Posted by: Tobias | July 31, 2008

Do Parasites Rule the World?

I read Carl Zimmer’s Parasite Rex some time last year and grossed out not just a few of my friends when telling them what I had learned during polite dinner conversation: From parasites that alter the brain chemistry of cats (and possibly humans) so as to make them more sexually promiscuous to the protozoa that cause malaria and almost completely reengineer the red blood cells in which they live to wasps that lay their eggs into living caterpilars, which are then slowly eaten alive by the hatching wasp larvae.

On the one hand, I found it absolutely fascinating what clever strategies parasites have evolved to get from one host to another and to defeat a host’s defenses (an ability which we may soon be able to harness to cure bad cases of allergy). But on the other, there were times when even I got so grossed out that I just had to put the book down for fear of not being able to live a normal life ever again.

And so, in the spirit of sharing, I thought I’d give you, too, a glimpse into the world of parasitism. There is a short Carl Zimmer essay, which is taken from the book and gives you a pretty good idea of how imporant but still underestimated a phenomenon parasitism is. The essay is provocatively entitled “Do Parasites Rule the World?“:

“Every living thing has at least one parasite that lives inside or on it, and many, including humans, have far more. Leopard frogs may harbor a dozen species of parasites, including nematodes in their ears, filarial worms in their veins, and flukes in their kidneys, bladders, and intestines. One species of Mexican parrot carries 30 different species of mites on its feathers alone. Often the parasites themselves have parasites, and some of those parasites have parasites of their own. Scientists have no idea of the exact number of species of parasites, but they do know one fact: Parasites make up the majority of species on Earth. Parasites can take the form of animals, including insects, flatworms, and crustaceans, as well as protozoa, fungi, plants, and viruses and bacteria. By one estimate, parasites may outnumber free-living species four to one. Indeed, the study of life is, for the most part, parasitology.”

Carl Zimmer also writes the excellent blog The Loom, which has become my go-to-blog for anything related to biology and the theory of evolution.


  1. Unbelievable!!

    Is Carl Zimmer’s Parasite Rex as captivating to read as the essay? This is some cool stuff written understandably. If the book is as good I might consider getting it.

    Although disgusted on the one hand (imagine I just went out for lunch with the others to have a nice big burger, thanks for spoiling the ‘after pleasure’ ) I am on the other hand really fascinated. Thos parasites are ‘awesome dude’, imagine what could be going on inside our bodies?! Is there some parasite steering my brains which makes me so attracted to meat in general and the spareribs in particular?! Maybe I just don’t want to know (or is that the parasite talking?)

    On the piece about to cure bad cases of allergy: I rather stick with the offer to be a subject in the research on the joys of ovulation than getting some of those parasites purposefully into my body. Even the comment: ‘’It was fairly itchy when they first go through the skin after that you don’t really notice them.’ hasn’t convinced me. Call me selfish by preferring lap dances over helping finding a cure for asthma, but the parasite running my brain has made up its / my mind!!

  2. If anything the book is even better than the essay. If you want I can lend it to you. I have no immediate need to read it a second time 😉

  3. Would be great!

  4. As a late addendum to your entry: SZ online publishes today an article (in German) on the small liver fluke, and its ability to manipulate the brain and behaviour of its host to ensure its own survival and reproduction.

    In brief: These tiny creatures normally live in the galls of sheep and cattle. To reproduce, they place their eggs in the guts of these animals, so that they exit with the dejection ‘through the backdoor’. After having made their then way through the digestive tract of snails, they enter ants.
    The larvae arising from the eggs here finally find their way into the brain of the ants, where they manipulate the ants’ behaviour. Instead of letting the ant return into its den, the larva induces the ant to climb up a blade of grass, in order to maximize the possibility of being eaten by another sheep. If successful, the whole reproduction procedure starts again.

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