms in most people who become infected – but acute cases of toxoplasmosis can be dangerous.
Healthy adults are generally thought to not be at risk from T. gondii infections, but children or people with suppressed immune systems can develop severe flu-like symptoms, in addition to blurred vision and brain inflammation.
Pregnant women need to be careful too, as the parasite can cause foetal abnormalities or even miscarriage.
Aside from the known physiological dangers, however, the stranger and more ambiguous risks associated with the parasite remain largely hypothetical – although a huge body of research suggests something weird is going on.
Causation remains very much disputable, but the brain-dwelling parasite – commonly carried by cats and present in their faeces – has been linked to a huge host of behaviour-altering effects.
Virtually all warm-blooded animals are capable of being infected, and when T. gondii gets inside them, unusual things happen.
In rodents, animals seemingly lose their inhibitions, becoming more exploratory and losing their aversion to cat odours.
The same kind of risky behaviour has been seen observed in human studies, where links have been documented between T. gondii infections and everything from car crashes to entrepreneurial activity.
Other research suggests the parasite could boost suicide rates, and numerous studies have drawn associations to conditions including a range of neurological disorders, including epilepsy, Alzheimer's, and Parkinson's, among others.
Many scientists are at pains to point out we can't yet show that the mind-altering cat parasite is actually producing these psychological changes itself – as opposed to merely being associated with them – but while the debate goes on, still more evidence of these alarming coincidences turns up.
In that vein, the new study, led by researchers from Copenhagen University Hospital in Denmark, analysed data from over 80,000 individuals who took part in the Danish Blood Donor Study – a giant cohort, providing the basis for what the team calls the "largest to date serological study" in this area.
To ascertain links between mental disorders and infections with T. gondii and another common pathogen, the herpes virus cytomegalovirus (CMV), the researchers identified 2,591 individuals in the blood study who were registered with psychiatric conditions, and analysed their samples to look for traces of immunoglobulin antibodies indicative of the two infections.
In terms of T. gondii, compared to a control group, the blood work revealed individuals with the infection were almost 50 percent more likely (odds ratio 1.47) to be diagnosed with schizophrenia disorders compared to those without an infection.
As the researchers explain, the link became even more evident when they filtered the data to account for 'temporality' - which meant only looking at participants who hadn't yet been diagnosed with schizophrenia when T. gondii was found in their blood.
"The association was even stronger when accounting for temporality and considering only the 28 cases who were diagnosed with a schizophrenia disorder after the date of blood collection," the authors write.
According to the researchers, this "corroborates that Toxoplasma has a positive effect on the rate of schizophrenia and that T. gondii infection might be a contributing causal factor for schizophrenia."
While the link between the parasite and schizophrenia has been observed in previous research, the researchers claim their study is the first to examine temporality of pathogen exposure like this.
Still, it's important to note, despite this new data, the researchers still aren't claiming to have definitive proof of causation, and they also acknowledge their study "did not control for socio-economic factors, which may influence the probability of pathogen infection [and] development of psychiatric disorders".
Despite these limitations, the researchers say their findings add to the growing scientific evidence linking pathogenic T. gondii infection with serious psychiatric disorders.
With researchers examining the microscopic mechanisms that may explain how these disturbing associations could come about, the picture is slowly becoming clearer, bit by bit.
In the meantime, to minimise your exposure to toxoplasmosis – let alone its hypothetical ramifications – always cook food to safe temperatures, wash fruit and vegetables thoroughly, wear gloves while gardening, and be really careful around kitty litter.
The CDC's official rundown on the parasite is a good resource for more information.
The findings are reported in Brain, Behavior, and Immunity.