In August 2008, Rosemary Alvarez from Arizona thought she had the flu. She felt chronically tired and irritable. But when she started feeling numbness in her left arm and her vision became blurred, she knew it was more than flu. She was taken to hospital where an MRI scan revealed that she had a brain tumor around her brain stem. Her neurosurgeon Peter Nakaji, went into this most delicate area in the middle of the brain expecting the worst. Tumors in this region are notoriously difficult to excise because this brain area controls many the major bodily functions that keep us alive. A slip of his scalpel could leave this woman on a ventilator for life. However, as he sliced the outer casing of the tissue around the tumor, he got a pleasant surprise that would make the rest of us wretch – it wasn’t a tumor, but a live tapeworm, taenia solium, that wriggled out. This little fella was far easier to remove than a tumor chuckled Dr Nakaji, and it was the fifth one he had removed that year.
Our bodies are full of parasites that most of us are blissfully unaware of. For example, our guts contain a multitude of bacteria that are essential for digestion and without them we would die. However, Rosemary’s tapeworm was an unwelcome guest that came from eating undercooked pork and poor personal hygiene. What is interesting in her case is the idea that another living organism invading her body had changed Rosemary. The tapeworm wasn’t literally eating her brain like a worm in an apple but it was causing an infection – the natural bodily response to cope with external agents. All manner of things can cause brain infections that lead to changes in personality. We are our brains and if these are damaged, then we are changed, but sometimes the effects can be very subtle.
In what must be one of the most outlandish propositions to appear in recent years, Kevin Lafferty is a scientist at the University of California, Santa Barbara who thinks that a common parasitic infestation may have influenced the whole of society. Toxoplasma gondii (T. gondii) is a parasite, typically hosted in cats, that has an interesting life cycle. When the cat defecates, the T. gondii is excreted in the cat feces. Along comes a hungry rat that is less than discerning and nibbles the cat litter containing T. gondii. Inside its new host, the parasite works its way into the brain of the rat where it creates an infection in the region of the amgydala disrupting the normal rat behavior by altering the levels of dopamine. The infected rat now becomes less fearful and adverse to the smell of cats – dangerous changes in behavior that increases the likelihood that the rat in turn, will be eaten by another cat thereby completing the T. gondii cycle.
But T. gondii is not restricted to cats and rats. Approximately one third of the humans on the planet have been exposed and Lafferty believes that this epidemic may have shaped human culture. He explains, “”We have a parasite in our brain that is trying to get transmitted to a cat. This changes an individual’s personality.” In fact, research on humans indicates that chronic T. gondii infection causes significant behavioral changes. It has been reported that in comparison to those who are not infected, women become more intelligent, outgoing, conscientious, sexually promiscuous, and kind whereas men show the opposite profile. It is not clear why the infection would affect women differently to men either. With the gradual domestication of the cat over the course of modern civilization, Lafferty argues that the associated chronic T. gondii infection in the human population is one factor that could have shaped cultural evolution.