Toxoplasmosis: How Parasites in Your Cat Can Infect Your Brain
Now this is a common stereotype: the singlelady who lives next door and you hear meowsechoing through the walls. Or maybe it’s your aunt, or grandma, witha whole clowder of cats. Whoever it is, she is known as the “crazycat lady. ” And stereotypes or not, there could be someactual psychological risks from hanging aroundso many kitties… if just one of them isharboring a parasite. See, there’s a parasite that only sexuallyreproduces only inside cats and it’s calledToxoplasma gondii. And if that parasite gets into a human, itcan result in a full-blown infection calledtoxoplasmosis, which could be bad news foryour body and your mind. T. gondii have really tough eggs. And when animals like rats or birds eat them,even just through contaminated water, theybecome intermediate hosts. Once that bird or rat makes it’s way insideof a cat — however that happens — in akitty’s gut, the parasite will mature, mate,and sexually reproduce. And, eventually, the cat will poop out moreeggs, 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 parasitescan enter your bloodstream and infect almostany kind of cell — including immune cells. An immune cell is like a Trojan horse, lettingT. gondii sneak around and invade more specializedtissues like muscle cells and brain cells. Once parasites get into your central nervoussystem, they can hide out in cysts, and infectyou 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 allhumans have toxoplasmosis without knowing it. As a human’s immune system gets weaker overtime, the parasitic cysts can cause some moresevere symptoms. An infected person can develop muscle weakness,poor coordination, seizures, or permanentdamage to the brain and eyes. Which is not great. And if a pregnant woman is infected, she couldpass the parasites onto her baby. On top of that, there’s evidence that linksT. gondii infection to psychological conditions, too. One study monitored the mental health of nearly46,000 women from Denmark who had their newbornbabies screened for T. gondii antibodies,which were passed on. The researchers then followed up on the mothers’medical and psychiatric information for upto 14 years, and found that infected womenseemed to have more risk of depression, anxiety,and self-harm later in life. Many studies suggest that T. gondii affectshow a couple different neurotransmitters work, too. These are the chemicals that travel betweenneurons 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 ofLeeds showed that T. gondii infection in braincells led to higher dopamine levels, becausethe parasite makes an enzyme that controlshow dopamine is made. Dopamine helps regulate parts of the braintied to pleasure, mostly the amygdala andnucleus accumbens. But too much dopamine flooding the brain hasbeen linked with illnesses that distort thoughtsand moods, like psychosis, schizophrenia,and bipolar disorder. A more recent study in 2016 showed that T. gondii infection can also mess up a proteinthat transports glutamate, which is usuallyan excitatory neurotransmitter that makesneurons more easily activated. Failed transporters lead to a glutamate build-upin the space between neurons all over thenervous system, like a chemical traffic jam. This is called excitotoxicity, and it basicallyoverstimulates the nerve cells, causing partsof them to go haywire and start breaking down. So dysregulation of glutamate is linked withneurodegenerative diseases like ALS, multiplesclerosis, Parkinson’s, and Alzheimer’s. And another study from 2012 found that parasite-hijackedimmune 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 makesomeone feel fearless — like how severelyinfected mice seem to be less afraid of cats,and get eaten more often. That way, the parasites can continue theirlife cycles. But even with all this research, other studieshave suggested there isn’t a significantassociation between toxoplasmosis and mentaldisorders. A study from Duke University took blood samplesfrom just over 800 people to look for T. gondiiantibodies, and did other surveys and teststo collect data about their behavior. About 28% of their participants tested positivefor parasite antibodies. But the researchers didn’t find a significantassociation with schizophrenia, depression,or other mental disorders, or any link toimpulsive activities like crime or car accidents. So research in this field is tricky, becauseanimal models like mice aren’t entirelycomparable to humans. But it’d also be super unethical to do controlledexperimental studies on humans, like givingpeople toxoplasmosis and monitoring theirbrains. But, there is no reason to ditch your cats. They bring happiness and cuddles!There is so many positive health outcomesassociated with having a fuzzy thing in your house. Just be careful around their poop, like, makesure 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 properlyso you don’t accidentally eat any uncookedparasite eggs. 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