January 17, 2006

The Return of the Puppet Masters – how parasites can alter behavior

ok… besides the “usual” zombie apocalypse crap (i really don’t like it and do not understand what people find so “fascinating” about such an “option”) what about a parasite that would make humans more empathic and less environmental destructive (e.g. make homo sapiens actually sapiens)? 😀


Posted by Carl Zimmer

src: http://web.archive.org/web/20150905181128/http://www.corante.com/loom/archives/2006/01/17/the_return_of_the_puppet_masters.php

Are brain parasites altering the personalities of three billion people? The question emerged a few years ago, and it shows no signs of going away.

I first encountered this idea while working on my book Parasite Rex. I was investigating the remarkable ability parasites have to manipulate the behavior of their hosts. The lancet fluke Dicrocoelium dendriticum, for example, forces its ant host to clamp itself to the tip of grass blades, where a grazing mammal might eat it. It’s in the fluke’s interest to get eaten, because only by getting into the gut of a sheep or some other grazer can it complete its life cycle. Another fluke, Euhaplorchis californiensis, causes infected fish to shimmy and jump, greatly increasing the chance that wading birds will grab them.

Those parasites were weird enough, but then I got to know Toxoplasma gondii. This single-celled parasite lives in the guts of cats, sheddding eggs that can be picked up by rats and other animals that can just so happen be eaten by cats. Toxoplasma forms cysts throughout its intermediate host’s body, including the brain. And yet a Toxoplasma-ridden rat is perfectly healthy. That makes good sense for the parasite, since a cat would not be particularly interested in eating a dead rat. But scientists at Oxford discovered that the parasite changes the rats in one subtle but vital way.

The scientists studied the rats in a six-foot by six-foot outdoor enclosure. They used bricks to turn it into a maze of paths and cells. In each corner of the enclosure they put a nest box along with a bowl of food and water. On each the nests they added a few drops of a particular odor. On one they added the scent of fresh straw bedding, on another the bedding from a rat’s nests, on another the scent of rabbit urine, on another, the urine of a cat. When they set healthy rats loose in the enclosure, the animals rooted around curiously and investigated the nests. But when they came across the cat odor, they shied away and never returned to that corner. This was no surprise: the odor of a cat triggers a sudden shift in the chemistry of rat brains that brings on intense anxiety. (When researchers test anti-anxiety drugs on rats, they use a whiff of cat urine to make them panic.) The anxiety attack made the healthy rats shy away from the odor and in general makes them leery of investigating new things. Better to lie low and stay alive.

Then the researchers put Toxoplasma-carrying rats in the enclosure. Rats carrying the parasite are for the most part indistinguishable from healthy ones. They can compete for mates just as well and have no trouble feeding themselves. The only difference, the researchers found, is that they are more likely to get themselves killed. The scent of a cat in the enclosure didn’t make them anxious, and they went about their business as if nothing was bothering them. They would explore around the odor at least as often as they did anywhere else in the enclosure. In some cases, they even took a special interest in the spot and came back to it over and over again.
The scientists speculated that Toxoplasma was secreted some substance that was altering the patterns of brain activity in the rats. This manipulation likely evolved through natural selection, since parasites that were more likely to end up in cats would leave more offpsring.

The Oxford scientists knew that humans can be hosts to Toxoplasma, too. People can become infected by its eggs by handling soil or kitty litter. For most people, the infection causes no harm. Only if a person’s immune system is weak does Toxoplasma grow uncontrollably. That’s why pregnant women are advised not to handle kitty litter, and why toxoplasmosis is a serious risk for people with AIDS. Otherwise, the parasite lives quietly in people’s bodies (and brains). It’s estimated that about half of all people on Earth are infected with Toxoplasma.

Given that human and rat brains have a lot of similarities (they share the same basic anatomy and use the same neurotransmitters), a question naturally arose: if Toxoplasma can alter the behavior of a rat, could it alter a human? Obviously, this manipulation would not do the parasite any good as an adaptation, since it’s pretty rare for a human to be devoured by a cat. But it could still have an effect.

Some scientists believe that Toxoplasma changes the personality of its human hosts, bringing different shifts to men and women. Parasitologist Jaroslav Flegr of Charles University in Prague administered psychological questionnaires to people infected with Toxoplasma and controls. Those infected, he found, show a small, but statistically significant, tendency to be more self-reproaching and insecure. Paradoxically, infected women, on average, tend to be more outgoing and warmhearted than controls, while infected men tend to be more jealous and suspicious.

It’s controversial work, disputed by many. But it attracted the attention of E. Fuller Torrey of the Stanley Medical Research Institute in Bethesda, Maryland. Torrey and his colleagues had noticed some intriguing links between Toxoplasma and schizophrenia. Infection with the parasite has been associated with damage to a certain class of neurons (astrocytes). So has schizophrenia. Pregnant women with high levels of Toxoplasma antibodies in their blood were more likely to give birth to children who would later develop schizophrenia. Torrey lays out more links in this 2003 paper. While none is a smoking gun, they are certainly food for thought. It’s conceivable that exposure to Toxoplasma causes subtle changes in most people’s personality, but in a small minority, it has more devastating effects.

A year later, Torrey and his colleagues discovered one more fascinating link. They raised human cells in Petri dishes and infected them with Toxoplasma. Then they dosed the cells with a variety of drugs used to treat schizophrenia. Several of the drugs–most notably haloperidol–blocked the growth of the parasite.

So Fuller and the Oxford scientists joined forces to find an answer to the next logical question: can drugs used to treat schizophrenia help a parasite-crazed rat? They now report their results in the Proceedings of the Royal Society of London (press release). They ran the original tests on 49 more rats. Once again, parasitized rats lost their healthy fear of cats. Then the researchers treated the rats with haloperidol and several other anti-psychotic drugs. They found that the drugs made the rats more scared. They also found that the antipsychotics were as effective as pyrimethamine, a drug that is specifically used to eliminate Toxoplasma.

There’s plenty left to do to turn these results into a full-blown explanation of parasites and personalities. For example, what is Toxoplasma releasing into brains to manipulate its hosts? And how does that substance give rise to schizophrenia in some humans? And even if the hypothesis does hold up, it would only account for some cases of schizophrenia, while the cause of others would remain undiscovered. But still…the idea that parasites are tinkering with humanity’s personality–perhaps even giving rise to cultural diversity–is taking over my head like a bad case of Toxoplasma.
Update 2/9: link to new PRSL paper fixed.

Comments (98) + TrackBacks (7) | Category: The Parasite Files



1. Geoff on January 18, 2006 01:58 PM writes…

Could this explain “cat ladies,” who shelter on the order of 100 cats in their homes, while ignoring all basic domestic hygiene?

Permalink to Comment

2. Apikoros on January 18, 2006 02:00 PM writes…

Fascinating post!

If there is a link between toxoplasmosis and schizophrenia, I wonder if people who get toxoplasma from their cat are sometimes driven to acquire more and more cats?

I should point out one small error: astrocytes are not neurons, but glial cells which surround and support neurons (and probably have other unknown functions).

Permalink to Comment

3. Apikoros on January 18, 2006 02:17 PM writes…

Wow–Geoff and I think alike.

Bummer for you, man.

Permalink to Comment

4. David on January 18, 2006 02:53 PM writes…

Absolutely chilling. One those great notions that seems so obvious upon reflection. Even if it turns out to be a false lead, the idea will certainly be exploited by writers of science fiction…
Perhaps my cat allergies are the side-effect of a competing parasite. My bet is that one is linked to excessive commenting on blog posts (aslo known as blogererha).

Permalink to Comment

5. David on January 18, 2006 02:57 PM writes…

Now I know why it is called a CAT scan.

Permalink to Comment

6. Britton Cole on January 18, 2006 03:29 PM writes…

The idea of parasites altering the behaviour of the host has already been broached in science fiction. Notably in Star Trek II, though these weren’t single-cell parasites causing subtle change, rather they were worm-like and caused a very strong desire to obey commands. It was not explained what evolutionary advantage this produced for the parasites. Maybe someone in the Star Trek community should write a backstory for these creatures (I’m not quite enough of a nerd to tackle that project).

Permalink to Comment

7. George on January 18, 2006 04:18 PM writes…

So… does this mean that an infection (toxoplasmosis) should be treated to rid the person of the parasite? While the symptoms are subtle, they might actually important.

Permalink to Comment

8. J. on January 18, 2006 05:51 PM writes…

Aha! — this explains some of the recent behavior coming from our political leadership …

Permalink to Comment

9. mazhar butt on January 18, 2006 07:02 PM writes…

still has much to be done. if fear has a link with physical elements, the it may be possible to overcome all diseases of mankind

Permalink to Comment

10. Henry on January 18, 2006 07:05 PM writes…

Britton, IIRC, Star Trek II is predated by Robert H. Heilein’s ‘The Puppet Masters’, which I suspect Carl is alluding to in the title of this post (and, by the way, is a very good book; one of the few books to ever give me repeated nightmares).

Permalink to Comment

11. Britton Cole on January 18, 2006 09:26 PM writes…

I was just responding to David, and didn’t even think of what other science fiction writing about mind-controlling parasites there were. I’ll have to look puppet-masters up. Admittedly, I don’t read that much science fiction.

Permalink to Comment

12. Jason Bobe on January 18, 2006 11:28 PM writes…

A friend recently alerted me a to parasite that induces grasshoppers to jump into a watery grave. National Geographic article here.

Permalink to Comment

13. Babbler on January 18, 2006 11:33 PM writes…

Reminds me of The Animorphs, a 90’s children’s book and TV series, about an alien force invading Earth by taking over people’s brains making them their slave, and the shape-shifting aliens coming to save humanity.

I never read any of the books, which considering the quality of the TV show, is most likely for the better.

Permalink to Comment

14. PavedWalden on January 19, 2006 12:26 AM writes…

HELP! What if they’re in my brain? GET IT OUT GETITOUT!

Seriously, can I have some of that pyrimethamine stuff, just in case?

Permalink to Comment

15. Chamaree on January 19, 2006 10:20 AM writes…

Wow, this is interesting. Thanks for the post!

Permalink to Comment

16. Cats are Snakes on January 19, 2006 11:17 AM writes…

Would this mean that a human infected would have a less averse reaction to the smell of cat urine? I’ve had a cat (or as many as two cats) for most of my life; I’m likely infected. Still, I cannot abide the smell of cat urine.

Permalink to Comment

17. John on January 19, 2006 11:58 AM writes…

“A parasite made me do it”!

Permalink to Comment

18. Brian S. on January 19, 2006 01:05 PM writes…

An identical twin study where only one twin was infected would be a great test of this theory (especially when only one twin is schizophrenic).

Permalink to Comment

19. cleek on January 19, 2006 01:50 PM writes…

i was hoping you weren’t going to say “…and then the researchers looked at the political orientation of the infected subjects and found…”

Permalink to Comment

20. Hai~Ren on January 19, 2006 03:18 PM writes…

One interesting thing I’ve thought about is how and why Toxoplasma has evolved such a bizarre life cycle, and also, when you consider that mice and rats are food for more than just cats, but also snakes, birds and lots of other creatures, and it does make one wonder about the potential losses that stem from the infected rodent being consumed by a non-feline predator.

Permalink to Comment

21. Apesnake on January 19, 2006 06:34 PM writes…

“and it does make one wonder about the potential losses that stem from the infected rodent being consumed by a non-feline predator.”

One of the drawbacks to being a parasite but it no doubt is able to distribute such a vast number of cysts (in a vast number of gardens and planter boxes) that the chances are good that some will make it into a rat and back into a cat. Plus these parasites can survive in a wide number of intermediate hosts, even ones where it might not be as adept at manipulating but might still be eaten or fed on by an opportunistic cat.

Can a tin-foil keep the parasites from communicating by radio?

Permalink to Comment

22. gaw3 on January 20, 2006 12:56 AM writes…

I would bet that this parasite has undergone quite a bit of natural selection since the rise of cities, with potential hosts–rats, cats and humans– all present at much greater density.

Permalink to Comment

23. Brenda on January 20, 2006 08:23 AM writes…

Surely this is what symbiosis and evolution is all about – we’ve reached the stage where we can recognise and evaluate what’s happening now but does this mean that we should interfere with the process?

Permalink to Comment

24. Lars Smith on January 20, 2006 11:19 AM writes…

There also seems to be a very interesting relationship between Borna Disease Virus infection and mental disorders, see e.g.

Clin Microbiol Rev. 2003 July; 16(3): 534–545.
doi: 10.1128/CMR.16.3.534-545.2003.
Copyright © 2003, American Society for Microbiology

Borna Disease Virus Infection, a Human Mental-Health Risk

Liv Bode1 and Hans Ludwig2*
Project Bornavirus Infections, Robert Koch Institute, 13353 Berlin,1 Institute of Virology, Free University of Berlin, 14195 Berlin Germany2


… This article supports existing correlative evidence for a pathogenic role of BDV infection in particular human mental disorders, in analogy to what has been proven for a variety of animal species.

Permalink to Comment

25. eric willemen on January 20, 2006 12:01 PM writes…

What most ppl dont mention is that after a cat develops antibodies for toxoplasmose she stops being infectious! a cat can be at most 3 weeks infectious for other living beings, unless the imune systems is too weak to combat the infection. and the spores in the cat droppings only become active after 2-5 days depending on the environmental temperature. the higher the temp the sooner they become active. since most droppings are flushed down the toilet once ot twice a day the risc of catching toxoplasmose through your cat is almost nihil.

Permalink to Comment

26. Filipe on January 20, 2006 06:04 PM writes…

Eric, on #25, are sure about those numbers? When there was that hype about Toxoplasma gondii making men more antisocial and women more promiscuous I remember reading that about 90% of cat owners in Fance and Germany were infected.

Permalink to Comment

27. Charack on January 20, 2006 07:13 PM writes…

Thank you for this insight. and the reference links. This is very interesting.

Permalink to Comment

28. vallen on January 20, 2006 08:19 PM writes…

this is a great commentary. I like the idea #18 by Brian S. to test this hypothesis.

Permalink to Comment

29. Zombie Apocalyps on January 21, 2006 12:43 AM writes…

With all this said and done. I think the real issue to debate would be the cover up of the zombie outbreak in Cambodia last year. Where it was reported on the BBC alittle over 1 year ago that a mutated form of a Malaria causing Protazoa was killing a large nuber of people within 24 hours of infection. They clamed that the mortality rate was 100%. Within 12 hours after death, the protazoa was clamed to reanimate the heart and brain of the dead. Resulting in dead people flailing around with limited nerve function long after post-mortem The next day it was said that the outbreak had been controled and U.S. forces where stepping in to clean up the situation.(The same artical now severly reduced. After all this there was not one trace of information to be found that even mentioned a Malaria quarentine in Cambodia in 2005. What are peoples feelings after reading about Toxoplasma virus altering the brain? How does the consept of a single celled protazoa mutating to alter the nerve center of a mammal host(on a very simple level), creating a “zombie like ” stasis sit? Does it not seeem a little more plausable? Is there anyone out there who can confirm the existance of this this international clam that was made in spring of 2005 on the BBC?

Permalink to Comment

30. chaka on January 21, 2006 01:19 AM writes…

I’ve never heard of the mutated malaria-zombies. If there is a sample of this mutated malaria it might be compared to the blowfish toxin-cocktail that is sometimes used in Haiti to induce the “zombie state”. That being said, is it really so frightening to learn that humans, the supposedly the only “thinking” species are pawns to a life form we can’t see. Perhaps we got a little too comfortable with our supposed position at the top of the food chain.

Permalink to Comment

31. animate5 on January 21, 2006 02:30 AM writes…

They should do research and find out if Toxoplasma infected individuals are, like the rats, less effected by the smell of cat urine. Most people I know who own a lot of cats never seem to notice the wretched smell of cat piss which pervades their house. My nose wrinles up and I want to run.

It would be a great benefit to Toxo if humans became immune to, or enjoyed the smell of cats. Humans would take care of them, perhaps take in more of them, the cats would live longer, and Toxoplasma would have an increased chance of infecting more cats, and more people.

Permalink to Comment

32. Anonymous on January 21, 2006 02:51 AM writes…

There are Lots of parasites that alter the behavior of their host. Guinea worms make their host seek water to sooth the burning sensation in their legs. Once in the water, the worm releases it’s eggs.

A gross little Cordyceps species of mushroom infects grasshoppers, then, when it is ready to fruit and drop spores, the grasshopper has the overwhelming urge to climb to the top of high grass (a dumb thing for a sane grasshopper to do, because it makes it easily visible to predators) and stay there, while the Cordyceps fruit (mushroom) pops out of it’s head, killing it, and then drops it’s spores. ick.

Who knows what we’ll find next – a dust mite that makes people become pack rats, a tick that makes people want to take their dog on lots of walkies?

Permalink to Comment

33. Paul on January 21, 2006 09:20 AM writes…

This is really all about how cats come to own humans. 🙂

The main reason cats benefit from this parasite appears to be that it makes them better hunters by making their infected prey easier to catch. That makes cats, infected or not, house-dwelling or not, more desirable to humans.

Ultimately infected cats should triumph over the uninfected, because the infected will sooner or later infect local rodent populations, for example by occasionally wounding but losing a victim. Ever wonder why a cat will sometimes play with wounded prey instead of delivering the coup de grace? Maybe spreading the parasite is part of it: if you sometimes inflict non-lethal wounds (pass on the parasite) instead of having a meal right away, hunting gets easier and easier with time.

If infected cats are also more docile and affectionate with humans (likely, considering the behavioral effects on humans and rats), and infected humans are more welcoming to cats (including their smell, but more importantly on an emotional level), the circle of symbiosis is complete and both cats and humans, but especially cats, derive benefits from populations of cats, humans and rodents all being infected. Even if the cats exterminate the local rodent population, humans still have reasons to keep cats around.

Hence this parasite is cats’ stepping stone to a life of leisure, with humans as their humble servants.

Permalink to Comment

34. Sean on January 21, 2006 12:26 PM writes…

Do STD’s increase sexual desire? The behavioural link would be pretty strong.

I have personally noticed a change in my behaviour prior to diagnosis and treatment.

Are some sex offenders merely more susceptible to latent unrecognised infections.

Permalink to Comment

35. nekultura on January 21, 2006 02:35 PM writes…

This must explain why, despite all evidence of its disastrous effects on humankind, collectivism in various forms (communism, socialism, naziism, fascism, tribalism, jihadism) continues to thrive, especially amongst the very uneducated and the over-educated.

Hmmm? Maybe the parasite wants to kill off the truly intelligent amongst us (conservatives, libertarians), so that it can propagate freely within the brainless masses of Dimocrats, socialists, university professors, et nauseum…?

Permalink to Comment

36. Wim L on January 21, 2006 02:42 PM writes…

I’ve often thought that the best way to understand housecats is as brood parasites (you know, like cuckoos). I mean, I like cats (or is that the toxoplasma speaking?), but the way that people take them in, and care for them like children, often to the exclusion of having children themselves, is pretty suggestive.

I recently read a YA vampire novel called _Peeps_ by Scott Westerfield. It’s a fun book. Every other chapter is a small parasitology infodump. It does mention T. gondii, as well as a bunch of other odd and creepy parasites. The infodumps do have something to do with the fiction chapters.

Permalink to Comment

37. McDLT on January 21, 2006 05:06 PM writes…

Check out the console game “Resident Evil 4” for the Nintendo Gamecube. The plot is that parasites have infected human brains, turning them into mindless zombie-like killers. The game even references Dicrocoelium dendriticum, those ants that allow themselves to be eaten, it mentions other parasite examples in nature as well. Who knew shooting zombies could be so educational.

Permalink to Comment

38. duh on January 21, 2006 05:13 PM writes…

Yeah nekultura,

If conservaties are so smart then why do the red states rank so poorly in intelligence?


Must be the liberal media making stuff up again…

Permalink to Comment

39. Sean on January 21, 2006 06:57 PM writes…

Zombie parasite story was an April fool written by and for a group of zombie film enthusiasts, which escaped onto the net. There is a write up by the the author out there but I have lost the link.

Permalink to Comment

40. musti on January 21, 2006 08:00 PM writes…

#34, Sean: I think just the opposite. I was just researching this when I stumbled upon here:


Permalink to Comment

41. Brian S. on January 21, 2006 08:19 PM writes…

Re #29:

Actually, I AM one of those zombie malaria mutants, and let me tell you, things are so much better this way. Believe me, once you go zombie, you’ll never go back! Eat all the kitty litter you can find!

Permalink to Comment

42. bllius on January 21, 2006 08:38 PM writes…

Wolbachia bacteria are endosymbionts that infect insect hosts. They are maternally transmitted from female to offspring. The interesting thing is that they affect reproduction. Infected males mating with uninfected females results in embryonic death, while infected females can mate with both infected and uninfected males. The end result is an increase in infected females in the population.

Permalink to Comment

43. Sean on January 21, 2006 08:59 PM writes…

I was thinking more of carriers (i.e. gross symptoms not evident) who represent a large proportion of the infected population in for example chlaymidia.

Sure once people are diagnosed as having an STD it affects their sex life.

Before conciousness and the awareness of the link between infectious organisms and disease, the link between occasional gentital sores and self esteem would I think not have been important, whereas a slight tweaking of testorone production in the host would have worked wonders for your ability to move around, (e.g. males take more risks to have sex with all and sundry and females who are only on heat once a month are more receptive).

Permalink to Comment

44. Me on January 21, 2006 09:02 PM writes…

Does your html editor make italic font for species names?

Permalink to Comment

45. Ivan Minic on January 21, 2006 11:05 PM writes…

Now.. that thingie doctor wrote is explained…

Permalink to Comment

46. jabaH on January 21, 2006 11:40 PM writes…

re: zombie apocalypse, a google reveals the article is dated April 1–it’s a joke.

Permalink to Comment

47. Clive Potter on January 22, 2006 05:08 AM writes…

Many scientists now know that humans can be hosts to Toxoplasma. Previous discussion of this topic has covered the subject well.

Permalink to Comment

48. Devin on January 22, 2006 09:43 AM writes…

This has already been used as a major plot point in Peter Watt’s novels Starfish, Malestrom, and Behemoth.

Permalink to Comment

49. Max on January 22, 2006 10:23 AM writes…

# 38 duh,

I note that your smart state list has California at 46. Is there a more blondly liberal state than California?

Permalink to Comment

50. Mike Smith on January 22, 2006 10:43 AM writes…

Uh, “uncultured”, you forgot a couple of collectivist institutions: churches, corporations, sports teams, the Marines, programming teams, theater casts, and the least voluntary of all, the family.

As for your lumping-together: I _wish_ Democrats were liberals.

Actually, you didn’t: you merely found another way of saying “what I don’t like” when you lost the ability to spell their names properly.

Permalink to Comment

51. Anonymous on January 22, 2006 11:23 AM writes…

max, shut up, you’re talking about the home of ronald reagan.

Permalink to Comment

52. Red on January 22, 2006 12:07 PM writes…

So… where do I get some of this stuff to infect my wife??

Permalink to Comment

53. Mengü Gülmen on January 22, 2006 01:08 PM writes…

I can’t seem to find the source now but there was a documentary i watched on the discovery channel 2 years ago, it was talking about mind-controlling single-celled creatures.

one of them was a parasite that lived in the amazon birds. it left its eggs to the urine of the bird so it fell down on the forest floor – where it is virtually impossible for any bird to pick it up.

so its eggs are “good food” to a type of bug. when the bug eats the eggs, the controller [i don’t remember its name so i’ll call it ‘the controller’] advances to the center of the bug’s neural system, it makes the bug want to climb on top of trees, so the bug does.

when it’s on top of a tree, the controller induces a reaction in the bug’s body and causes some pink color to show on the bug’s skin. so it’s highly detectable by the birds against a green background .

and inevitably, the bird eats the bug, and the parasite’s home again.

anyone watched that documentary or has some knowledge about this little guy? i’d appreciate it .

the main point is, if some parasite can make a bug do thee stuff, can there be trillions of parasites which induces certain behaviours in any creature’s nervous system?

Permalink to Comment

54. Zombie Jesus on January 22, 2006 04:12 PM writes…

This story strikes me as particularly interesting because of its astounding ramifications. For instance, the myth of the Cartesian mind-body split comes under fire when considered with respect to a story such as this. While the idea that an organism can, through biological and chemical means, alter what many used to consider the sacred and bodily independent personality of someone isn’t unprecedented — some have mentioned Star Trek and Heinlein as being an example of this ‘life imitates art’ pattern, and I’d add that the films of David Cronenberg anticipate this in some way, notably the parasites in “Shivers” which cause people to become sex-crazed walking zombies, and to a metaphorical extent, the idea of a person’s will entering someone’s mind without any bodily contact whatsoever in “Scanners” — this bacteria is striking because of how subtle and potentially widespread its effects are.

Interesting how we take for granted that we often refer to emotions “taking us over” in everyday metaphorical language, as though they were externalized substances entering our brains…

Permalink to Comment

55. pissed on January 22, 2006 06:24 PM writes…

it took me 4 years of therapy (and a #@!%load of money) to isolate certain traumatic childhood events, and it turns out im insecure because cleaning the litterbox was one of my childhood chores…

i want a refund

Permalink to Comment

56. Merrill Pye on January 22, 2006 10:50 PM writes…

Dear #53. (Mengü), I think that was a parasitic worm called Leucochloridium varidae. Inside a snail, it grows tubes that it sends into the snail’s two eye-tentacle-thingies. They swell, pulsate, flash bright colors, and look like fat, wriggling caterpillars. This prevents the snail from retreating into its shell, and also attracts the attention of birds.

The snail’s behavior changes as well, so it crawls out in the open instead of seeking shade and shelter.

When a bird eats the snail, it contracts the parasite and scatters its eggs in droppings (birds don’t have separate urine & faeces) to be eaten by more snails. If you ever see the video of the effect on the snail, it’s extraordinary.

Permalink to Comment

57. SJ on January 22, 2006 11:07 PM writes…

Just a note for the person interested in the cambodia thing:

They talk about it there. Talk about something getting swept under the rug….thats fuckin scary man…..

Permalink to Comment

58. SJ on January 22, 2006 11:24 PM writes…

hm, alright april fools joke. n/m then 🙂

Permalink to Comment

59. mega on January 23, 2006 12:24 AM writes…

yaa, i just finished playing resident evil 4…definately should check that game out.

Permalink to Comment

60. Ruth on January 23, 2006 01:09 AM writes…

Yeah, I am sure that the nasty fungus, candida albicans also makes its human hosts to crave sugar. And what about plants manipulating our brains. I am sure the “farmers friends” weed whose seeds stick to your clothes have a way of creating an impulse to pick them off, only after moving away from the site they stuck, and especially, before one gets to the house where they can be disposed of into the garbage.

Well, maybe I am infected with Toxoplasma, but I think that the mind of lifeforms are connected beyond a conscious level and they can manipulate and affect each other in ways we have not yet discovered.

Permalink to Comment

61. DCM on January 23, 2006 08:56 AM writes…

Perhaps the parasite can be genetically engineered as a means of improving women’s behavior and personalities.

Permalink to Comment

62. Ken on January 23, 2006 11:25 AM writes…

Let’s not forget the most terrifying “puppet master” infectious agent.


It targets the mammal brain, maxing out its agressiveness and then spreading through saliva into the bite wound of another mammal.

Permalink to Comment

63. jenn on January 23, 2006 11:30 AM writes…

Funny how this entire statement can be stood on it’s head and it will make the same sense:

This must explain why, despite all evidence of its disastrous effects on humankind, collectivism in various forms (communism, socialism, naziism, fascism, tribalism, jihadism) continues to thrive, especially amongst the very uneducated and the over-educated.Hmmm? Maybe the parasite wants to kill off the truly intelligent amongst us (conservatives, libertarians), so that it can propagate freely within the brainless masses of Dimocrats [sic], socialists, university professors, et nauseum…?

Permalink to Comment

64. Caate on January 23, 2006 12:25 PM writes…

Hooray for a Carl Zimmer site! I read Parasite Rex last year and have utilized that book to explain to friends why parasitology is awesome. I’m sure your book sales have gone up slightly in Wisconsin…

Permalink to Comment

65. Eek on January 23, 2006 01:26 PM writes…

#55, you’ve been to psychoanalysis. It’s been, basically, shown not to work. Get somebody that does real therapy.

For feeling insecure, I’d guess cognitive-behaviour therapy or hypnosis to be the best choices.

Permalink to Comment

66. tim on January 23, 2006 03:38 PM writes…

This could explain those guys who stick their heads in lions mouths.

Permalink to Comment

67. sheila on January 23, 2006 07:02 PM writes…

I blame t. gondii for this: http://snltranscripts.jt.org/99/99blittercritters.phtml

Permalink to Comment

68. PARASITE ADVOCACY NEXUS 7 on January 23, 2006 07:10 PM writes…







Permalink to Comment

69. Owlmirror on January 23, 2006 08:17 PM writes…

#56 & #53, you can see an image of a snail infected with Leucochloridium varidae (as well as other disturbing parasites & the results of parasite infection) here:

There is also this earlier article by Carl Zimmer, which also makes for interesting parasitology reading:


Permalink to Comment

70. John on January 24, 2006 01:18 AM writes…

Aren’t you glad you posted this article, Carl? Actually, the response has been fascinating – and for such a wide variety of reasons!

Permalink to Comment

71. Anonymous on January 25, 2006 08:47 AM writes…

“Are brain parasites altering the personalities of three billion people?”

Very interesting article. I have a question: why 3 billion? why not everyone?

Permalink to Comment

72. Filipe on January 25, 2006 04:09 PM writes…

The number of people infected with this thing is quite high. In the US the number is close to 25%. Anyone to join me in promoting cat erradication?

Jones JL, Kruszon-Moran D, Wilson M. Toxoplasma gondii infection in the United States, 1999–2000. Emerg Infect Dis [serial online] 2003 Nov.

Available from: URL: http://www.cdc.gov/ncidod/EID/vol9no11/03-0098.htm

Permalink to Comment

73. Filipe on January 25, 2006 04:16 PM writes…

Ooops, I touched the wrong key, it’s 15% not 25%.

Permalink to Comment

74. dbpitt on January 28, 2006 02:15 AM writes…

Just thought I’d mention: I bought your book after reading this.

Permalink to Comment

75. Dan S. on January 29, 2006 12:51 PM writes…

1. Amazing. I would love to learn, if this holds up, what little Toxi is doing to rodent & human brains – and the on-average gender difference is fascinating – what’s going on there? (And is it in fact pulling opposite switches, so to speak, in many men and women, or are these specific behaviors actually connected in some fashion & differently expressed due biological/social differences?) And how widespread is T. gondii among the cat familiy – any nifty examples of host-parasite co-evolution, co-radiation [?], etc.?

2. Arrrgh!! [grabs head]. Get them out! GetthemOUT! Stop looking at me, cat! You planned this, didn’t you!!

Permalink to Comment

76. MinaW on January 30, 2006 03:11 PM writes…

On the same National Geographic page referenced above (#13?) for the grasshopper parasite is a recent news photo of a whale swimming up the Thames, 060120 date. If I remember correctly there have been some autopsy reports of beached whales, especially those who return to the beach when pushed off, which implicated parasites. (Internal ones that is. External marine parasites are killed by fresh water, which would be a beneficial reason for swimming upriver.) So what would the parasite get from causing the whale to beach or swim upriver? Or are they just randomly interfering with a sense of location, which was the previous hypothesis I’d seen.

By the way, those who are thinking that the parasite Toxoplasmosis interferes with the cat lovers’ ability to smell cat urine don’t need to stretch their hypothesizing that far. Habituation easily accounts for the phenomenon. As shown with my experience this last week with skunk smell coming into the house. I guess the coyote got it (or didn’t) right outside. Three days of all the doors and windows open, because outside the smell had blown away by morning, didn’t get it out of all the corners. But I don’t smell it until I go outside and come back in. Apparently anything can be habituated to. It’s been raining now for days….no more airing out.

Permalink to Comment

77. fusion on February 1, 2006 04:38 PM writes…

Perhaps this theory can be used to change men’s behavior, minds and personalities. Then more of them might actually be useful for something besides foot soldiers, corrupt politicians, overpaid/poor performing business leaders and violent criminals.

Permalink to Comment

78. LJRphoto on February 3, 2006 06:02 PM writes…

MinaW, THANK YOU! I’ve been reading through all of these responses trying to figure out how we got from a rat losing it’s self preservation response to people enjoying the smell of cat urine. I’ve had cats my entire life so I am quite possibly infected if the numbers in the article are correct and now spend a small fortune on fancy cat litters to keep that odor at bay. And why do people always want to destroy the cats? Why aren’t they on a mission to kill all the rats so that the cats don’t get infected?

Permalink to Comment

79. Neurontic on February 3, 2006 08:25 PM writes…

So, what types of schizophrenia are thought to be isolated from toxoplasma? And why?

Also, have there been any studies done on cat ladies to substantiate the theory that they may be hooked on the side effects of Toxoplasma?

Permalink to Comment

80. Snarkle on February 4, 2006 10:19 PM writes…

Does this explain all those “previously thought to be morons” who jump into big cat enclosures at zoos . . .

Permalink to Comment

81. No One of Consequence on February 6, 2006 01:05 AM writes…

Holy crap.

There is a short story I read a very, very long time ago, written in the 60s I belive. It posits a very, very unlikely scenario. Cats are the most intelligent sentient species on earth.

You see, a long time ago, cats and dogs were both sentient (and primates weren’t even close). Both species lived on the moon and Earth both. There was a war. Cats won. The moon was blasted. Dogs were reduced to savages and still howl for their moon home today. End the modern mythology bit.

Fun part here. Cats learn how to become immortal. They need a place to store their consciousness while making a new body for themselves (natually, through birth). They found one. Remember those primates?

This was back when people believed you only used 20% of your brain. The author asserted that the other 80% was — you guessed it — hyperintelligent cat spirit hotel. When a cat dies, it sits in your head until it grabs a new body. The protagonist is clued in when he notices cats leaving war zones . . . before the battles begin.

Of course, knowing this information pretty much dooms you. After all, cats are right there in your head. You can’t stop them.

Not exactly the same, of course, but it was an unnerving and entertaining story, and this makes it all the more. . . unnerving.

Schitzophrenia is one of the most horrible diseases I have ever had the misfortune to deal with, and if cats are even remotely linked to it, their unpleasant reputation is earned. And I like cats, by the way.

Permalink to Comment

82. sara oyarce on February 7, 2006 05:21 AM writes…

fascinating article! i’m very interested to see what continues to come from this research. thanks for sharing.

Permalink to Comment

83. Marcel on February 9, 2006 02:23 PM writes…

How do you get rid of the Toxoplasma gondii infection totally… if anyone knows please email me at mpamphile at gmail dot com.

Permalink to Comment

84. Britton Wingfield on February 10, 2006 03:29 PM writes…

I’m just happy to see another Britton in the world 😀

Permalink to Comment

85. Enoch on February 10, 2006 05:29 PM writes…

I wish “Zombies in Cambodia” was a true story.

Too bad it’s an urban legend…


Permalink to Comment

86. joanne on February 11, 2006 02:54 PM writes…

Hmmmm, when I was a kid, our family had a Siamese cat for several years and I was in charge of taking care of the litter box. I never directly handled the stuff, but you never know. Meow!

Permalink to Comment

87. boinkie on February 11, 2006 09:49 PM writes…

1) Toxoplasmosis infection during pregnancy can affect the child and cause mental retardation.
2) Intercranial parasites are known to be a common cause of seizures in the third world.
3) If this fad gets publicized, I’ll have a million neurotic ladies in my office attributing their aches and pains to toxoplasmosis
4) Now, if you really want to be paranoid, how about noticing lack of B vitamins can cause psychosis, mental retardation, etc…
5) pollution, chemical and heavy metals can also cause psychosis, mental retardation, dementia etc.
6) Conclusion: We’re all gonna die…

Permalink to Comment

88. Gagdad Bob on February 12, 2006 10:55 AM writes…

Good point. In fact, the concept of mind parasites is fully explained in my book:


Permalink to Comment

89. Don Meaker on February 12, 2006 01:54 PM writes…

Here we find a reasonable backing for Muslim’s detestation of pets. Of course Muslim males often have a significant dose of paranoia and jealousy already.

Permalink to Comment

90. sonnylove on February 17, 2006 02:06 AM writes…

IF the saying “THe avious answer is usully the right
one” this article explains a whole lot of questions

Permalink to Comment

91. Hector Guinness on February 18, 2006 07:51 PM writes…

Did they not think of the fact that people who have cats are probably insane, so could then catch toxoplasmosis.
more seriously, women who have cats probably tend to be more warm-hearted and outgoing.

Permalink to Comment

92. Kane on February 21, 2006 12:09 PM writes…

Don’t teach me on parasites anymore. I’ve just finished studying biology and I don’t want to bang my head anymore. Ha-ha!

Permalink to Comment

93. Stryder on February 23, 2006 03:00 PM writes…

Interesting read. Makes me think about how ancestral hominids Hunted animals, then ate their kills and potentially adopted superstitious beliefs over how their “warriors” might have inherited the soul or spirit of the animal they killed.

If the animal contained parasites that then became a preportion of the hunter through ingestion, it could attribute to some interesting psychological changes.

Permalink to Comment

94. Stryder on February 23, 2006 03:08 PM writes…

Another thing I just thought of after posting, is in relationship to something someone posted about parasites being rampant in the third world and again is also relative to ancesteral hominids.

“Cannabilism”. In some instance of Cannabilism, human organs were eaten raw to supposedly grant the person eating the soul of that of his victim. Suggestibly the raw human organ could contain parasites, which the person eating would then ingest. (Obivously cooked meat would kill the parasites off)

Perhaps such cultures were suffering “parasicosis”.

Permalink to Comment

95. KJ on March 12, 2006 01:02 PM writes…

“if Toxoplasma can alter the behavior of a rat, could it alter a human? Obviously, this manipulation would not do the parasite any good as an adaptation, since it’s pretty rare for a human to be devoured by a cat.”

Humans not being eaten by (large) cats is only a very recent development.

Permalink to Comment

96. wholesomedick on March 19, 2006 04:42 AM writes…

I think there’s a parasite that makes people ignore the cherps from smoke alarms as the battery dies. If you don’t, then you don’t listen to enough Loveline.

Permalink to Comment

97. mabranich on March 23, 2006 04:13 PM writes…

I was diagnosed with congenital toxoplasmosis in 1955, I have chorioretinitis with a large macular lesion in one eye (and some amblyopia in that eye also) and some smaller peripheral lesions in the other eye. I have led a useful, productive life, even though I am essentially a one-yeyed person (Only peripheral vision in the “bad” eye). Throughout my life my parents and I were terrorized by half-truths, falsehoods, and speculations such as your article. For the sake of those of us who must live with this diagnosis, please be kind and truthful.

Permalink to Comment

98. phatkhat on April 23, 2006 01:33 PM writes…

#89, I have read more than once that the prophet Muhammed was a great lover of cats. Interesting that his followers do not have many pets. (Persian cats came, after all, from Iran.)

Interesting article, though.

Permalink to Comment

TRACKBACKSListed below are links to weblogs that reference The Return of the Puppet Masters:

Carl Zimmer over at The Loom has post discussing Toxoplasma, and how it can affect behavior–and may play a role in the development of schizophrenia. (Check out his post for the details). As Zimmer mentions, this is a controversial hypothesis…. [Read More]

Tracked on January 20, 2006 08:48 AM

Spooooky – and existential from Backseat driving

Carl Zimmer posts about how a parasite present in half the human population may be altering our psychology….The existential part of this comes from my experience many years ago in Southeast Asia when I contracted typhoid, a very unpleasant disease…. [Read More]

Tracked on January 21, 2006 09:46 PM

Return of the Puppet Masters from The Burden of Proof

Is your cat controlling you via its parasites? Different parasitic infections can cause noticeable behavioral changes, e.g. rats that no longer fear cats… [Read More]

Tracked on January 22, 2006 07:49 AM

I’ve always been disturbed by parasites that can take over your body and alter your physiology and behavior in subtle ways. Wasps, for instance, lay eggs on the bodies of caterpillars, which then alter the caterpillar’s behavior so that i… [Read More]

Tracked on January 22, 2006 10:15 AM

If you find yourself craving more more more Adam Roberts than The Valve alone can provide, I encourage you to pop over to Acephalous, where Rich and I will commence stoning him tomorrow. (Translation: We’re book-eventing his novel Stone. Yo…

[Read More]

Tracked on January 22, 2006 09:48 PM

I love cats. I really do. I’m definitely a “cat person”, as opposed to a “dog person”. All the more disturbing to me, then, was finding out via Battlepanda that scientists appear to be hot on the trail of an apparent caus… [Read More]

Tracked on January 25, 2006 12:39 PM

Puppet Masters From Felines from Dean’s World

Oh, now this will get you thinking: there’s some evidence that the toxoplasma parasite, which up to 3 billion humans carry, may alter huma…

[Read More]

Tracked on February 11, 2006 10:03 AM


Remember Me?

Talking at Woods Hole
Invisible Gladiators in the Petri Dish Coliseum
Synthetic Biology–You are There
Manimals, Sticklebacks, and Finches
Jakob the Hobbit?
Grandma Manimal
Hominids for Clinical Trials–The Paper
The Neanderthal Genome Project Begins