Can brain-parasites influence human culture?

Or: mmm, this cat poo is delicious.

The claim that parasitic organisms are influencing our culture may initially seem quite far-fetched, and the idea stems from the somewhat worrying claim that brain-parasites are able to control our behaviour. Could any of your actions really be influenced by a microscopic organism living in your brain? Could there be more to this than science fiction? Believe it or not, science says (probably) yes.

Toxoplasma gondii is a protozoan species that has intrigued biologists and psychologists all over the world. Everything we know about this parasite is interesting, so deciding where to begin is a challenge. The best place to start is probably by explaining how wildly successful T.gondii really is. Specialised parasites are often fascinating as they have evolved incredible adaptations, but many parasites become so specialised that they’re only effective in a specific host species, or sometimes even a specific organ within a specific host. But T.gondii is able to use many animals as hosts. Perhaps the most amazing fact about T. gondii is that most people reading this article probably haven’t heard of it before, which is quite incredible considering the parasite is thought to have infected up to half of the entire human population. It varies from country to country: in the US, there are approximately 50 million carriers; in France, only 12% of the population are not carriers.  It is so benign and causes so little obvious damage to the host, that half of the humans on Earth can be infected and most people haven’t even heard of it.

Regarding the “little obvious damage”, Toxoplasmosis can kill in some circumstances. The infection can harm individuals with weakened immune systems, including AIDS sufferers or a fetus if the mother becomes infected. For most people, the parasite gets in, causes a benign infection, and the host is none the wiser. Becoming harmless is a generally good evolutionary strategy for parasites. If they aren’t trying to kill us, what the are they doing inside our bodies? Where do they go? Before we get to that, attention must be brought to the parasite’s life cycle. Unfortunately, this brings us to cat excrement.

T.gondii is capable of infecting a huge number of warm-blooded species including birds, rodents, us primates, and even marine mammals. But the parasite does have a primary host; the cat. In all the host species (including cats), T.gondii is able to reproduce asexually and relatively slowly. Even this reproduction is interesting as T.gondii undergo endodyogeny: two daughter cells are created inside the parent cell and then they eat the parent cell. The strategy changes in cats, however, as the parasite can also reproduce sexually and this is why it all comes back to our feline friends. Cats can become carriers in all the same ways we can such as eating an infected animal or by contracting the infection from the mother during pregnancy. The difference in cats is that T.gondii can survive the journey through the cat’s stomach and has the ability to infect cells in the small intestine. This is the environment where T.gondii can reproduce sexually and a huge number of oocysts are produced. From a single host cat, over 100 million oocysts are deposited in its droppings. From there, the parasite may enter new hosts as they contaminate drinking water. Human infections can occur if we eat unwashed vegetables or we eat uncooked meats from animals that have been infected. Needless to say, a cat is a great target for this parasite as it provides an environment that allows mass reproduction. This raises the question, what exactly is happening when humans become infected? What is T. gondii doing to our own bodies?

The life cycle differs depending on whether the parasite is in a cat (its primary host) or another species. Image © LadyofHats

Biologists have known about the parasite for a while. It was first discovered in rabbits and the gundi (small African rodents from the family Ctenodactylidae) back in 1908. As our tools have improved over the years, biologists have learned a lot more about T.gondii and many other microscopic organisms. Perhaps unsurprisingly, most of the major advancements in our understanding of this protozoan (and the infection it causes) have occurred within the last decade. When biologists first took a good look at the bodies of infected animals (including humans), there didn’t appear to much evidence of T.gondii at all. This was because the parasites were hidden within immune cells of all places. Our bodies contain dendritic cells, which play an important role for our immune system. There are dendritic cells in several areas of our bodies, but mostly in the gut since this is a common place to find potentially harmful foreign molecules and organisms. When a dendritic cell finds an antigen like T.gondii, it transports it to the lymphoid node where T and B cells begin the proper immune response by telling the dendritic cell what to do with the antigen. In this case, the dendritic cell is told to commit suicide (taking T. gondii with it), but something goes wrong. The dendritic cell ignores the message, then it leaves, moving out of the gut and around our body. Somehow, T.gondii has hot-wired the cell, is behind the wheel, and has turned the communications radio off.

The parasite has not only avoided being destroyed by our immune system, but has taken over a cell that is essentially an invisible getaway vehicle since the immune system is unable to detect the parasite when it’s inside the cell. The dendritic cell moves around our bodies, under control of the parasite. This is an exceptional method for spreading around the body. Geneticists have engineered the parasite with a gene from the firefly which allows us to visually track the spread of the parasite. If they simply inject T.gondii into an animal, the parasite spreads quite slowly; but if they first infect some dendritic cells with T.gondii and then inject those cells into the animal, the spread occurs at a significantly higher rate. The parasite is able to jump into nearly any type of cell in all the compatible hosts and hide. If they aren’t going to kill us. what has T.gondii evolved to be able to do in our bodies?

Next stop: the brain

T.gondii attempts to get into animal brains. Why? Because it helps them fulfil their goal of reproducing as much as possible. As we all know, the brain is a remarkable organ. Nearly everything we do is regulated by the brain, whether we consciously realize it or not. The brain is remembering to do all those auto-pilot tasks. The brain is making all those conscious decisions too. The brain helps make us who we are, and is responsible for maintaining our personality, our memories, and even our behaviour. Here is an example of what T.gondii can do in an animal brain: Say a rat drinks some contaminated water or eats some infected meat. Before long, the infected dendritic cells of the rat are moving around and heading for the brain. The brain has a highly effective barrier that should keep out unwanted potentially harmful substances and organisms, but T.gondii gets a free ride in the dendritic cells and then takes up residence in the brain.

In a cat, the strategy of T.gondii is to get into the small intestine and reproduce. In any other animal, the goal is to get into a cat, and that is where the brain comes in. This astonishing parasite is capable of surviving for a year in soil, capable of infecting an incredible number of species, and capable of easily entering the brain, but still hasn’t revealed its ultimate ability. Rats have a complex brain that executes many different behaviours, and they have many fears. Rats fear large open spaces, unfamiliar foods, and cats. Scientists have studied rats and mice that have been infected by T.gondii and discovered some very interesting behaviours. The infected rodents are still scared of large open spaces. They are also still wary of unfamiliar food scents. Most of their behaviours are normal, including their fears, but the infected rats and mice no longer exhibit a fear of cats. Surprisingly, many infected rodents are attracted to areas that smell of cat urine. When mice and rats are infected by T.gondii, they actually seek out cats, the very animals they are naturally fearful of. They follows the scents, often find cats, get eaten, and the parasite enters the small intestine of the cat and reproduces sexually. It all comes back to cats. Just to recap, we know these parasites have evolved the ability to alter the behaviour of some animals and they have infected more than half of our global human population. Are they capable of doing something similar to us? Can they affect who we are and what we do? Can a parasite influence our behaviour? The answer appears to be yes.

How T.gondii actually modifies behaviour isn’t fully understood. We know cysts are created in the brain and it is thought that these may produce neurotransmitters. Targeted increases of dopamine are likely to be involved. Hopefully we will have a more complete understanding over the next few years. While we don’t fully understand all the details of how this behaviour modification actually occurs, we can study what behavioural changes those might be. Psychologists have studied infected humans and identified some behavioural traits that are typical. These are correlations, which obviously may or may not be causations. Studies seem to suggest that infected humans of either gender are more insecure and less novelty-seeking. There is also a very strong correlation between infected individuals and neuroticism. There are many correlations between the infection and gender-specific traits as well. Infected men appear to be more jealous and infected women are seemingly more loving and open. Again, it is important to note that while some of these are very strong correlations, they are just that. More studies need to be conducted, ideally on the behaviour of individuals before and after being infected, but this has ethical implications. However, it should also be noted that many of these behaviours are to be expected if the cysts function as they are postulated to. An increase or decrease in dopamine levels at specific areas of the brain would cause changes like many of those observed.

There are several other interesting correlations that have less to do with behaviour. Studies have shown that infected mothers are more likely to have male offspring. Also, a person that both carries the infection and has Rh-negative blood is 2.5 times more likely to be involved in a car accident. Similarly, there have also been reports of reduced reaction time in infected individuals. Some psychologists have even suggested a link between the infection and schizophrenia. Some studies have suggested that infected males have shorter attention spans and a more risk-taking attitude, while infected females are more outgoing and open. These studies also claimed that males found infected females more attractive than uninfected females, while females are more attracted to uninfected males rather than infected males. A lot more research still needs to be done, but some studies on mice do seem to confirm these preferences. If T.gondii really can influence behaviour, we are faced with an even more intriguing question. By contributing to the behaviour of individuals, could T.gondii actually contribute to human culture on a global level?

T. gondii and global human culture

The culture of a region is partly an aggregation of the individual personalities in that region. Not only have psychologists been studying infected individuals to identify personality traits potentially caused by the infection, they have for many years studied the differences between personality traits in different countries or geographical locations. Scientists also have a good idea of the prevalence of the infection in many countries. The prevalence of the infection in populations is affected by many factors, including climate and regional diet, so it is no surprise that there are significant differences between countries. As briefly mentioned earlier, 88% of the population of France carries the infection. Many countries are around the 60-80% mark. But only about 4.3% of the population of South Korea are infected.

We must be careful not to jump to causation but it is possible to compare countries to see if there is a correlation between the prevalence of the infection and the prevalence of the personality traits thought to be caused by the infection. For some traits, psychologists have identified very strong correlations in recent studies. Not all countries have been included, but the correlations are interesting. For example, almost every country that has a high proportion of infected individuals also has a high proportion of neurotic individuals. These are interesting and bizarre questions that are being raised. Can T.gondii influence human culture? And here we are merely focussing on a single species of parasite. Other species are also known to alter human behaviour in varying degrees. How much of our global human culture is contributed to by little monsters in our brains? Could cultural differences between South Korea and France be partly explained by the life-cycle of a parasite? We won’t know for sure until more studies are completed and we have a stronger understanding of the exact mechanisms involved. Clearly it’s a small part of a larger picture but interesting nonetheless.

Upon learning that parasites can alter human behaviour, all sorts of questions may be raised, from global culture to behaviour at home. Take extreme cat-lovers (and hoarders) for example. Are they really choosing to be obsessed with cats? Some people keep scores of cats in their homes and many see them as children. We see countless avatars online dedicated to the owner’s favourite feline companion. The internet is cat-crazy. Sure, most species have fans (I love reptiles) and I’m not claiming that all (or any) cat-lovers are definitely being influenced by parasites when they love their pets. However, cat obsession is more prevalent in certain countries and cultures than others. Is it possible that many humans could be affected by T.gondii in such a way that causes them to be attracted to cats? As I mentioned earlier, mice and rats usually avoid cats and their scent, but infected rodents will seek them out. And we’re not talking about reducing the overall level of fear or making them more risk-taking, we’re talking about something very specific. They were still scared of open spaces, for example. Could T.gondii cause a similar change in human brains to make us more attracted to our feline friends? Could “crazy cat lady” be a symptom of an infection? It would be interesting to see if there is a strong correlation between cat-lovers and infected individuals but such a study would be difficult. If there was a strong correlation, it wouldn’t necessarily mean that infected individuals become more likely to love cats. It could be that people who already love cats are simply more likely to become infected since T.gondii is can be picked up from cat faeces. It all comes back to cats.

Anyway, what an incredible species T.gondii is. Absolutely marvelous. I just can’t get over how wonderful the species is. Seriously, this is one great organism. It’s truly excellent. If I had to pick my favourite parasite, this would be it. And cat excrement is delicious.

Wait, did I just say that?

Main image © Jitinder P. Dubey