It's human nature to read about a disease and then, like a hypochondriac, be convinced (or at least wonder if) you've got it. So, apologies in advance: You're about to wonder whether you're infected with a cat parasite.
It's long been known that the microbe in question, Toxoplasma gondii (T. gondii or Toxo for short), can harm people with weakened immune systems, such as people with AIDS. It's also been known that pregnant women should avoid cat litter so they don't catch T. gondii, lest they pass it on to their babies, causing brain damage in the infants or even death.
However, new research from an unconventional scientist is showing that even if we catch it and our bodies overcome it, the parasite can, in its dormant phase, when it lodges in our brain cells, alter our basic personalities -- making us more or less outgoing, trusting and fearful, and even making us more prone to schizophrenia, car crashes and suicides.
As the researcher, the Czech evolutionary biologist Jaroslav Flegr, told The Atlantic, which wrote a much-talked-about feature on him and his theory, when you consider all its impacts, “Toxoplasma might even kill as many people as malaria, or at least a million people a year.”
The basic premise
Just why would a cat parasite in humans cause personality changes? Basically, because, as the Atlantic put it, "Once inside an animal or human host, the parasite then needs to get back into the cat, the only place where it can sexually reproduce."
Generally, he hypothesizes that the parasite, which commonly ends up in rodents that cats prey on, causes changes in the rodent that will put the rodent in a position to get the parasite back into a cat.
For instance, an Imperial College London parasitologist Joanne Webster and other researchers before her showed that rats infected with T. gondii were not only less cautious in areas where they were more likely to be preyed upon but that they were actually more active in those spots. She then showed that infected rats actually preferred the smell of cat urine over other smells such as their own smell, the smell of water or the smell of a non-predator's urine. Another study showed that the parasite actually makes the smell of cat urine sexually arousing to rats.
All of these effects would make the rat more likely to get caught by the cat, putting the parasite back in a place where it can reproduce.
How the parasite affects humans
Until Flegr's studies, it was thought that when healthy children and adults get the parasite, it only gives them brief flu-like symptoms and then lies dormant inside brain cells.
But when Flegr gave personality and reaction-time tests to Czech subjects who had the parasite and subjects who didn't, he found significant differences:
- Subjects with the parasite had significantly delayed reaction times.
- Compared to men without the parasite, infected men were more introverted, suspicious, oblivious to others' opinions of them and more dismissive of rules.
- Infected women were just the opposite: more extroverted, trusting, image-conscious and careful to follow rules than uninfected women.
Before we launch into why men and women showed such different characteristics, let's dive deeper into the slowed reaction times, which inspired Flegr to launch two epidemiological studies in the Czech Republic. These showed that people who tested positive for the parasite were about 2.5 times likelier to be in a traffic accident than the uninfected; subsequent studies in Turkey replicated his results. As the Atlantic concludes:
"With up to one-third of the world infected with the parasite, Flegr now calculates that T. gondii is a likely factor in several hundred thousand road deaths each year. In addition, reanalysis of his personality-questionnaire data revealed that ... many other people who have the latent infection feel intrepid in dangerous situations. 'Maybe,' he says, 'that’s another reason they get into traffic accidents. They don’t have a normal fear response.'"
As for why men and women with the parasite show such different personalities, Flegr hypothesizes that the parasite makes them more anxious, and men deal with increased anxiety by withdrawing, whereas women who are more anxious seek solace through social bonding.
Links to schizophrenia
More seriously, a small study that Flegr did of the brains of 44 schizophrenic patients showed, via MRI scans, that 12 of them had reduced gray matter in the brain, and this size decrease was almost exclusively found in the subjects who tesed positive for the parasite. Researchers who looked at a subset of the 70 epidemiology studies since the 1950s that have examined the link between schizophrenia andT. gondii found the these studies also showed the schizophrenic patients with Toxo have reduced gray matter in their brain. Other researchers also found that people with the parasite are two to three times more likely to have the disease. All of these findings might explain why schizophrenia runs in families.
Another English parasitologist (Glenn McConkey at the University of Leeds) found that the parasite boosts dopamine production in the host brain. Coincidentally, dopamine tends to be unusually high in schizophrenics. Furthermore, they found that a petri dish of T. gondii will actually stop growing if antipsychotic medicines are added to the petri dish.
Links to suicide
Toxo may also be associated with increased suicide. As the Atlantic put it, "A 2011 study of 20 European countries, the national suicide rate among women increased in direct proportion to the prevalence of the latent Toxo infection in each nation’s female population." Other studies of the general population as well as of groups suffering from bipolar disorder, severe depression and schizophrenia also support the link between T. gondii and higher incidences of suicidal behavior.
How to protect yourself
First, don't get rid of your cat if it's an indoor cat. They don't have the parasite. And if you have an outdoor cat, just be wary of it when it's young and has just begun hunting. During that period, for about three weeks of the cat's life, it may shed the parasite. So, during that short time, you'd just want to keep your kitchen counters and tables clean.
Even more importantly, scrub all vegetables and take care to drink purified water, especially when you're in developing countries. Humans can catch the parasite by drinking water contaminated with cat feces, eating unwashed vegetables or, the most common point of contamination for Europeans, eating raw or undercooked meat. Flegr says that the French, who love very rare steak, can have infection rates as high as 55%. (Americans, on the other hand, have infection rates in the range of 10%-20%.
Joanna Webster, the parasitologist at Imperial College london, says:
“I don’t want to cause any panic. In the vast majority of people, there will be no ill effects, and those who are affected will mostly demonstrate subtle shifts of behavior. But in a small number of cases, [Toxo infection] may be linked to schizophrenia and other disturbances associated with altered dopamine levels—for example, obsessive-compulsive disorder, attention-deficit hyperactivity disorder, and mood disorders. The rat may live two or three years, while humans can be infected for many decades, which is why we may be seeing these severe side effects in people. We should be cautious of dismissing such a prevalent parasite.”
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via: The Atlantic