Parasites Use Sophisticated Biochemistry to Take Over Their Hosts


In the rain forests of Costa Rica lives Anelosimus octavius, a species of spider that sometimes displays a strange and ghoulish habit.

From time to time these spiders abandon their own web and build a radically different one, a home not for the spider but for a parasitic wasp that has been living inside it. Then the spider dies — a zombie architect, its brain hijacked by its parasitic invader — and out of its body crawls the wasp’s larva, which has been growing inside it all this time.

The current issue of the prestigious Journal of Experimental Biology is entirely dedicated to such examples of zombies in nature. They are far from rare. Viruses, fungi, protozoans, wasps, tapeworms and a vast number of other parasites can control the brains of their hosts and get them to do their bidding. But only recently have scientists started to work out the sophisticated biochemistry that the parasites use.

“The knowledge that parasites can manipulate their hosts is old. The new part is how they do it,” said Shelley Adamo of Dalhousie University in Nova Scotia, a co-editor of the new issue. “The last 5 to 10 years have really been exciting.”

In the case of the Costa Rican spider, the new web is splendidly suited to its wasp invader. Unlike the spider’s normal web, mostly a tangle of threads, this one has a platform topped by a thick sheet that protects it from the rain. The wasp larva crawls to the edge of the platform and spins a cocoon that hangs down through an opening that the spider has kindly provided for the parasite.

Written By: Carl Zimmer
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  1. Fascinating.   Loved reading more about SuperRat.  He knows no fear, he’s sexy, the ratettes adore him.  He’s a super rat hero!  Oh, look.  The cat got him….

    Imagine a cautionary tale told by rat parents to their offspring (Yes, I know, it’s  anthropomorphist.  Get over it), about the rat who sold his soul to the devil.  He wanted to be big and strong, fearless, and attractive to the females.   Devil showed him some Good Shit to eat, and his wish came true, for the rest of his short life.

    I wonder if having some of these super-rats around could somehow be beneficial to the uninfected?  Gives the cats someone else to eat instead of me, and generally serves an educational purpose, as a counter-example of how to behave.

  2. All parasites will have some positive effect on the uninfected to some extent. They inevitably will move the animal from the niche it has been prepared for by evolution – it could simply be using up some resources better used elsewhere, therefore just making it slightly less effective at living – whether that be a bit slower, producing a few fewer sperm or eggs, or having to spend more time eating than it otherwise would, or it could be directly modified by the parasite to its detriment – wasp larvae eating insects before they reach adulthood, or the super-rat referenced. Inevitably the other animals competing in the niche benefit from the lack of competition, and, save species whose individuals are extremely isolated, the other competing animals are the uninfected from the same species.
    Of course there may be a down-side for the uninfected too – but I suspect that is only likely in (a) very small surviving groups where the loss of an individual could push the group to extinction (b) very complex social groups dependant on certain individuals to some extent for success. But the effect is likely to be minimal (in the case of (a) they’re likely to be on the way out anyway (b) they’d have to be very unlucky to end up with the more important individual being infected rather than the plebs, particularly as important individuals are likely to have best access to food etc and therefore less likely to contract or succumb to infection. 

    Obviously it could have a downside for those who remain (in human terms – what if Churchill died from malaria in 1940?) but is much more likely to be an upside (for example, the black death caused a blossoming of wealth and culture as the survivors gained more resources).

  3. Thanks for the thoughtful response.  Though you did raise the spectre of an Important Individual being infected by some psycho-active parasite.  Stuff of science fiction, I hope.   Though – what if there was a parasite that wanted lots of human corpses, and its way to get them was to compel its hosts into setting up a genocidal regime….

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