2012 was the year of peak Zombie. The Walking Dead TV show first aired in late 2010, and zombies started to get hot in 2011. In May 2011, the normally tight-laced CDC even put out a press release highlighting the importance of disaster preparation using the Zombie Apocalypse as an example. In 2012, The Walking Dead season 3 set viewing records, and inspired zombie pub crawls and zombie fun runs. But how long can zombie hysteria last? There are more zombie movies set to be released In 2013, but as with vampires, eventually the public will tire of zombies and will move on to some other mythical creature. Perhaps minotaurs will be next? Or unicorns?
But while everyone’s attention is focused on zombies, I’d like to point out examples of so-called “zombies” in nature – no, not the undead, brain-eating kind that are transformed by a zombie virus, but people whose minds are being controlled in much more subtle and insidious ways.
First, there is the classic example, rabies. This virus is passed through the saliva, usually from a bite, and travels up the nerves to the brain. The exact mechanism that the virus uses to change an infected animal’s behavior are unknown, but the disease can severely alter an animal’s personality. During the later stage of the infection, the infected animal eperiences “hypersalivation” and the inability to swallow, as well as hallucinations and aggression. These tactics help the virus to encounter and be transferred to a new host when the infected animal bites.
A physical and psychological disease that was just recently identified, which has an inappropriately cutesy name is called PANDAS, which stands for Pediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcal Infections. When a child suddenly develops symptoms of obsessive-compulsive disorders – sometimes in the span of 48 hours – the cause may be a bacterial infection caused by certain streptococcus bacteria, or other viral or bacterial infections. Though the phenomenon is not currently well-understood, it is believed that the child’s immune response to the infection also attacks parts of the brain, which results in tics, humming, anxiety, bed-wetting, aggression and behavioral problems. Remarkably, treatment with antibiotics can stop or reduce the OCD behavior. Some children take preventative antibiotics during cold and flu season to keep the tics under control. It is unknown if the OCD behavior increases the transmission of the strep bacteria, or if it is just an overactive autoimmune response that happens in certain people who are genetically predisposed to the disease.
Speaking of the flu virus, have you had your shot this year? Did you go out to the club afterwards? A group of biomedical anthropologists at Binghamton University wanted to know if exposure to the flu makes you more extraverted. So they monitored the social lives of people, before and after they received the flu shot, which contains dead or weakened flu particles. They found that their test subjects interacted with more people and in bigger groups in the 48 hours after they received the shot, than the 48 hours before. Since the flu spreads through close-contact, if the virus can evolve a way to make its host interact with more people, then it will have a better chance of infecting new hosts and spreading more widely.
But perhaps the best-studied example of mind-controlling infectious agents is the single-celled parasite Toxoplasma gondii. The protozoan prefers to live in cats, but can also infect people, rats, cattle, pigs and other species. When it inhabits a mouse, the parasite infiltrates the brain and causes the animal to become sexually aroused when it smells cat urine in a phenomenon called “fatal feline attraction.” This causes the mouse to spend time in areas frequented by cats, which often gets them eaten, and helps the parasite to get back to its preferred host.
The poster child for human Toxoplasma infection is a Czech researcher named Jaroslav Flegr, who is himself positive for the infection. He realized that when other people were terrified – such as when he was hiding from gunfire while doing a postdoc in Turkey, or when walking out in front of moving traffic – he was only mildly concerned. He wondered what was wrong with him, and if the parasite living inside him could have something to do with his. He has spent the past 20 years performing personality and motor skills tests on students, people in the military. Since 30-40% of the Czech population is infected, he has plenty of subjects.
People with a healthy immune system can usually keep the parasite in check, but it can wreak havoc on individuals with a compromised immune system, and can be lethal to fetuses in utero. Toxo is the reason that pregnant woman are banned from cleaning the litter box. It might comfort you to know that the Toxoplasma infection rate in the US is only about 10%, but in some parts of the world, that rate climbs to 90%.
When a person is first infected, he will often experience flu-like symptoms, which soon subside as the infection moves to the chronic stage. The parasite forms clusters of cells, called cysts, inside of the brain and other tissues. For decades, the chronic phase was believed to be “asymptomatic,” but a handful of researchers are finding subtle, but troubling differences in the brains of people infected with Toxo.
Men who have the infection are usually antisocial, sloppy dressers who care little about what people think of them. Infected women, however, tend to be more social and to dress nicely, and to be less suspicious than uninfected women. Why the sex-related differe
nces? It’s likely that these are just gendered responses to changes in important brain chemicals. The parasite increases dopamine release in the brain, and even carries genes on its chromosome that are part of the dopamine-synthesis pathway.
A recent study also found that Toxo can hijack certain immune cells called dendritic cells, and forces them to secrete GABA, which functions in the central nervous system as an inhibitor. Excess GABA can have a relaxing, anti-anxiety effect. The Toxo also caused the dendritic cells to become hypermigratory so that they spread the parasite all over the body and deliver it to the brain. High GABA levels may be the reason why infected individuals show less fear and anxiety in dangerous situations.
Both men and women were found to have slower reaction times, and those times got slower with more advanced levels of infection. Several studies have also found that people infected with Toxo are about 2.5% more likely to be involved in a traffic accident. If you consider Toxo’s role in traffic deaths, the parasite may kill more people than malaria.
Perhaps even more concerning is Toxoplasma’s role in the onset of schizophrenia. Though not everyone with schizophrenia is infected with Toxo, it is still the highest risk factor, and the connection has been known since the 1960’s. Higher levels of dopamine are believed to play a role in the onset and progression of schizophrenia, and Toxo may just be nudging the disease along in genetically-susceptible people.
Currently, it is hard to separate out the behavioral changes that Toxo have created as adaptations that help it to find new hosts, and changes that are simply a side-effect of the infection. For example, infected men find the scent of cat urine to be more attractive than uninfected men, but the same effect is not seen in women. Also, infected men have higher testosterone levels and are rated as more “masculine and dominant” than uninfected men. Is this an evolved effect that makes women want to have sex with infected men, thus becoming infected themselves? Or does this mechanism, which functions in mice, affect humans by accident?
Whether a parasite intentionally causes behavior changes in its host, or if the psychological symptoms are just side-effects of the infection, doctors and scientists would be wise to look at disease symptoms in the mind AND the body. In Flegr’s recently published review of the mental effects of Toxoplasma infection, he writes “A large number of parasitic organisms probably exist in helminths, protozoa, fungi, bacteria, archea and viruses that may influence the [behavior] of their human host even more than the Toxoplasma. These organisms are, however, still waiting for research teams to engage in a systematic study of their influence on the human host.” I wouldn’t be surprised if once we start looking for them, we’ll find all kinds of organisms that are subtly manipulating our minds. And that phenomenon is scarier than imaginary flesh-eating zombies.