Can a parasite control your mind?
A single-cell parasite, Toxoplasma gondii, is perhaps best-known for its connection to cats. The parasite can move from its feline host to humans, most commonly through contact with cat feces. And while the parasite typically only causes mild infection (people may have flulike symptoms), in people with weakened immune systems, infections can cause serious problems, from seizures to severe lung problems.
But a T. gondii infection can also have some downright bizarre effects. Over the years, the parasite has not only surprised and stumped researchers, but has also led to new insights into how human behaviors work.
Here are seven strange facts about T. gondii.
T. gondii has been shown to increase fearlessness in rats.
Rats infected with the parasite seem to lose their typical fear of cats, and more specifically, their fear of cat urine. A 2011 study in PLOS ONE suggested that infected rats start to feel a type of "sexual attraction" to the smell of cat urine, rather than their usual defensive response to the scent.
This might be because a T. gondii infection changes neural activity in certain areas of the brain, the researchers said. The parasite “overwhelms the innate fear response” so that rats no longer avoid the scent of cat urine, they wrote. Instead, the amorous rats are drawn to it — and to their deaths.
T. gondii can leap between almost all warm-blooded animals.
Cats and rats aren't the only animals that can host the parasite. T. gondii is an exceptional parasite in that it can leap from almost any warm-blooded creature to another. Although an estimated 6 in of 10 infectious human diseases are zoonotic (meaning they can hop from animals to humans), hardly any share T. gondii’s wide versatility of host organisms, according to the Centers for Disease Control and Prevention.
Scientists haven’t pinpointed exactly why this is, but some research, such as a 2011 study published in the Proceedings of the National Academy of Sciences, suggests that at least part of the answer may be found in the parasite’s proteins: When a certain type of protein is removed, the parasite is no longer virulent. Researchers speculate that this is because those proteins disrupt key proteins in the host’s cell that are key to the host’s immune response. [10 Deadly Diseases That Hopped Across Species]
Others suggest that T. gondii could contribute to other types of mental illness, even suicide.
Schizophrenia isn't the only psychological disorder that is possibly linked to T. gondii. A 2011 study that was done in mice and published in the journal PLOS ONE showed that the parasitemay cause infected brain cells to release high levels of the neurotransmitter dopamine.
Increases in dopamine could play a hand in certain mood disorders, such as bipolar disease, which has been linked to dopamine irregularities, according to the study. Other research done in humans suggests that Toxoplasma could be connected to impulsivity, and even suicide.
The parasite wouldn’t be the first pathogen to alter people's brain and behavior: The rabies virus, which is deadly in people, has long been shown to have devastating neurological effects.
Some studies suggest that T. gondii is linked to schizophrenia.
One of Toxoplasma’s most frightening — and most controversial— possible effects is its impact on the mind. If a woman is infected with the parasite while she is pregnant, her fetus may be at risk for developing schizophrenia and other mental disorders, according to a 2006 study of people living in Denmark, published in the journal Biological Psychiatry.
And that’s not all. A 2014 study that was published in The Journal of Nervous and Mental Disease found Toxoplasma antibodies in patients with schizophrenia, providing further evidence for a potential link between the parasite and the mental illness.
T. gondii has no cure.
There are no medications available to treat T. gondii. Other than remaining vigilant to try to prevent infection, there’s not too much else to do.
In a 2005 study in the American Journal of Obstetrics and Gynecology, health care providers advised pregnant women and women looking to become pregnant to get screened for T. gondii. The authors also suggested that women understand the risk factors for infection, such as exposure to cats (and particularly litter boxes and feces), consuming improperly cooked meat and drinking untreated water.
Up to half of the world may be infected.
Researchers can't seem to agree on just how widespread T. gondii is, in part because many cases may go unreported. A 2002 study in the journal Clinical Microbiology and Infection estimated that between 16 to 40 percent of people worldwide are infected, but in certain regions, such as parts of Central America, South America and continental Europe, the rates are between 50 to 80 percent.
A 2011 study in the Journal of Global Infectious Diseasesgave a much higher estimate of the numbers of people infected: about 6 billion people worldwide, or more than 80 percent of the world.
The Centers for Disease Control and Prevention estimates that about 60 million people in the United States — or nearly 20 percent — are infected. A simple blood test can determine whether you have T. gondii antibodies, which indicate infection with the parasite. [7 Absolutely Horrible Head Infections]
Most of the time, T. gondii has no symptoms.
Though T. gondii can cause flulike symptoms in mild cases, a 2014 study in the American Journal of Tropical Medicine and Hygienesuggested that in the vast majority of cases, people experience only mild symptoms, or no symptoms at all. Indeed, this may be why the parasite has been largely neglected by researchers, the authors wrote.
The parasite is most dangerous to those with preexisting conditions, such as diseases that contribute to a suppressed immune system. But most people infected with T. gondii will never even know they have it.
Originally published on Live Science.