Have you ever felt guilty and not understood why? Or felt that you're easily distracted? Most have. A study by the U. S. Geological Survey shows that a parasite that invades human brains and cat intestines could be to blame.

Here's how the bug works: Humans ingest Toxoplasma gondii, a common relative of malaria, which, at first, only makes itself known through mild flu symptoms. These symptoms have been known to linger in individuals with compromised immune systems, such as those affected by AIDS, but, in the majority of cases, the symptoms pass, the host feels better, and the bug is forgotten.

But Toxoplasma gondii stays on...

Dormant in the host's brain, it forms cysts. Research suggests that the parasite begins to affect how the brain responds to dopamine, a hormone that works as a neurotransmitter. And, gradually, it changes the host's personality.

Scientists have recorded four ways in which T. gondii affects the host. First, individuals with the parasite are more likely to experience guilty feelings, classified in studies as a type of neuroticism, than those who are not infected.

Second, researchers have found that the parasite's human hosts are more likely to be uncoordinated. One study demonstrated that both drivers and pedestrians who have been in traffic accidents are three times more likely to have been exposed to T. gondii.

Third, those who are infected with T. gondii are less likely to seek out new experiences. In researchers' words, they are less prone to "novelty seeking" and are more likely to demonstrate characteristics of "uncertainty avoidance."

And fourth, and perhaps most frighteningly, the presence of Toxoplasma gondii in human hosts shows a correlation with schizophrenia. People who suffer from schizophrenia are, in fact, three times more likely to carry T. gondii than those who do not. While unsettling, the correlation is not altogether surprising; schizophrenia is related to abnormalities in the way the brain receives dopamine, the hormone that Toxoplasma gondii also affects.

Scientists have observed other trends as well, including skewing sex ratios so that in heavily-affected populations, more males are born.

So what is Toxoplasma gondii, and what does it want with us? Most often described as a "cat parasite," the bug has three hosts: humans, cats, and rodents.

In cats, the parasite lives in the wall of the small intestine and is spread through the cats' feces. Then other animals ingest it, including rodents and humans; in both, it forms cysts in the brain, liver and muscle tissues.

The brain cysts affect the rodents even more dramatically than they affect humans. Research conducted by Joanne Webster at Imperial College, London seems to indicate that T. gondii causes rodents to draw predators' attention by wandering in the open; in some cases, it seems that they may even seek out the smell of cats. Of course, if a cat eats the rodent, Toxoplasma gondii's lifecycle starts over within the cat's intestines, so it's in the bug's best interest to hijack these rodents.

Interestingly, the dopamine-blocking Haloperidol, the drug used to treat schizophrenia in humans, reverses the fatal effects of T. gondii in rodents.

Over the last five years or so, evidence has been building that some human cultural shifts might be influenced, or even caused, by the spread of Toxoplasma gondii. Kevin Lafferty, the author of the USGS study, said in the September 2006 issue of soundwaves, the USGS newsletter, "The geographic variation in the latent prevalence of Toxoplasma gondii may explain a substantial proportion of human population differences we see in cultural aspects that relate to ego, money, material possessions, work, and rules."

In other words, the values and practices of different societies around the world might be shaped by the prevalence of Toxoplasma gondii among the members of that society. So if a large percentage of a particular population has the parasite, then that population is more likely to exhibit certain behavioral characteristics ranging in seriousness from unfounded feelings of guilt to schizophrenia.

This is certainly a frightening observation, given that, in same geographic areas, nearly 60 percent of the population is infected!

In some countries, pregnant women are tested for the bug before giving birth. In the United States, 12.3 percent of women tested carried the parasite, and in the United Kingdom only 6.6 percent were infected. But in some countries, statistics were much higher. 45 percent of those tested in France were infected, and in Yugoslavia 66.8 percent were infected!

It's not yet known whether Haloperidol, the drug used to suppress schizophrenia and block T. gondii's effects on rodents, also works to reverse the parasite's less-serious effects in humans. In fact, not a lot is clear at this point. As most of the scientists looking into T. gondii have pointed out, a lot more research will have to be conducted before we fully understand how much the bug has influenced human culture.

So for now, we'll just have to wonder: how much has the bug affected our lives today?

John Light is a contributor to several Ohio news and arts publications. He is currently enrolled as a senior at Oberlin College, where he works as Editor-in-Chief of the Oberlin Review, an independent, student-run web and print news source.

User Comments:

Yet another stab in the dark on the etiology of psychiatry's flagship disorder, schizophrenia. There has been more research to bear on the study of schizophrenia in the last century, when it was first "discovered", than any other putative mental illness. And to this day, no one has come any closer to any disease marker. The speculative science presently circulating, that mental illness is a neurotrasmitter or receptor issue, is based on the equally speculative science as proffered by the big pharma. The fact is, people fair much worse on Haloperidol, than if left unmedicated (and this goes for most every other so-called mental illness and disorder.) So much for the pharmaceuticals' self-serving studies!

Wow. Interesting comment.

The history of entire continent development such as political movements can be traced to parastites, bacteria and viruses that infect vast populations and that have infected them for thousands of years. As an example in Africa, Malaria, (five forms) still kill millions per year. Tuberculosis is another. Then there is the biblical reference to Leprosy, a bacterial infection, as well as the ever present flat worms, round worms, and ameoba. Many being deadly and most promoting permanent lethergy and arrested mental development in infected populations such as in certain native black African tribes. If that were not enough, there is AIDS, Ebola and Montazuma's revenge... again, an ameoba. Typhoid, Small Pox and influenza are others that can and have caused death and destruction of whole native populations.
During my biology studies, years ago, at the University of North Texas, my professor of Biology at the time, Dr. JKG Silvey stated there was only one way to safely see Africa and that was to fly over it at ten thousand feet in an aircraft. He said there was no safe way to visit vast areas of the continenet on the ground. I think his advice today still holds true for areas of Africa and for many tropical countries or regions such as the Amazon.
Domestic dogs, cats, certain domestic and wild animals and rodents are primary or secondary carriers of many vectors of disease and much more attention by medical doctors, clinicians and researchers should be given to that fact.
One should be aware the animal one casually may pet or that may be sitting in one's lap at home could be signing your death warrant if it is carrying an infectious agent of disease.

Thanks you for bringing to light this fascinating topic. However, you do your readers a disservice by failing to mention that the most common way that humans contract T. Gondii is through the consumption of undercooked meat (especially port and lamb) and raw shellfish. Everyone blames the cat, but it is (possible but) less common that people get it from their cats as the cats, like us, only show symptoms (and shed the infectious eggs) once during their lives, and only for a couple weeks, often when young.


A most interesting article.

What is the procedure to detect this most insidiuos parasite, in cats and humans?

Is there a treatment to get rid of T. Gondi?

Sincerely, Nick Alaniz

It has long been known that toxoplasmosis infection can cause the fetus in a pregnant woman to be born blind or with serious vision impairment. Women who are carrying a child are cautioned not to have a cat, or at least to have someone else change the cat litterbox.

In reading the article, I was hoping and now feel that the information given is incomplete without explaining how to rid our bodies of the parasite. What tests are used to detect the parasite in humans?

I've read that people who have been raised with at least one cat and one dog in the home are less likely to have asthma than people who grew up without. Cats also help keep the mouse population down in our homes. Dogs help keep the bad guys away, and both are excellent for our mental health and happiness. I've had cats and dogs in my home since I was born, and in my elder years, I'm in robust health and have none of the symptoms described in this article. (I wonder about some of my relatives though). Ha!

I wonder whether some of your relatives have been infected while perhaps you have been spared?
It is possible I suppose that some areas are less affected as well. Maybe the rodents your cat enjoys are healthy meals!
Thanks for sharing.

Thank you for sharing your thoughts and knowledge.
My dream vacation of an African Safari may be on hold... regardless of the various vaccinations, etc recommended for travel.

I have often wondered about people who say things like "my dog is my best friend", and "my pet loves me unconditionally". They even put their pets ahead of human relations, such as when their lover is allergic to their pet but they will not even consider getting rid of the pet.

"Unconditional love" means "they don't care what I am like, or what I do, they still love me". Why is that a good thing?? Isn't it better to love someone for who they are?

So I conclude that it is neurotic behavior brought on by parasites that affect pet owner's minds.

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