The Myth of “Mind-Altering Parasite” Toxoplasma Gondii?

By Neuroskeptic | February 20, 2016 5:00 am

Toxoplasma gondii is a tiny organism that lives inside cells. It may well live inside your cells – the parasite up to 50% of the world’s population, along with cats and many other animal species.

toxoplasma

This is worrying, because many researchers believe that T. gondii infection, or toxoplasmosis, can alter human behavior. Among other organs, the parasite infects the brain, and it has been blamed for making people more impulsive, and more prone to mental illness, including schizophrenia. The idea of ‘behavioural’ toxoplasmosis has driven a huge amount of research and media interest. But in a new PLoS ONE paper, Duke University researchers Karen Sugden et al. suggest that there may be nothing to worry about after all. They report that toxoplasmosis is associated with essentially no behavioural abnormalities in humans.

Sugden et al. examined the Dunedin Longitudinal Study, a sample of over 1,000 people born in New Zealand in 1972-73 and followed up from birth. Sugden et al. tested the participants blood samples, taken at age of 38, for antibodies against T. gondii. Of 837 people who gave blood samples, 28% tested positive, indicating that they were infected with the parasite.

 

journal.pone.0148435.t002

Toxoplasmosis was not associated with any personality traits, nor with rates of schizophrenia or depression. There was also no evidence that was linked to ‘poor impulse control’, e.g. criminal convictions, driving offenses, and accident claims on insurance. The one possible exception was that suicide attempts were more common in T. gondii positive people, but this difference was only of trend significance (p=0.06). T. gondii also wasn’t correlated with IQ or other measures of cognitive performance, except on one memory test (the RAVLT, p=0.04).

Sugden et al. conclude that

Our results suggest that a positive test for T. gondii antibodies does not result in increased susceptibility to neuropsychiatric disorders, poor impulse control or impaired neurocognitive ability…  this is, to our knowledge, the most comprehensive assessment of the possible link between T. gondii infection and a variety of impairments in a single cohort.

Yet many previous studies have reported an association. How do Sugden et al. account for the inconsistency between their results and those others?

This is, to our knowledge, the most comprehensive assessment of the possible link between T. gondii infection and a variety of impairments in a single cohort. Previous positive associations have been reported across different studies, often in selected or clinical samples; for example, one study will examine the link to violence, another the link to schizophrenia, and yet another the link to self-injury, and so forth.

Sugden et al. say that their result is unlikely to be a false negative

Although our cohort is of only modest size, it is adequately powered to detect small effect sizes (r = 0.1).

n=837 people is fairly large, but other T. gondii studies have been even bigger. For example, this study had n=7440, although the results were mostly negative also.

The authors conclude that the hype train around this media-friendly microbe may have gone too far:

T. gondii is a microorganism whose source of transmission [cats] is common and relatable, as evidenced by numerous recent popular opinion pieces (e.g., “How Your Cat Is Making You Crazy” [48]). It has been observed that the ‘hotter’ the topic and as more studies are reported and accumulate, replication becomes more difficult [49]. If we accept that the findings reported in the present article represent scenario two, then views of the link between T. gondii and aberrant behavior may need to be tempered accordingly.

Cats are part of the T. gondii story because, in order to complete its life cycle, the parasite must be ingested by a feline (such as a cat). So the theory goes that T. gondii evolved to cause behavior changes in its hosts in order to make them get eaten. If T. gondii infects, say, a mouse, the parasite would only breed if the mouse got eaten by a cat. Reckless mice are more likely to get eaten than cautious ones, the theory goes.

Note that even if T. gondii doesn’t cause human behavioural changes, that doesn’t mean it’s harmless. Especially in people with impaired immune systems, and in pregnant women, it can cause serious illness.

Hmm, does all this mean that I should become a Toxoplamoskeptic?

toxoplasmaskeptic

ResearchBlogging.orgSugden K, Moffitt TE, Pinto L, Poulton R, Williams BS, & Caspi A (2016). Is Toxoplasma Gondii Infection Related to Brain and Behavior Impairments in Humans? Evidence from a Population-Representative Birth Cohort. PloS one, 11 (2) PMID: 26886853

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  • John Mclaren

    The harmless parasite chewing it’s way through your brain and eyeballs. What excellent news! Thank you for this analysis.

    • http://blogs.discovermagazine.com/neuroskeptic/ Neuroskeptic

      I’ve added a sentence to acknowledge that T. gondii can cause serious illness in some cases, which is a separate issue from the claimed behavioural effects of chronic infection.

      • Dangus McFinghin

        It is good for balance purposes that you updated it, but frankly the negatives about it are a lot better reported than anything else. So, while you may not have made a point of talking about them, plenty of others have.

      • MagpieSkogsra

        I’m wondering if it played a role in animal domestication, or even human civilization where cannibalism occurred like in New Guinea.

        • Buster

          I have a similar theory about mental illness. Most view it as a detriment. Until basically 60 years ago there was so much death and physical disability that human life was much more depressing and full of misery. Inherited mental illness would, in theory provide an inherited ability to disassociate themselves from the horror of everyday life. Historically it hasn’t been an illness but a defense mechanism to protect humanity from all the horror of war, disease and poverty.

    • s k

      why john, its only one of a vast empire.

  • http://blogs.discovermagazine.com/neuroskeptic/ Neuroskeptic

    Thanks for the comment, by the way I’m not sure why your comments always get caught in the spam filter. I think it may be because you have “smut” in your name and Disqus thinks you must be a smut spammer.

    • MagpieSkogsra

      Today it said I was blocked by you despite not having posted here ever before. so I used a proxy & now its fine.

      • http://blogs.discovermagazine.com/neuroskeptic/ Neuroskeptic

        How bizarre – sorry about that! I’ve no idea why this happens.

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  • Trader Joe

    Meow!

  • Rhonda Pieroni

    Has their been a study done of not just those that tested positive for the parasite but those that have had an active infection at one point in their life?

  • JJ McKibbin

    60 million Americans and half the world’s population overall have contracted this parasite. I hardly think anyone is exaggerating the ease with which one can become infected.

    More studies obviously need to be done. The 40 previous studies could all be wrong, but they may not be. And there is no question about the health risks to pregnant women and the immuno-compromised. And with the increasing senior population in the U.S. the number of people who are considered immuno-compromised will be rising rapidly.

    The best remedy? Keep pet cats indoors and do not feed them raw meat. Remove feral and stray cats from the environment.

    Another study just released finds a connection in our closest living primate, the chimpanzee. http://www.cell.com/current-biology/abstract/S0960-9822(15)01517-1?_returnURL=http%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS0960982215015171%3Fshowall%3Dtrue

    • Dangus McFinghin

      Perhaps it would be better to say that the ease of contracting it directly from a cat is exaggerated. There are so many other vectors. Its my understanding more people get it from gardening and improperly cooking their food than from any other sources.

      • JJ McKibbin

        No matter how you get it, the original source is cats. Cats are the ONLY source.

        Toxoplasma gondii can only complete its life cycle in the intestine of a member of the cat family. Only cats. The cat then sheds the oocysts (eggs) by the tens of millions in its feces. Again, only cats. No other animal sheds the oocysts. The oocysts are very durable and can remain viable in soil or water for 18 months or more. In dry conditions they can become aerosolized and inhaled. Oocysts can also be spread by insects or rodents or even the wind from feces to food sources. Water sources can become contaminated when rain washes the oocysts into them. The reason the soil, water, or plant material is contaminated with oocysts is because cats are shedding them. There would not be environmental contamination of oocysts if the cats weren’t there to shed them by the tens of millions. Recent research now indicates that cats can become reinfected a second time and go through a second oocyst shedding cycle. Yes, any warmblooded animal, including humans, can become infected by ingesting oocysts from contaminated food or water sources, or even by inhaling them in dry areas where they have become aerosolized. But none of these other warmblooded animals can shed more oocysts.

        Humans and animals can also become infected by eating the undercooked meat of infected animals. But those infected meat animals became infected by ingesting oocysts shed by cats. No matter how humans or animals become infected, the original source is cats. Now, it is true that any member of the cat family can shed oocysts, and this includes native cats such as bobcat, mountain lion, and lynx. But since the population of all of these native species combined comprises only 2% of the total number of cats nationwide, and since most human populated areas have almost zero native cats living among people, the source of oocyst contamination is almost exclusively domestic house-cats.

        Environmental contamination of oocysts from cats is so pervasive in areas with high concentrations of cats that scientists fear toxoplasmosis infection may become almost impossible to avoid. 60 million Americans are already infected. There is no vaccine and no cure for toxoplasmosis. Once you have it, you have it for life. We really don’t know at this point what all issues this parasite presents in humans. The potential problems from this parasite should not be minimized. To do so is irresponsible.

        For more information go to the “files” section of the toxoplasma gondii facebook page and read the 2013 Torrey and Yolken study entitled “Toxo Oocysts Public Health”. https://www.facebook.com/groups/toxoplasmagondii/files/

        • Randy Johnson

          But WHY only cats?

          • JJ McKibbin

            I do not know the answer to that question. Toxoplasma gondii and felids evolved together over millions of years to develop this relationship. Scientists have studied and tested many different species from many different animal families, and felids are the only animal that toxoplasma gondii is able to complete its life cycle in.

          • Randy Johnson

            Well reasoned JJ McKibbin.

    • Randy Johnson

      I would guess that the bonobo is closer, based on behavior.

  • Dangus McFinghin

    Thank you for putting up this article! The fear of cats when it comes to Toxo is so greatly exaggerated, and far too much blame is put on them for something that they are also victims of. Toxo won’t kill a healthy adult cat, but it can be outright deadly to kittens and elderly cats.

    All in all it was a pretty treatable disease until Martin Shkreli decided to raise the price of the drug from 13 dollars a pill to something like 750 a pill.

    • JJ McKibbin

      You do not seem to understand the life cycle of toxoplasma gondii or its potential consequences.

      Toxoplasma gondii can only complete its life cycle in the intestine of a member of the cat family. Only cats. The cat then sheds the oocysts (eggs) by the tens of millions in its feces. Again, only cats. No other animal sheds the oocysts. The oocysts are very durable and can remain viable in soil or water for 18 months or more. In dry conditions they can become aerosolized and inhaled. Oocysts can also be spread by insects or rodents or even the wind from feces to food sources. Water sources can become contaminated when rain washes the oocysts into them. The reason the soil, water, or plant material is contaminated with oocysts is because cats are shedding them. There would not be environmental contamination of oocysts if the cats weren’t there to shed them by the tens of millions. Recent research now indicates that cats can become reinfected a second time and go through a second oocyst shedding cycle. Yes, any warmblooded animal, including humans, can become infected by ingesting oocysts from contaminated food or water sources, or even by inhaling them in dry areas where they have become aerosolized. But none of these other warmblooded animals can shed more oocysts.

      Humans and animals can also become infected by eating the undercooked meat of infected animals. But those infected meat animals became infected by ingesting oocysts shed by cats. No matter how humans or animals become infected, the original source is cats. Now, it is true that any member of the cat family can shed oocysts, and this includes native cats such as bobcat, mountain lion, and lynx. But since the population of all of these native species combined comprises only 2% of the total number of cats nationwide, and since most human populated areas have almost zero native cats living among people, the source of oocyst contamination is almost exclusively domestic house-cats.

      Environmental contamination of oocysts from cats is so pervasive in areas with high concentrations of cats that scientists fear toxoplasmosis infection may become almost impossible to avoid. 60 million Americans are already infected. There is no vaccine and no cure for toxoplasmosis. Once you have it, you have it for life. We really don’t know at this point what all issues this parasite presents in humans. The potential problems from this parasite should not be minimized. To do so is irresponsible.

      For more information go to the “files” section of the toxoplasma gondii facebook page and read the 2013 Torrey and Yolken study entitled “Toxo Oocysts Public Health”. https://www.facebook.com/groups/toxoplasmagondii/files/

      • Jane Citizen

        Can you tell me if only cats shed the oocysts?

        • JJ McKibbin

          Toxoplasma gondii can only complete its life cycle in the intestine of a cat. Only members of the cat family (felids) can shed oocysts in their feces.

          Any member of the cat family can shed oocysts, and this includes native cats such as bobcat, mountain lion, and lynx. But since the population of all of these native species combined comprises only 2% of the total number of cats nationwide, and since most human populated areas have almost zero native cats living among people, the source of oocyst contamination is almost exclusively domestic house-cats.

          • R Sherman

            If this is true, why does the CDC website recommend avoiding the consumption of raw or under-cooked meat?

          • JJ McKibbin

            From my earlier comment:

            Humans and animals can also become infected by eating the undercooked meat of infected animals. But those infected meat animals became infected by ingesting oocysts shed by cats. No matter how humans or animals become infected, the original source is cats. Now, it is true that any member of the cat family can shed oocysts, and this includes native cats such as bobcat, mountain lion, and lynx. But since the population of all of these native species combined comprises only 2% of the total number of cats nationwide, and since most human populated areas have almost zero native cats living among people, the source of oocyst contamination is almost exclusively domestic house-cats.

      • True_Blue

        Another cat hating alarmist. My family and all of its members had adopted many stray cats all of our very long lives. We have also had blood work confirming that none of us has ever had Toxo, and none of us is a toxo-paranoid alarmist (must be some other critter causing the paranoia). Moreover, we have had our (ususally stray) cats tested for toxo upon every adoption and they have never tested positive.
        I love birds too, and think you must be one of the “cats kill all songbirds” fanatics to post such rubbish. We do not let our cats out to roam and they don’t kill songbirds or other stray animals (cats have always helped humans by killing rodents throughout history).
        PS, We are all scientists, so we are not uninformed about this subject.

        • JJ McKibbin

          Name-calling. Typical response from someone with no valid argument. Are you actually disputing any of the facts in my post?

          You call me an alarmist. 60 million Americans have toxoplasmosis. And you and all of your scientist family members have had yourselves tested for toxo (hardly typical). And all your cats too (also hardly typical). Sounds to me like you’re far more concerned about it than the average American.

          Thank you for keeping your cats contained. That is the ONLY way to keep them from killing native birds and other wildlife.

  • angelos angelos

    Branding all previous research as myth because 2 new studies say toxoplasma is not to be blamed doesn’t inspire much confidence for your article.
    Regarding the first study from Duke University people should know that the study took the sample from a long-running study of 1037 people born between 1972-73.(The Dunedin Longitudinal Study) So at the age of 38 they examined only the people who wanted and were still alive, 837 in number. So if the rest have committed suicide or had a car accident or didn’t want to participate because they were depressed to fly from around the world to that town, that we don’t know.
    The second study didn’t found a link for Major depression and dysthymia, however, there was a significant relationship between T. gondii and bipolar disorder type I for respondents in which both manic and major depression symptoms were reported.
    So the study didn’t examined for schizophrenia and also found a significant relationship for bipolar disorder and you still referencing it as pro argument evidence?
    Mythical article!

    • http://blogs.discovermagazine.com/neuroskeptic/ Neuroskeptic

      Regarding the fact that 1037 people were in the study but only 837 gave blood samples, the authors say

      “We compared people with missing (N = 200) versus non-missing (N = 837) T. gondii seroprevalence data at age 38. Seroprevalence data were missing because a) for cultural reasons, Maori-ancestry cohort members’ blood was not studied (7% of cohort), b) Study members either did not consent to phlebotomy (2%), did not take part at age-38 assessment (5%), or died before age 38 (3%), or c) immunoassay data did not pass quality control
      (2%).

      Across all 27 outcome variables tested in our analysis, we found only three differences: Study members with missing T. gondii data were more likely to meet a diagnosis of schizophrenia (N = 13, 10.7%; OR (95% CI) = 4.07 (2.01–8.24), p < .01), a diagnosis of major depression (N = 28, 24.1%; OR (95% CI) = 1.77 (1.11–2.83), p = .02), and were more likely to have an adult criminal conviction (N = 54, 39.1%; OR (95% CI) = 1.60 (1.04–2.46), p = .03)."

      • angelos angelos

        I don’t understand your argument. With your more detailed explanation i still only see that the people who didn’t participate would have changed the results a lot if they had.

        • Randy Johnson

          I applaud your use of your brain!

  • Bahar Gholipour

    Even in mice the behavioral effects of infection with the Toxoplasma parasite is unclear: https://www.braindecoder.com/maybe-cats-arent-making-us-crazy-1115138833.html (shamelessly plugging in my own story from last year 😉 )

    The story of toxo suffers from at least four classic red flags in science reporting: 1) Studies have failed to replicated behavioral effects in mice 2) There’s no powerful evidence pointing to a mechanism by which the parasite may be making rodents fearless 3) We can’t draw on findings in mice and extend to humans, especially in this case where their immune system responses appear to be different. 4) Correlational studies have been small and inconsistent in methodologies, and well, correlational.

    • Nemo_of_Erehwon

      But it’s CREEEEPY! That’s sufficient for much of what passes for modern science journalism.

    • herecomesjohnny321

      No powerful evidence? How about – T. gondii produces dopamine on its own, and does so directly inside the brain?

  • tomrees

    Hi Neuroskeptic, unrelated, but I thought you might like to know that your blog is name-checked in this research article https://scholar.google.com/scholar?q=Blog+Citations+as+Indicators+of+the+Societal+Impact+of+Research

    • http://blogs.discovermagazine.com/neuroskeptic/ Neuroskeptic

      thanks very much! I have seen that paper, but thanks for keeping an eye out!

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  • Arnold Linder

    So if we kill all cats we eliminate this parasite?

    • Randy Johnson

      Gotta watch out for the mutations, mutant.

  • Geoffrey Brent

    I work in statistics and I’m unfamiliar with the expression “trend significance” (which does not appear in the paper on which this is based). I’ve seen other work representing values slightly above the cutoff as “trending towards significance” but that is an abuse of language; “trend” implies a pattern of movement, which this isn’t.

    Without wanting to get into an argument about the merits or otherwise of p-value-based hypothesis testing: if one is going to use that method, then a value of 0.06 should be reported as “not significant at p=0.05”.

  • Zhang Tingyu

    Obviously the authors of the study are infected themselves, and this is their toxobrains making them deny its existence.

    • bwana

      Darn, sounds reasonable to me :)

  • reed1v

    Now about those tiny people that inhabit our brains….

    • eirikr1

      explains those voices in my head..

  • Mike Bozart

    me mind is gone with gondii.

  • Randy Johnson

    I’ll bet Sheldon told you that.

  • Odin Matanguihan

    damn, there goes my alibi.

  • Michael Duke

    What about the Chimps that lost their fear of leopards? What happened to that scientific study? Hey! What became of the zombie ants that had fungus explode out of their heads? Have we been scammed by another ambitious, sub-contracting copywriter temporarily employed by Discover?

  • http://www.detroitrugby.org DarkHorseSki

    While it may not be THIS one… we do know that some of these do have clear effects on critters, like that recent disclosure about the one that made crickets want to breed more.

  • SILENTHAMMER

    In Michigan several years ago, the sun was accused of grave and grotesque mutations in pond frogs. Later analysis discovered that it was T. Gondii that was causing the mutations, as a result the frogs had too many legs and couldn’t evade the herons. Guess what the secondary host is? Yup, Herons, which crapped out T. g. cysts, and the cycle perpetuated. I’ll bet there’s some “scientist” somewhere who will “find” that N. fowleri isn’t really a danger to humans. Won’t that be special.

    • JJ McKibbin

      There are several factual errors in your comment. First of all, only warm-blooded animals can become infected with toxoplasma gondii, so the frogs would not have had toxo.

      Second, while the herons could be infected with toxoplasmosis, they were not shedding oocysts in their feces. ONLY cats shed the oocysts in their feces. No other animal does this.

      Where did you hear this misinformation?

      • SILENTHAMMER

        You are absolutely correct, J.J. With age, my biology is fading. Thanks for correcting me.

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  • http://qpr.ca/blog alqpr

    I don’t see felophilia listed as one of the abnormalities that was tested for. But in any case, regardless of whether it is parasitically induced, I don’t expect my own mental ‘illness’ to be problematic unless I visit Africa – in which event I already know that I will have to hire a guide to restrain me from an uncontrollable urge to cuddle with all the lions and leopards.

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  • Daniel Mahler

    I do not see how they get to claim their tests are not underpowered.

    According to the inline table, for driving conviction they obtained the the rates 21.6% vs 14.9%. That is almost a 1.5x in the rate of occurrence, yet it only scores p=.22, very far from usual standards of statistical significance. For suicide attempts the rate increase is more that double yet it only scores p=0.06.

    Differences of this size would certainly be of importance if they were real, yet their sample is not large enough to discriminate this. That sounds like the very definition of underpowered.

  • Janet Graham

    I have 5 cats, so I enjoyed reading this article, but thanks to #Iasodetox tea, that rids my body from all heavy metals, parasites and more I am living a healhy life.
    You guys and ladies can do this as well, you can get you #Iasodetox tea at http://www.totallifechanges.com/5589731

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  • herecomesjohnny321

    So you took ONE paper that claims there is nothing to worry about, and disregarded tons of research that claims exact opposite. Way to go! I remember many other similar “scientific” studies that claimed there was nothing to worry about… Like “Don’t worry, once cured, ebola does not come back (it does, it hides in eyes and testicles). T. gondii does alter the mind, and we even know some of the ways it does so. More than one study has proven that people infected with it are 2.6 times more likely to be involved in a car accident. In addition, prevelence of schizophrenia and bi-polar disorder is much higher in those with t. gondii then those who are not infected.
    But, everyone always likes to hear “oh, don’t worry, it will all be fine”. Fools paradise.

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About Neuroskeptic

Neuroskeptic is a British neuroscientist who takes a skeptical look at his own field, and beyond. His blog offers a look at the latest developments in neuroscience, psychiatry and psychology through a critical lens.

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