Toxoplasma gondii is a single-celled protozoan. Yet, it has the ability to subvert ‘free will’ of its warm-blooded hosts and alter their behavior. It is capable of infecting all mammals—humans included—although the way to true love is through the cat’s stomach. By this one means that this protozoan can only sexually reproduce in feline species. What one does not mean is that it can only ‘affect’ cats. When Toxo finds itself inside a mouse, it brainwashes it into being a cat-lover. Despite the dire consequences such behavior would portend for the mouse, a Toxo-infected mouse will run towards the smell of a cat—indeed, get sexually aroused by it. Keeping aside the colloquial references to the term ‘mouse brain’, given the complex cognitive reconditioning required to override the instinctive cat aversion mice have and turn it into something akin to sexual arousal, this is no small feat for something that is all of one cell. Toxoplasma gondii represents perhaps one of the most impressive and sinister examples of a parasites that have evolved to manipulate their hosts’ nervous systems and behavior. Needless to say, Toxo is a neurobiologist’s wet-dream.
Here is a factoid for you: As many as one in three people around the world have Toxo in their brains. It sits around in a dormant state and does not pose a health risk for most people. However, it has been suggested that Toxo manipulates our behavior with parasitic mind-hacking. We would like to think our brains to be way too complex for a puny single-cell organism to be able to control them, but consider. Not too long ago, humans were as apt as a food source for the larger cats as mice are to cats, and therefore as apt an intermediate host for Toxo to brain-fuck. There’s evidence—suggestive although inconclusive at this juncture—that the parasite is truly influencing our behavior: infected individuals score different on personality assessments, have correlational tie-ups to risk tolerance and schizophrenia, and find the smell of cat urine affable!
..given the complex cognitive reconditioning required to override the instinctive cat aversion mice have and turn it into something akin to sexual arousal, this is no small feat for something that is all of one cell.
A study published last month offered new insight into the possible manipulative effects of Toxo on our closest primate cousins, the chimpanzees. The study assessed how Toxo-possessed chimps react to the smell of a big cat that they are afraid of—the urine of a leopard.
Clémence Poirotte undertook a journey from France to Gabon in Africa to find out the answer. She and team assessed how often 9 Toxo-infected chimps sniffed out and approached leopard pee compared to another 24 noninfected chimps. Turns out that while the non-infected chimps had a healthy dislike for the smell of leopard urine and actively avoided it, Toxo-infected ones were way more likely to sniff out and walk upto the pee, with apparent disregard to the possible ‘encounter’ that proximity to leopard pee may lead to.
The behavior of infected chimps suggestively speaks of Toxo effectively manipulating chimp behavior to get them into close quarters with leopards. But the study is hamstrung by the ever-present problem of correlation studies. The results of the study could very well be explained by an inherently strong streak of risky behavior in these 9 chimps, their reckless abandon explaining both them approaching leopard pee and being infected with Toxo because they previously sniffed out leopard pee.
Not too long ago, humans were as apt as a food source for the larger cats as mice are to cats, and therefore as apt an intermediate host for Toxo to brain-fuck.
However—this is the proposed clincher—this pee predilection existed only for leopards in these 9 infected chimps. They were not more likely to sniff out pee from lions, tigers, or humans. Now while leopards and chimps share an antagonistic relationship, neither tigers nor lions are a natural predator of chimpanzees. And us humans, we are practically cousins! The authors tout this as convincing proof of the ‘host-specific’ mind-bending capabilities of Toxo—yes the single-celled protozoan. If this is true, then this is beyond exciting, remarkable, stupendous. If this is true then ‘This is so toxo’ will become the urban English slang for something that is out-of-the world stupendously awesome. But whether or not it is true remains to be answered. And this work on 9 infected chimps cannot be the definitive answer. What it can be and is is a call as loud as can be for us human neuroscientists to delve into and tease apart the whether, the where, and the how of Toxo’s effects on intermediate host behavior. Because to understand Toxo would be so damned toxo!
Artwork: Dr. Leslee Lazar