There were once two rats named Ratniss and Raticus. Like any self-respecting rats, Ratniss and Raticus enjoyed exploring, socialising, and eating garbage. Life was good, until one day, Ratniss noticed something odd about Raticus – he didn’t seem quite right in the head. While Ratniss was rightly terrified of cats and disgusted by the smell of cat urine, Raticus didn’t seem to mind either. So unconcerned was he that one day, Raticus climbed into a cat’s food bowl and found himself face to face with Mr Floof, who promptly gobbled him up. Like many before him, Raticus had sadly fallen victim to the life cycle of a rather unusual parasite: Toxoplasma gondii.
Toxoplasma gondii (T. gondii) is a pathogen that is neither a bacteria nor a virus, but rather a protozoan: an organism composed of a single eukaryotic cell, similar in nature to the cells from which animals are composed. While T. gondii is capable of infecting virtually all warm-blooded animals, including humans, it can only reproduce sexually when it infects felines. What makes this parasite remarkable, however, is its apparent ability to alter the behaviour of infected rodents – or, as sensationalists might put it, mind control.
Before we get to the whole ‘mind control’ subject, it helps to understand T. gondii’s life cycle. When a feline is infected by T. gondii, the parasite first reproduces inside the animal’s small intestine, giving rise to millions of new parasites contained within thick-walled sacs called oocysts. These oocysts make their way into the cat’s faeces. If another animal (such as the late Raticus) consumes food or water contaminated by oocysts, they too will become infected. However, T. gondii faces a problem once it finds itself in the small intestine of a non-feline. You see, in order to reproduce sexually and form oocysts, T. gondii needs an omega-6 fatty acid called linoleic acid. It turns out that felines are the only host lacking a key enzyme that degrades this fatty acid, allowing it to build up in their intestines. In order to complete it’s life cycle, T. gondii must therefore find its way back to a feline host.
T. gondii is not a particularly dangerous pathogen for organisms with healthy immune systems, meaning that it is quickly supressed by the host’s T cells. In order to survive, T. gondii leaves the gut and hides itself from the immune system by slowing its growth and forming cysts throughout the body, including in the brain. These cysts are between 5 and 50 micrometers in diameter (for comparison, most human hair is between 50 and 100 micrometers thick). This stage of infection is called latent toxoplasmosis, and is permanent. If another animal eats meat containing T. gondii cysts, it can become infected. This is where ‘mind control’ comes into play.
Studies have found that rodents with latent toxoplasmosis exhibit behavioural changes that make them more likely to be preyed upon by felines – for example, they have a decreased aversion and even an attraction to cat urine. The mechanism used by the pathogen to achieve this is impressive to say the least: research suggests that T.gondii can actually make epigenetic changes to key rodent genes, leading to the ‘rewiring’ of certain circuits in the brain. This could be an evolutionary adaptation by the parasite to increase its chances of finding its way back to its preferred host. But now for the question you’re probably most concerned about: can toxoplasmosis alter human behaviour?
We mentioned earlier that Toxoplasma gondii can infect humans, but do humans get infected often? The answer is yes – very often. In fact, studies estimate that between 30% and 50% of the global population has been exposed and may be chronically infected. The exact prevalence varies significantly from country to country – in France, for example, it was estimated that up to a staggering 84% of the population and 44% of pregnant women may have been infected as of the years 2000 and 2003 respectively.
The good news is that in most people, T. gondii is pretty harmless. It occasionally causes mild flu-like symptoms shortly following initial exposure, but then progresses to latent toxoplasmosis as mentioned above, which though permanent, is asymptomatic and prevents a person from being infected a second time. Serious and potentially deadly infection can occur in children and the immunocompromised – T. gondii causes approximately 750 deaths annually in the United States. Toxoplasma gondii also has a small risk of causing miscarriages if contracted during pregnancy, but latent toxoplasmosis does not appear to be a risk to the foetus unless the mother already has a weakened immune system.
Ever since T. gondii’s ability to alter rodent behaviour has been known, scientists have been interested in whether the parasite might be able to do something similar in humans. The effect, if it exists, must surely be mild in comparison to rodents (as evidenced by the rarity of cat pee lovers), which is why scientists have turned to statistics to tell us whether humans exposed to T. gondii behave differently to the uninfected. And the answer? Yes, they do. Studies have found associations between T. gondii infection and slower reaction times on average, more car accidents, a reduced sense of fear and self preservation, increased aggression, increased suicide rates, and increased probability of being schizophrenic.
Alarming stuff, right? Well, not exactly. We have to remember that correlation doesn’t mean causation – in other words, we still don’t know whether toxoplasma causes behavioural changes, or the reverse, or whether they are both being influenced by a third factor. It’s not hard to imagine that people with a reduced sense of fear and self preservation might be more likely to expose themselves to infection, for example. Even if a causal relationship does exist, it’s pretty mild compared with many other factors, and while there does seem to be a trend, there’s also some conflicting research. Human behaviour is influenced by vast number of different factors interacting with each other in complex ways, so no single factor is going to determine behaviour, especially not an effect as minor as toxoplasma infection. In other words, exposure to T. gondii isn’t going to turn you into an aggressive, suicidal bad driver.
Mr Floof has made a grave mistake: he ate Raticus, a rat infected with T. gondii, and now he’s infected too! Should Mr Floof’s owners roll him up in a carpet and put him on the next boat to Antarctica? Absolutely not! While cats play an important role in the transmission of T. gondii, catching the disease from a cat directly is quite unlikely if proper precautions are taken. This is because a cat only passes oocysts in their faeces for two weeks after being infected, after which they cannot usually be infected again. That’s a very small window relative to the lifetime of the cat. What’s more, it takes 1-5 days for the oocysts in faeces to actually become infective. If the cat’s litter is emptied frequently, and you wash your hands after handling the cat or its poop, you are rather unlikely to catch T. gondii. Infection is more likely to come from consuming cyst-containing meat or otherwise contaminated food/water. The risk of this can be reduced by avoiding undercooked meat (especially shellfish, pork, lamb, and venison).
Toxoplamsa, despite being relativey obscure to many people, is one of the most prevelant infectious diseases on the planet. Yet while it can certainly be dangerous in the wrong circumstances, the jury is still out on whether it has any effect on human behaviour.
Centers for Disease Control and Prevention: https://www.cdc.gov/parasites/toxoplasmosis/epi.html
Toxoplasmosis – A Global Threat. Correlation of Latent Toxoplasmosis with Specific Disease Burden in a Set of 88 Countries: https://dx.doi.org/10.1371%2Fjournal.pone.0090203
Toxoplasma gondii Infection in the United States: Seroprevalence and Risk Factors: https://dx.doi.org/10.1093%2Fschbul%2Fsbl074
Congenital toxoplasmosis in France in 2007: first results from a national surveillance system: https://doi.org/10.2807/ese.15.25.19600-en
Fatal attraction in rats infected with Toxoplasma gondii.: https://dx.doi.org/10.1098%2Frspb.2000.1182
Masterpiece of epigenetic engineering – how Toxoplasma gondii reprogrammes host brains to change fear to sexual attraction: https://doi.org/10.1111/mec.13006
Correlation of Suicidal Thoughts and Toxoplasmosis in Patients With Depression: https://dx.doi.org/10.7759%2Fcureus.13369
Toxoplasma gondii infection: relationship with aggression in psychiatric subjects: https://doi.org/10.4088/jcp.14m09621
Effects of Toxoplasma on Human Behavior: https://dx.doi.org/10.1093%2Fschbul%2Fsbl074
Risky business: linking Toxoplasma gondii infection and entrepreneurship behaviours across individuals and countries: https://doi.org/10.1098/rspb.2018.0822