National Geographic

The Great Puppet-Master Debate

Life is rough for parasites. Say you’re a tapeworm that only lives in the gut of one species of shark. You start out as an egg inside an adult tapeworm. Your parent releases you and a bunch of other eggs from its body, and its shark host shoots you out of its own body. Now you float in the vast ocean, stretching out on all sides. You are not made for the free-living world. If you don’t get into another host, you will never reach adulthood. Not just any host, but a fish. And not just any fish, but one species of shark. Chances are good, in other words, that you’ll die.

The miserable odds for individual parasites can potentially drive the evolution of something remarkable: the ability of parasites to manipulate their hosts. By controlling their hosts, the parasites can raise their odds of surviving and reproducing.

Scientists have gathered a number of examples of host manipulation over the years. I dedicated a chapter of my book Parasite Rex to some of them, and a group of scientists published a whole book on the topic last year. A lot of these parasites are cool just in and of themselves, whether they infect caterpillars or spiders, but inevitably people want to know whether parasites control us. I put it down to a combination of our species’s narcissism and love of zombie movies. There are a few potential cases of parasites influencing–if not outright manipulating–human behavior. And the most intriguing one of them all involves a single-celled parasite called Toxoplasma gondii.

Toxoplasma gondii forms cysts in people’s brains. Unless their host has a weak immune system, their cysts cause no apparent harm. But they make up in numbers what they may lack in deadliness. Perhaps a billion or more people carry Toxoplasma cysts in their brain. They pick up the parasites in the soil, undercooked meat, or cat litter.

It’s in cats that the Toxoplasma life cycle gets its start. The parasites mate in the intestines of cats and then produce egg-like offspring, which are passed out with cat droppings. The durable eggs can stay viable for months as they wait for their next host–which can be any species of mammal or bird. Those hosts swallow the parasites, which migrate out of their gut and wander their body; the ones that make it to the brain form protective cysts and play the waiting game again. Only if they can get back into a cat’s gut will they be able to take the next step in the Toxoplasma life cycle.

As other cases of parasite manipulation came to light, some scientists wondered whether Toxoplasma might have some tricks of its own. After all, there’s one obvious opportunity for increasing its odds of getting into cats: make its host easier for cats to catch.

Starting in the late 1990s, a number of researchers published evidence indicating that the parasite does, in fact, do this. Most of the work has been carried out on rats and mice. In a number of experiments, infection with Toxoplasma appears to make the rodents less frightened by the smell of cat urine. Some studies even hint at an attraction to the scent of their killer. (I’ve written about some of the research in recent years here and here.)

Now, in the journal Trends in Parasitology, a team of Australian parasitologists have challenged the Toxoplasma puppet-master hypothesis, in a commentary entitled “Adaptive host manipulation by Toxoplasma gondii: fact or fiction?”

“In our opinion,” the authors declare, “the accepted dogma that T. gondii manipulates host behavior to increase transmission to cats, tells an appealing story but does not stand up to scrutiny.”

They make their way systematically through the published record of experiments on Toxoplasma. They observe that the parasites produce a range of effects on the behavior of their hosts. In some studies, animals become more active, while in others they become less active, and in others they experience no change at all. In one study, scientists found that Toxoplasma impaired a host’s ability to learn, and in another, it didn’t. The same split in results turns up in tests on memory, a preference for exploring new things, time spent near cat urine, and anxiety.

The scientists also attack the elements that make up the manipulation hypothesis. Just because a parasite does something that appears to make it easier prey does not mean that natural selection produced that change to improve its odds of completing its life cycle. Toxoplasma could be altering its hosts as a side effect of infection, not as an evolved adaptation. The authors note that another single-celled parasite called Eimeria also robs mice of their fear of cats, despite the fact that its life cycle takes it from mouse to mouse, not mouse to cat.

If natural selection really had been at work here, it would be necessary for manipulations to actually increase the number of offspring Toxoplasma produced. But no one has ever done a large-scale trapping experiment to see whether cats catch more Toxoplasma-infected prey than healthy ones. This sort of experiment has been done for other species. In an experiment on fluke-infected fish, researchers found they became easier targets for birds because they jumped around near the surface of the water.

On the other hand, some behaviors that seemed like they ought to make hosts more likely to be killed turned out not to. A tapeworm called Hymenolepis dimunata alternates between beetles and rats. When it infects beetles, they spend more time out in the open, where they ought to be easier for rats to find and eat. Despite such expectations, scientists found that infection didn’t actually raise the odds of a beetle getting killed. In the case of Toxoplasma, rodents may lose their fear of cat urine but might still avoid other scents from their predators, such as the smell of cat fur.

The Australian scientists also point out that Toxoplasma has a more flexible life cycle than it’s often given credit for. It has to get into cats in order to sexually reproduce. But it can also clone itself in other species. The parasite can even spread from mothers to their offspring. When scientists survey the DNA of Toxoplasma, they see evidence for a lot of cloning, and not a lot of mixing genes through sex. If the parasite isn’t moving much between cats and their prey, the natural selection for manipulation should be weak.

A lot’s been made of the fact that Toxoplasma winds up not just in rat brains, but in human brains. Some scientists have argued for a whole host of changes to behavior in people who carry the parasite. It’s tempting, for example, to see “cat ladies” as being in the thrall of cat-infecting parasites. The critics think all this speculation is completely unwarranted at this point.

“Given that research into human behavior is based at least partly on findings in rodents,” the authors conclude, “it is vital that we have a good understanding of how rodent behavior is affected by T. gondii, before we extrapolate to other species.”

I reached out to some of the scientists who have done the most prominent work advancing the manipulation hypothesis. While they all agreed that scientists should be careful to avoid assuming adaptations that may not actually exist, they dispute a lot of the claims made by the Australian critics.

“I firmly believe Toxoplasma is a clear case of actual manipulation and that their attempt to dismiss this is a little too naïve and simplistic,” says Joanne Webster of Imperial College. Webster herself is no “manipulation fundamentalist,” as it were–she was the one who did the beetle experiment that the Australian critics use as evidence against manipulations. But she thinks that the evidence for Toxoplasma is strong, and the criticism against its power to manipulate are weak.

Mathematical models of evolution, for example, show that it’s not necessary for there to be a huge boost in cat attacks in order for natural selection to favor manipulation. And while it’s true that Toxoplasma doesn’t need sex to reproduce, sexual reproduction in the long run has many advantages–such as mixing genes together into better combinations.

Webster agrees that it would be great to see whether cats are more likely to kill infects prey than uninfected ones, but that’s a hugely challenging undertaking for all sorts of reasons (such as the rules about animal welfare). Michael Eisen, a Berkeley researcher who has done experiments on Toxoplasma in mice in recent years, put it this way:

It’s an almost impossible experiment to do right. Are you going to infect mice and release them and a control group and see which get eaten? Where would you do that? How would you know the results wouldn’t be different in a different setting?

As for the different results from some studies on Toxoplasma infection, Webster argues that a lot of that may come down to the fact that scientists have run some experiments on lab rodents and others have studied wild ones. Lab animals have been bred for decades away from their natural threats–including cats. If these differences are accounted for, Webster still sees the evidence for manipulation as being strong.

Eisen, on the other hand, thinks that scientists have yet to do enough experiments to make any strong statements about Toxoplasma. “We’re still far away from having done truly definitive studies to characterize the behavior itself,” he said. “Let’s talk about this before we start calling Toxo the parasitic king of behavior manipulation.”

Still, he agrees with Webster that the critics make a weak case when they try to downplay the importance of cats to Toxoplasma’s long-term survival. If Toxoplasma can do so well without having sex inside of cats, he asks, why does it still carry so much genetic machinery for having sex and producing offspring? Eisen calls this argument from the critics “very poor evolutionary thinking.”

Finally, I got in touch with Ajai Vyas of Nanyang Technological University. He’s done some of the most detailed work on how Toxoplasma affects the brains of rodents, finding it zeroes in on emotional circuits. Vyas pointed out that scientists have debated for a long time how you can tell whether the effects of a parasite are an adaptation for manipulating their host. A few criteria have emerged. Is the effect complex? Is it something that well-fitted to a parasite? Does it turn up in different parasite species, suggesting natural selection has favored it repeatedly. By these standards, Vyas argues, it makes sense to look at Toxoplasma as a manipulator.

But it’s also important to bear in mind that the cats and rodents that we see today–even in the so-called wild–are living in a human-dominated world. Both predator and prey in this case have benefited from our company; we’ve moved them around the world, we’ve given them shelter, and we’ve given them–intentionally or not–an abundant supply of food. The manipulations that Toxoplasma originally evolved may have been less ambiguous before its hosts underwent so much rapid change.

“It could be a historical legacy rather than a present adaptation,” says Vyas. “I am not yet clear about this.”

There are 13 Comments. Add Yours.

  1. Mike Lewinski
    October 2, 2013

    If it does manipulate host behavior, changes to human behavior don’t seem too far-fetched since we were once more regularly prey to large cats and may have been involved more directly in the life cycle.

    Also I understand Toxo can be sexually transmitted in some species including sheep, dogs and rats. That might account for some of the altered behaviors.

    “Infected men have lower IQs, achieve a lower level of education and have shorter attention spans. They are also more likely to break rules and take risks, be more independent, more anti-social, suspicious, jealous and morose, and are deemed less attractive to women.

    On the other hand, infected women tend to be more outgoing, friendly, more promiscuous, and are considered more attractive to men compared with non-infected controls.”


    The fact that infected women are reported as more attractive (even to uninfected men) is quite interesting.

  2. David Bofinger
    October 2, 2013

    They write, “Just because a parasite does something that appears to make it easier prey does not mean that natural selection produced that change to improve its odds of completing its life cycle. Toxoplasma could be altering its hosts as a side effect of infection, not as an evolved adaptation.”

    Is this a real distinction? There’s no teleology in evolution, in some sense side effects is all it is. (Perhaps they’re saying that it doesn’t affect evolutionary fitness, it’s ambiguous.)

  3. JMW
    October 3, 2013

    Easy to conceive, expensive and time consuming to carry out:

    Place a large number of rodent traps across a large geographic area. Trap the rodents, and check them for T. gondii. The establishes the prevalence of T. gondii in the rodent population.

    Then, deploy a large number of cat traps to catch feral domestic kitties. Obtain and analyze samples of their droppings. If their last meal was a rodent, count it in the study. Check for the presence of T. gondii in the droppings. This establishes the prevalence of infected rodents getting eaten by feral cats.

    Compare the two numbers.

    To be sufficiently rigorous, the study would have to be carried out with a high density of traps, over a large area. Hence, it would be expensive and time consuming to carry out.

    The weakness is that T. gondii might make infected rodents more prone to behaviour that leads to be trapped. This might create an artifically high figure for the “prevalence of T. gondii in target rodent populations”. But at this moment I can’t think of a way around this limitation…

  4. Glenn Scheyd
    October 3, 2013

    Good point and good question. It doesn’t have to be teleological. Evolved adaptations are adaptations that have been shaped in response to specific ancestral challenges. The formation of the adaptations inevitably creates by-products which ought not to be considered adaptations. The logic and importance of the distinction is all laid out elegantly by George Williams in his (1966) Adaptation and Natural Selection. The umbilical cord is an evolved adaptation; the navel is not.

  5. Robert C Brooke
    October 3, 2013

    Does Toxoplasma gondii only infect mice?Or can other rodent species carry it?

  6. Fernando Pereira
    October 6, 2013

    What about alternative hypotheses for the brain tropism, such as getting away from some immune responses?

  7. gseattle
    October 8, 2013

    Great, we discovered that Toxoplasma gondii dramatically alters the mind of humans and one third of us are infected.

    How many of the other 1000+ known species of human microbes also affect thought and behavior?

  8. ARW
    October 13, 2013

    Even if cats are necessary for the long term survival of Toxoplasma, is this evidence that the parasite should therefore manipulate rodent behaviour to enhance transmission back to cats? Rodents are already a large part of a cat’s diet. The fact that the parasite ‘finds a way’ to infect rodents (via oocysts in the soil) may be enough to ensure transmission back to cats and continuation of the life cycle.

    In reply to Robert C Brooke- yes other rodent (and non rodent!) species can carry toxoplasma. Any warm blooded vertebrate can be a host.

  9. David Bump
    October 16, 2013

    Sadly, the real king of the puppet masters is the amoebic colony organism which infects our brains in such a way that, every time we discover it, we immediately forget about it….
    Muah ha hah ha haaaaaa!
    Happy Halloween!

  10. SRT
    October 26, 2013

    Toxoplasma gondii is not merely a latent infection. Within families where many cycles of infection have occurred, the parasite infection tends towards what can be considered “symbiotic” as its presence dampens down the “tuned for its presence” immune system. UNfortunately, the host ultimately can be manipulated in extreme cases to act in the best interest of the parasite in human form, i.e. transmission paramount. Such hosts exhibit idiosyncratic and stereotypical behaviors that can best be described as oppressive, controlling, sexual, etc. Systems have evolved that include money, religion, hierarchies, etc. are slowly ordering the species by this external biological pressure that could be termed “insectualization”, e.g. nepotism. Homo Sapiens has only been around in evolutionary terms for the blink of an eye; insects such as ants, bees, etc. much longer. They can be organized as so-called “social” species but “totalitarian” would be more appropriate. Brain parasite infections can be linked to war, cognitive dissonance, paedophilia, schizophrenia, paranoia, military/pharmaceutical/prison-industrial-complex, psyops, psychology, cancer, autism, royalty, oppression, torture, etc. Many systems are corrupted because, like magic, the effect of brain parasite infection is invisible but profound and often determining. Those most affected as hosts will ensure that this remains so often by secrecy and idiopathic behavior hidden in the shadows, e.g. infection cycles via paedophilia. The puppet master is king but the king is the puppet.

    [CZ: Um--evidence?]

  11. SRT
    October 27, 2013

    Open your eyes!

  12. SRT
    October 27, 2013

    Toxoplasma gondii has been linked to a myriad of different behavioral effects. Take for instance it being linked to suicide tendencies. An assumption might be that extremely risky behavior may be linked to an inherent infection. The effect of toxo on the immune system is quite profound especially in respect to cytokine production. So-called healthy volunteers for dangerous drug trials may all be infected with toxo. Who knows? It is not tested for. Why not? It may show up how corrupted the pharmaceutical industrial complex is. Money-centric magic: cancer, immune deficiency, obesity, schizophrenia, paranoia, etc. Look up the TGN1412 so-called “elephant man” drug trial and the effect of testing drugs that affect cytokine production on persons whose “toxo status” is not predetermined. A toxo infected individual may have a suppressed immune system that allows “unusual microorganisms” to be present in biofilms that can produce cancer causing chemicals. The TGN1412 drug trial demonstrated that these microorganisms are many and varied and unique to each individual. It is no wonder that the volunteers all suffered from a so-called “cytokine storm”.

  13. Collin Merenoff
    November 26, 2014

    Impossible! It’s just a virus! It has none of the linkages by which a brain thinks. It does, however, have lots of DNA strands on which data can be stored as patterns of methylation. And it has them in a much higher density than brain matter. We store an enormous amount of data in our brains — much more than could be explained by our head size. Toxo may be where we’re putting it.

    It’s not our master; it’s our slave. A personal hard-drive in our heads. It’s the most easy-to-manage species we’ve ever domesticated. It doesn’t demand anything except merely to exist. It allows us to write data all over it and thread axons and dendrites through it and never complains.

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