Disease-Spreading Zombies

Are you a disease-spreading zombie?

You may have read about the fungus that can infect an ant and turn it into a zombie, making it climb up the stem of a plant and hang onto it, then die and release spores from a stalk that grows out of its head.

But this isn’t the only parasite that controls the behavior of its host.

If you ever got sick, had diarrhea, and thought hard about why, you’ll understand what I mean. You were helping spread the disease… especially if you were poor and didn’t have a toilet. This is why improved sanitation actually reduces the virulence of some diseases: it’s no longer such a good strategy for bacteria to cause diarrhea, so they evolve away from it!

There are plenty of other examples. Lots of diseases make you sneeze or cough, spreading the germs to other people. The rabies virus drives dogs crazy and makes them want to bite. There’s a parasitic flatworm that makes ants want to climb to the top of a blade of grass, lock their jaws onto it and wait there until they get eaten by a sheep! But the protozoan Toxoplasma gondii is more mysterious.

It causes a disease called toxoplasmosis. You can get it from cats, you can get it from eating infected meat, and you can even inherit it from your mother.

Lots of people have it: somewhere between 1/3 and 1/2 of everyone in the world!

A while back, the Czech scientist Jaroslav Flegr did some experiments. He found that people who tested positive for this parasite have slower reaction times. But even more interestingly, he claims that men with the parasite are more introverted, suspicious, oblivious to other people’s opinions of them, and inclined to disregard rules… while infected women, are more outgoing, trusting, image-conscious, and rule-abiding than uninfected women!

What could explain this?

The disease is carried by both cats and mice. Cats catch it by eating mice. The disease causes behavior changes in mice: they seem to become more anxious and run around more. This may increase their chance of getting eaten by a cat and passing on the disease. But we are genetically similar to mice… so we too may become more anxious when we’re infected with this disease. And men and women may act differently when they’re anxious.

It’s just a theory so far. Nonetheless, I won’t be surprised to hear there are parasites that affect our behavior in subtle ways. I don’t know if viruses or bacteria are sophisticated enough to trigger changes in behavior more subtle than diarrhea… but there are always lots of bacteria in your body, about 10 times as many as actual human cells. Many of these belong to unidentified species. And as long as they don’t cause obvious pathologies, doctors have had little reason to study them.

As for viruses, don’t forget that about 8% of your DNA is made of viruses that once copied themselves into your ancestors’ genome. They’re called endogenous retroviruses, and I find them very spooky and fascinating. Once they get embedded in our DNA, they can’t always get back out: a lot of them are defective, containing deletions or nonsense mutations. But some may still be able to get back out. And there are hints that some are implicated in certain kinds of cancer and autoimmune disease.

Even more intriguingly, a 2004 study reported that antibodies to endogenous retroviruses were more common in people with schizophrenia! And the cerebrospinal fluid of people who’d recently gotten schizophrenia contained levels of a key enzyme used by retroviruses, reverse transcriptase, four times higher than control subjects.

So it’s possible—just possible—that some viruses, either free-living or built into our DNA, may change our behavior in subtle ways that increase their chance of spreading.

For more on Jaroslav Flegr’s research, read this fascinating article:

• Kathleen MacAuliffe, How your cat is making you crazy, The Atlantic, March 2012.

Among other things you’ll read about the parasitologists
Glenn McConkey and Joanne Webster, who have shown that Toxoplasma gondii has two genes that allow it to crank up production of the neurotransmitter dopamine in the host’s brain. It seems this makes rats feel pleasure when they smell a cat!

(Do you like cats? Hmm.)

Of course, in business and politics we see many examples of ‘parasites’ that hijack organizations and change these organizations’ behavior to benefit themselves. It’s not nice. But it’s natural.

So even if you aren’t a disease-spreading zombie, it’s quite possible you’re dealing with them on a regular basis.

6 Responses to Disease-Spreading Zombies

  1. daedalus2u says:

    This is interesting. I wonder if some of the neurodegenerative disorders are evolved compensatory responses to brain infections?

    Maybe that is why the protein inclusions that are characteristic of neurodegenerative disorders are so toxic, it isn’t a bug, it is a feature.

    Amyloid is a pretty good antibacterial agent.


  2. romain says:

    I had never heard about this before, that’s a great story! I very much liked the idea that T. Gondii may increase the dopamine production. Dopamine is ubiquitous in the brain and is much involved in decision-making. To my knowledge, most studies in neuroscience about the mechanisms of decision-making involve dopamine.
    Here’s a short account by the BBC:

    Dopamine levels in brain ‘influence decision making’

    Also, a short comment: for a disease to spread, it seems like a very good idea to act differentially on genders. Even if the description of the change in behaviour in the article by Kathleen MacAuliffe is not quantified, it seems that it makes women more attractive (or more visible) and men much less. So that males will be more attracted to infected females who will then select only healthy males, therefore optimizing the infection rate.
    If the effect was the same on both genders, either it would decrease the infection rate (if infected subjects are less visible) or increase the rate of infection of already infected subjects (if infected subjects are all more visible).

    This somehow makes a link with evolutionary game theory!

    • John Baez says:

      That BBC story is interesting, thanks!

      They talk about how in decision-making we use dopamine signals to estimate the pleasure to be gained from our choices, and near the end they say:

      Professor John Maule, an expert in decision making, at Leeds University Business School, said that in recent years people had begun to realise emotional or “gut instinct” decision making was just as important in human choices as analytical decision making.

      Just in recent years? If so, it’s about time! We are not disembodied ‘rational agents’ who primarily think by manipulating symbols according to rules; we’re a complex stew in which chemical signals play a big role!

      People are just starting to learn how hormones like testosterone and cortisol affect decisions in the financial markets.

      A truly scientific approach to economics would ultimately need to take this kind of thing into account. Economists do talk about ‘animal spirits’, but I’m not sure how carefully they try to model them.

  3. Nathan Urban says:

    Carl Zimmer’s book Parasite Rex may be of interest to readers of this blog post.

  4. Very interesting topic, and nice examples! Another face of parasitism is that sometimes the parasite is very adapted to its host, and sometimes it’s the fact that humans are the wrong host that makes it deadly for us. Usually the problem is that while in the usual host the parasite tricks the immune system in a specific way, in humans it will make the immune system go crazy and cause anaphylactic attacks. This is the case of Echinococcus and I think of Telazia (a worm living in the eyes of dogs, transmitted to humans by a fly who drinks the tears of the dogs…. in humans it lives ok, but if it is disrupted in the attempt to take it out, the worm causes anaphylactic attacks).

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