Mycology fans may be familiar with the harrowing fate of insects infected by the zombie fungus. Scientifically known as Ophiocordyceps unilateralis, this entomopathogen survives within a host much like a parasite and alters the behavior of its host. The carriers find themselves drawn to humid, fungus-friendly environments where they attach themselves to the underside of the leaf. Shortly after, a fruiting body erupts from their heads and spews spores of O. unilateralis all over the shop. Grim stuff.
The concept of such a parasite influencing our behavior might seem like something constrained to science fiction, but such a pathogen exists and, if you’ve got a cat, it might be in your home. Toxoplasmosis is the disease caused by infection from the Toxoplasma gondii parasite, and it’s one of the most common parasites in the world. We can catch it from eating poorly prepared meat or by coming into contact with infected cat feces.
Symptoms are usually mild or absent in healthy people, but those with a compromised immune system or who are pregnant can suffer more severe disease. Disease experience can range from flu-like symptoms to seizures and respiratory distress, but some research indicates that toxoplasmosis may influence our decision making (don’t worry, you won’t end up with a mushroom bursting from your forehead).
Toxoplasmosis begins its life in the stomach of the cat, which is also the only place it can reproduce so its mission once expelled is to find its way back in there. The bizarre path it has adapted goes via rats, who eat cat feces and become infected. Usually, a rat will steer well clear of a cat, running at even the faint whiff of their urine. A toxo rat, however, suddenly finds the stuff delectable and will seek it out, increasing its chances of ending up in the stomach of the cat and taking the toxo with it. Clever parasite.
So how does it do it? It takes around six weeks after munching on some infected cat feces for toxoplasmosis to get to the brain, where it forms cysts in the amygdala region. Here, it shrivels dendritic nerve cell endings linked to neural pathways that govern the rat’s response to predation. At the same time, the toxo also works away to rewire the area of the brain in charge of sexual attraction and effectively links together the two paths. Now, rather than smelling urine and feeling fear, the rat smells urine and feels… frisky. The crossover sees the randy rat actively seeking out cat urine and soon, the toxo is right back where it wanted to be.
For all its ingenuity, toxoplasmosis is going to have a tougher time trying to get the body of a human into the stomach of a cat, and if you’re wondering how infected patients feel about cat piss I’m delighted to inform you that this study tried to find out exactly that by doing precisely what you expect. They rounded up 34 toxoplasma-infected and 134 non-infected students to sniff wee samples from a cat, horse, tiger, brown hyena, and dog and rate them on their intensity and pleasantness. The sniffers didn’t know if they were infected or not, nor which species the pee-perfumes were from. Olfactory function wasn’t found to differ between participants, but there was a strong, sex-dependent effect of toxoplasmosis on the pleasantness attributed to cat urine. Infected men liked the cat pee more than noninfected men, yet there was no difference between infected and non-infected women in their perception of the piss. Not a sentence I thought I would be writing this week.
The bizarre finding sits within a small body of research that suggests some infected men become more impulsive when infected with toxoplasmosis, and infected people are three to four times more likely to be involved in a fatal car crash. Interestingly, men with schizophrenia are more likely to present with a history that indicates an increased risk of contracting toxo, such as their parents having owned a cat while the child was in utero. Schizophrenic patients are treated with dopamine blockers and if you give these drugs to a toxo rat it stops being attracted to cat urine.
While a grim thought, the reality is that, for most of us, toxoplasmosis isn't something to fear and there are plenty of steps you can take to reduce your risk of transmission. "Although cats do harbor the disease, the most common transmission is from undercooked meat," said infectious diseases specialist Dr Dominic Sparkes in an email to IFLScience. "It is also worth noting that it is only cats who forage outdoors and eat raw meat that are at risk, indoor cats who eat dry or canned processed cat food pose no risk. Even if infected with toxoplasma, the vast majority of people will be asymptomatic and those that become unwell get nothing more than a fever and so immunocompetent people usually do not require any treatment. It can be important in patients who are immunocompromised who may require treatment, or in pregnant women, as it can be passed onto the unborn fetus, which can cause congenital abnormalities."
The headache-inducing story of toxoplasmosis is enough to make us question our concept of free will. While the human race exhibits an almost-unique degree of consciousness, there are still biological underpinnings to some of our behavior. Who knows what else might be influencing our decision making? *cue the Twilight Zone opening*