[INTRO].

Now this is a common stereotype: the single lady who lives next door and you hear meows echoing through the walls. Or maybe it's your aunt, or grandma, with a whole clowder of cats.

Whoever it is, she is known as the "crazy cat lady" [tox-oh-plaz-mah gone-dee-eye] And stereotypes or not, there could be some actual psychological risks from hanging around so many kitties... if just one of them is harboring a parasite. See, there's a parasite that only sexually reproduces only inside cats and it's called. Toxoplasma gondii.

And if that parasite gets into a human, it can result in a full-blown infection called toxoplasmosis, which could be bad news for your body and your mind. T. gondii have really tough eggs. And when animals like rats or birds eat them, even just through contaminated water, they become intermediate hosts.

Once that bird or rat makes it's way inside of a cat -- however that happens -- in a kitty's gut, the parasite will mature, mate, and sexually reproduce. And, eventually, the cat will poop out more eggs, which continues the cycle. In a human, or other intermediate host, T. gondii can still hatch but only reproduce asexually.

They're dangerous though, because parasites can enter your bloodstream and infect almost any kind of cell -- including immune cells. An immune cell is like a Trojan horse, letting T. gondii sneak around and invade more specialized tissues like muscle cells and brain cells. Once parasites get into your central nervous system, they can hide out in cysts, and infect you for life.

But you might not show any symptoms. Or if you do, it's just like a mild flu. Some estimates even say over a third of all humans have toxoplasmosis without knowing it.

As a human's immune system gets weaker over time, the parasitic cysts can cause some more severe symptoms. An infected person can develop muscle weakness, poor coordination, seizures, or permanent damage to the brain and eyes. Which is not great.

And if a pregnant woman is infected, she could pass the parasites onto her baby. On top of that, there's evidence that links T. gondii infection to psychological conditions, too. One study monitored the mental health of nearly 46,000 women from Denmark who had their newborn babies screened for T. gondii antibodies, which were passed on.

The researchers then followed up on the mothers' medical and psychiatric information for up to 14 years, and found that infected women seemed to have more risk of depression, anxiety, and self-harm later in life. Many studies suggest that T. gondii affects how a couple different neurotransmitters work, too. These are the chemicals that travel between neurons all over the body, not just in the brain.

And when neurotransmitters get imbalanced, that can lead to physical and mental illnesses. One 2011 mouse study from the University of Leeds showed that T. gondii infection in brain cells led to higher dopamine levels, because the parasite makes an enzyme that controls how dopamine is made. Dopamine helps regulate parts of the brain tied to pleasure, mostly the amygdala and nucleus accumbens.

But too much dopamine flooding the brain has been linked with illnesses that distort thoughts and moods, like psychosis, schizophrenia, and bipolar disorder. A more recent study in 2016 showed that T. gondii infection can also mess up a protein that transports glutamate, which is usually an excitatory neurotransmitter that makes neurons more easily activated. Failed transporters lead to a glutamate build-up in the space between neurons all over the nervous system, like a chemical traffic jam.

This is called excitotoxicity, and it basically overstimulates the nerve cells, causing parts of them to go haywire and start breaking down. So dysregulation of glutamate is linked with neurodegenerative diseases like ALS, multiple sclerosis, Parkinson's, and Alzheimer's. And another study from 2012 found that parasite-hijacked immune cells start releasing a bunch of GABA.

GABA is usually an inhibitory neurotransmitter, which means it can keep neurons from firing as much. When it interacts with the amygdala, for instance, GABA can help control feelings of fear and anxiety. So way too much GABA could presumably make someone feel fearless -- like how severely infected mice seem to be less afraid of cats, and get eaten more often.

That way, the parasites can continue their life cycles. But even with all this research, other studies have suggested there isn't a significant association between toxoplasmosis and mental disorders. A study from Duke University took blood samples from just over 800 people to look for T. gondii antibodies, and did other surveys and tests to collect data about their behavior.

About 28% of their participants tested positive for parasite antibodies. But the researchers didn't find a significant association with schizophrenia, depression, or other mental disorders, or any link to impulsive activities like crime or car accidents. So research in this field is tricky, because animal models like mice aren't entirely comparable to humans.

But it'd also be super unethical to do controlled experimental studies on humans, like giving people toxoplasmosis and monitoring their brains. But, there is no reason to ditch your cats. They bring happiness and cuddles!

There is so many positive health outcomes associated with having a fuzzy thing in your house. Just be careful around their poop, like, make sure it doesn't go in your mouth. Try to keep them from eating mice and birds, if that's possible.

And make sure your food is cooked properly so you don't accidentally eat any uncooked parasite eggs. Thanks for watching this episode of SciShow Psych, which was brought to you by our patrons on Patreon. If you'd like to help us make more episodes like this, which we love to do and we love that you love for us to do it, you can go to patreon.com/scishow. And also don't forget to go to youtube.com/scishowpsych and subscribe! [OUTRO].