How a zombifying cat parasite called Toxoplasma gondii conquered the globe - Salon

Don't freak out, but the human body — and probably your body — is swarming with tiny parasites. If we were to count every cell in your body, only about 43 percent would be human. The rest are bacteria, viruses, parasites and other single-celled organisms. Not all of them are bad guys — some can even be beneficial for your health. But one hitchhiking microbe called Toxoplasma gondii really gets around, with around one-third of the planet encountering this tiny pathogen in their lifetimes.

T. gondii isn't a virus or a bacteria. It's a protozoan, similar to the malaria parasite. In most people, T. gondii doesn't cause any problems. However, in other mammals, especially rodents, it can change the behavior of its host, causing it to approach predators like cats. The felines appreciate an easy meal and the T. gondii appreciates being able to breed in the cats' guts, spreading through its feces and repeating its life cycle. It's a real world example of zombies, which is why this tiny bug has long fascinated people.

If toxoplasmosa gondii sounds familiar, that's probably because you've seen it mentioned in wild headlines about cat owners being infected by their cats, and having their brains (possibly) altered by the parasite. Hence, toxoplasmosa gondii has been linked to psychosis, schizophrenia, and bipolar disorder in humans. 

But since toxoplasmosa gondii evolved to move between rats and cats, its neurological alterations are most prominent in those creatures. 

"If you're a rat, and you get infected with it, it makes it so that cat urine doesn't smell bad. In fact, it smells good," Athena Aktipis, an associate professor at Arizona State University's department of psychology, told Salon in a recent interview. "It's sexually arousing to the rodents, and makes them approach the territory of cats, which makes it much more likely that they'll get consumed by the cat."

Humans, especially those with pet kitties, are also susceptible to these infections, but in healthy people it's usually not an issue. Those at greatest risk are pregnant people, unborn children and immunocompromised individuals, especially folks undergoing chemotherapy or with HIV.

Researchers at Stockholm University recently discovered how the parasite targets immune cells, hijacking their identity and using it to exist undercover. It's perhaps the smallest example of a Trojan horse, but it can have big effects on its host.

"There's been some work showing that there's greater susceptibility to some mental disorders, especially if your mother was infected with it while you were in the womb," Aktipis said. "There have also been a number of studies looking at changes in personality and behavior with Toxoplasma gondii infection. There's some controversy around those, about whether the methods were sufficient to rule out alternative hypotheses. … It's likely that it's having some effect on humans, but I think that the research is still kind of early as to what exactly the nature is of those effects. Like, there's no evidence that if you're infected by Toxoplasma gondii, that makes you more likely to be a person with too many cats in your house."

Now, scientists have uncovered the unique trick T. gondii uses that made it one of the most dominant lifeforms on earth. Researchers at Stockholm University recently discovered how the parasite targets immune cells, hijacking their identity and using it to exist undercover. It's perhaps the smallest example of a Trojan horse, but it can have big effects on its host.

By understanding how T. gondii became so stealthy, we can exploit this relationship to develop new therapies. That's potentially good news, because even though T. gondii infection — colloquially known as toxoplasmosis — is generally harmless, that isn't always the case.

T. gondii uses a protein called GRA28 as a sort of disguise to "reprogram" the immune system and do its bidding.

"More than 40 million men, women, and children in the U.S. carry the Toxoplasma parasite, but very few have symptoms because the immune system usually keeps the parasite from causing illness," according to the Centers for Disease Control and Prevention, which describes toxoplasmosis as a "neglected" parasitic infection because it has received little public health response. "However, women newly infected with Toxoplasma during or shortly before pregnancy and anyone with a compromised immune system should be aware that toxoplasmosis can have severe consequences."

Such consequences include miscarriage, stillbirth or a baby born with abnormally enlarged or small heads. In some cases, it can cause severe eye infections that can lead to blindness if untreated. While humans can't easily spread the parasite to each other, it can spread through cat feces, contaminated food and organ transplants.

So how does it get around our immune systems so well? New research in the journal Cell Host & Microbe elucidates this process, showing that T. gondii uses a protein called GRA28 as a sort of disguise to "reprogram" the immune system and do its bidding.

First, it encounters a phagocyte (a name which just means "cell eater"), a beneficial microbe like a white blood cell that protects the body from foreign invaders. The specific phagocyte T. gondii likes to attack is called a dendritic cell. T. gondii parasitizes the dendritic cell and tells it to migrate, hitching a ride on its back. In this way, T. gondii can spread throughout the body undetected, almost like getting its own police escort.

"It is astonishing that the parasite succeeds in hijacking the identity of the immune cells in such a clever way. We believe that the findings can explain why Toxoplasma spreads so efficiently in the body when it infects humans and animals," Prof. Antonio Barragan, who led the study, said in a statement.

Other recent research published in the journal Nature Communications sequenced the genome of T. gondii and found that the domestication of cats and the globalization of trade both played a significant role in spreading this parasite around the globe. But we're still only beginning to understand how T. gondii conquered the body. By taking a deeper look at how this stealthy protozoan travels through our anatomy, we can develop better tools at disrupting its devastating pathways.

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