Doesn’t it seem like when it comes to nutrition there are more opinions than facts to go around? Every day we hear new theories about diets, and supplements, and the best foods to eat. My role is to take the mystery out of good nutrition, and look at the science. Welcome to the Nutrition Facts Podcast. I’m your host Dr. Michael Greger. And I’m here to bring you an evidence-based approach to the best way to live a healthier longer life.
Today we go all Toxoplasmosis. And yes, your correct reaction should be –“Yuck! That sounds horrible!” And truth be told, it is. Nearly one quarter of Americans have already been infected with the brain parasite toxoplasma. Let’s break it down.
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 resident” 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 symptoms 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, “for 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 the 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.
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.
In our next story we learn how toxoplasma brain parasites can cause personality alterations.
“Studies have now revealed associations between toxoplasma infection and the presence of various psychiatric disorders in humans” schizophrenia, bipolar, suicide, self-harm, and memory impairment when we get older. How can a tiny parasite alter our very behavior?
I talked about how the rabies virus hangs out in the saliva, while specifically targeting the emotion center of the brain to drive animals into a fury, so they’ll effectively do the virus’s bidding to bite others to transmit the virus; or the famous zombie-ant brain fungus that takes over the animal completely. These are examples of so-called parasitic manipulation, where the parasite manipulates the host to “enhance its own transmission by altering host behavior.” And toxoplasma is “perhaps one of the most convincing examples of a manipulative parasite of higher animals, like us.”
Since the parasite thrives in cats, “chronically infected rodents no longer respond to cat odour with fear and indeed the physical response is reversed to attraction.” Mice become attracted to the smell of cats, serving the parasite up on a silver platter. The parasite manipulates the rodent’s brain to turn “their innate aversion to cats into a ‘suicidal’ ‘fatal feline attraction.’” Mice become attracted to cat pee, and such fatal feline attraction appears specific towards cats. They don’t become attracted to pee in general. They remain indifferent to rabbit pee, and continue to be turned off by other predator pee. So, on one hand, the parasitic manipulation appears incredibly specific, but the parasite doesn’t just want the mouse to seek out the cat, but get eaten as well. And so, there are these general effects too: impaired motor function, slower response times, “memory, and coordination.” And so, when the cat pounces, the parasite tries to make sure the mouse doesn’t get away. It’s like when California sea otters get toxoplasma, they’re more likely to get eaten by a shark. It’s not that the parasite wants to get into the shark, it may just be a by-product of the kind of general cognitive deficits that is so helpful for the parasite in other contexts.
It’s like when humans get toxoplasma, we start liking the smell of cat pee more, too. Isn’t that wacky? The parasite knows just what strings to pull. But it’s the more general effects we’re concerned about. We don’t need to worry our newfound appreciation for saber-tooth tiger urine is going to get us eaten, but mucking with our reaction times, that could be a problem. That could be why multiple studies have shown more traffic and worksite accidents among those that are chronically infected. But it may not just be our slowed reaction time. The parasite appears to also affect “subtle behavioural alterations,” like personality alterations that make us more likely to take risks. Great for the parasite in the cat-and-mouse game, but not so much if we’re driving a car, or wondering whether or not to take that next drink. Maybe one reason people with this brain parasite get into so many car accidents is that it may make people engage in riskier behaviors, like excessive alcohol consumption.
We typically think of malaria as being humanity’s greatest killer parasite. “However, when we take into account the hundreds of thousands of deaths that occur due to the increased probability of traffic accidents, working accidents, suicides, and possibly also other side effects of the infection,” maybe this supposed “’asymptomatic’ latent toxoplasma infection that has infested one-in-four Americans could easily take malaria down a notch.” Before I get into how to prevent and treat the darn thing, what might these other side effects be?
How exactly does toxoplasma manipulate behavior? Well, one clue we got decades ago is the rise in dopamine levels in the brain. You can show it right in a petri dish of infected brain tissue. Turns out that these parasites actually have an enzyme to make dopamine from scratch, which they then release into the surrounding brain tissue. Why do we care? Because elevated dopamine is a characteristic of schizophrenia. That’s how nearly all modern antipsychotic drugs work, by trying to bring dopamine levels back down, ”either inhibiting dopamine receptors or decreasing the level of dopamine in the brain.”
Is it “possible that the increased dopamine accumulation and release observed during toxoplasma infection” might increase the risk of schizophrenia? Well, that should be easy to figure out. I mean, do schizophrenics have an increased prevalence of infection? “The increased prevalence of toxoplasmosis in schizophrenics has been demonstrated by at least 50 studies.”
Finally today we discover the risk of contracting the brain parasite toxoplasma from kitty litter vs. meat.
How do you prevent it?
Well, the parasite can get into the muscles. So, from food-animals, people can get it “through meat consumption.” But in “a non-food animal like a cat,” you get infected through contact with feces. Thankfully, in cats, the “danger of infection exists only when the animal is actively shedding the parasite.” They get it from eating infected rodents. And so, cats that are kept indoors, that don’t hunt, and are not fed raw meat shouldn’t pose a threat. Though if feral cats are turning your local playground sandbox into a litter box, that could be a problem. As many as 6 percent of stray cats or those with outdoor access may be actively infected at any one time. They only shed the parasite for a few weeks, though. So, if you adopt a cat at a shelter, it should be safe as long as they didn’t just come in.
Many women have heard about the cat connection, but may be less aware about the risk of foodborne infection. “Only about one in three may be aware that toxoplasma may be found in raw or undercooked meat. Nevertheless, a high percentage of women indicated that they do try to practice good hygiene such as washing their hands after handling raw meat, gardening where cats may be pooping, or changing cat litter.”
What’s the riskiest type of meat? “Cattle are not considered important hosts for the parasite;” it’s more pigs and poultry, as well as sheep and goats. The prevalence of infection among factory-farmed pigs varies from 0 to over 90 percent, though, ironically, the likelihood of toxo infection in organic meat may be higher because the animals have outdoor access.
Who under cooks pork and poultry, though? Surprisingly, about one in three Americans may undercook meat across the board, in terms of reaching necessary pathogen-killing temperatures, and a single slice of ham can end up with more than a thousand parasites per slice.
Current meat inspection at the slaughterhouse can’t detect them. There are tests you can do, but there is no widespread testing. The risk from a single serving of meat, though, is really small. The average probability of infection per serving of lamb, for example, was estimated to be 1 in like 67,000. The reason there are 16 times more the number of cases attributed to pork is not because pigs are more affected; in the U.S., we just happen to eat a lot more pork chops than lamb chops.
Is there anything we can do if we’re one of the approximately one in four Americans already infected? Well, one of the problems with having these parasites in our brain is accelerated cognitive decline as we age. This study evaluated older adults every year for five years, and the executive function of those testing positive for toxoplasma seemed to drop quicker over time, as did a measure of their overall mental status.
Another thing that’s associated with cognitive decline is reduced folate availability, and the two may actually be related, as recent evidence suggests that toxoplasma may harvest folate directly from our nerve cells, sucking up folate from our brain. So, beyond dopamine production, which is why we think toxo increases the risk of schizophrenia, the parasite may be sucking folate out of our brain. But enough to affect our cognitive functioning? Perhaps so. Here’s a measure of cognitive function across a range of folate concentrations. Among those uninfected, it doesn’t seem to matter whether they have lots of folate or little. They obviously have enough either way. But those who are infected have worse scores at lower levels (higher is worse on this test). The same thing with vitamin B12. So, it’s important to get enough B12 and folate. For B12, the official recommendation is that all people aged 50 or over start taking a vitamin B12 supplement, or eat vitamin B12-fortified foods every day. And, anyone on a plant-based diet should start taking that advice at any age. And folate is found concentrated in beans and greens. So, following my Daily Dozen recommendations will get you more than enough, as, for example, a half-cup of cooked lentils gets you half the way there, as does three-quarters of a cup of cooked spinach.
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