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Toxoplasma is a brain parasite that may infect a million Americans every year, making it “a leading cause of severe foodborne illness in the United States.” “Nearly one quarter of adults and adolescents in the United States have [already] been infected.” Newly acquired infections in a pregnant woman can be devastating. But in most people with intact immune systems, these parasites just sit there in your brain in “a fine-tuned balance between the parasite…and your immune system,” just laying in wait, hoping you’ll get AIDS or something, and your immune system will slip, and it can come raging back and spread throughout the brain.

But in healthy non-pregnant individuals, the parasite just sits there long-term, as a “potentially lifelong residen[t]” of your brain, kept at bay by your immune system “without any significant clinical consequence.” And so, “chronic toxoplasma infection has been viewed as a benign condition”….until now. “This absence of overt symptom[s] has contributed to the view that [the] cysts [in your brain formed by the parasites] are dormant entities,” but “recent developments… directly challenge the notion that chronic toxoplasma infections are without consequence.” “An emerging body of evidence suggests that the presence of an established chronic infection may contribute to the [development of a variety of] neurological conditions including schizophrenia, epilepsy, and neurodegenerative conditions.” Hmm; that’s not good.

Now these effects may not be “a direct consequence of the parasite,” but rather driven by a persistent “low level [of] inflammation in the infected brain.” But, either way…not good. So, let’s explore this new data that’s gotten neurologists so worried, and review strategies on how not to get infected in the first place, and what one might be able to do to mediate the effects if you are one of the one in four Americans that’s already infected.

Suspicions that infections might play a role in mental health date back more than a century—for example, this editorial from 1896 asking whether insanity might be due to a microbe. Well, “[f]or millions of years, parasites have altered the behavior of their hosts.”

Take, for example, the “diabolical effects of [the] rabies” virus, which is “usually transmitted [through] saliva.” So, it makes sense that the virus taps specifically into the limbic system of the brain, turning its victims from Fido into Cujo to facilitate “transmission of the virus.” But brain parasites can do more than just switch behaviors on and off. “Some parasites can adaptively take over and completely control the behavior of their hosts,” like the famous “zombie ants dying” from an infection with an “enslaver” fungus that steers “their insect hosts to die perched in [just the] position that favors the dispersal of [fungal] spores by the wind.” Here’s the head of a manipulated ant, colonized by tendrils of the fungus. “Ants infected by [this] fungus…die in a dramatic way.” Once the ant is positioned just right, “dying is preceded by biting behaviour where ants clamp [down] onto plant surfaces” to keep it stable while the fungus bursts out of the back of the ant’s head and grows this long stalk. Here’s a picture—how crazy is that?

There are also aquatic parasites that can cause their insect hosts “to drown themselves,” or others that can “cause bees to bury themselves alive,” or cause spiders to build special webs. That’s these parasitic wasps that lay their eggs into the spider’s abdomen, and then right on the night the larva eats its way out, it marionettes the spider to build it a little home. How creepy is that?

Okay, but come on, these are insects with simple brains. Surely, brain parasites couldn’t affect complex behaviors in higher animals…which brings us to toxoplasma.

“Toxoplasma is known to manipulate the behavior of their hosts to increase the probability that the host is captured by a predator.” For example, toxoplasma can reproduce in cats, but how is it going to get itself from the brain of an infected mouse into the cat? It can hijack the mouse’s brain and hijack their “native, inborn fear of the odor of cats into an attraction to that odor.” The parasite causes the mouse to develop a fatal attraction to cats, which is good for the parasite, not so good for the mouse. What does this all have to do with human mental illness? That’s what we’ll explore next.

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