Parasite Rex

It might surprise and horrify you to learn that your whole body is crawling with parasites. This is nothing about not having washed your hair properly! Most of the micro-organisms are beneficial, indeed necessary, to the proper functioning of ‘your’ body. I parenthesise ‘your’ because the organism you prefer to call ‘you’ is really an eco-system. The proportion of purely human cells in the body, if concentrated in one segment, would occupy the left leg from knee to toe. Most of the other (non-human) cells constantly switch between offering beneficial effects and sucking resources.


The whole subject of parasitism is fascinating. Charles Darwin spent eight years researching barnacles, which he saw as a degenerate form of arthropod. Barnacles settle as larvae on the side of another animal, like a crab or whale which thenceforth offers locomotion. The barnacle cements itself to the body, loses most of its front end and spends the rest of its life beating its legs to bring food particles into its tough little volcano-shaped shell. What an opportunistic little bastard!


Darwin had only just glimpsed the devious horror of nature. Take the crustacean parasite called ‘sacculina’ ( This begins life as a free-swimming larva. It then settles on a crab, crawls to a joint in its leg, poking a hole in it. Squeezing her soft parts through it into the crab, she then makes her slug-like way to the abdomen and begins to feed on the nutrients. She then forms a bulge in its shell as she grows sending extensions of her own body, called ‘roots’, throughout the crab’s body, even to its eye stalks. Soon the crab can’t grow nor make eggs or sperm. The ‘walking corpse’ lives only to serve its pitiless assassin. A pinhole opening in the crab’s abdomen, made by the female sacculina, attracts the tiny male, who similarly injects himself into the host. They fertilize each other for the rest of their lives; but to intensify the horror, they manipulate the crab’s hormonal system so that the crab periodically climbs a high rock, squeezes out the parasite’s larvae and waves its claws in the water to speed them on their way, as they would do for its own offspring. Its helpless waving, directed by an internal puppeteer, is ignored by the other marine creatures who are battling out their own savage existence.


Horrors like this pile up through page after page of Carl Zimmer’s ( book, Parasite Rex. Repellent though it all is, it’s also an appreciation of the clever strategies used by this life form. We know comparatively little about the evolutionary history of most parasites; worms, leeches and flukes don’t fossilize well! But we do know that although they have lost many body parts and the structural complexity of their relatives, they have retained a formidable behavioural complexity. That behavioural flexibility became the main strategy of parasite evolution: it allowed them to become insidious. They can so change a host’s personality that rats will boldly sniff hay stained with cat urine, for example.



The detective work behind Zimmer’s story is fascinating and frequently ingenious. He follows medics treating sleeping sickness in Sudan, fieldworkers assessing internal parasites in Costa Rica, curators tending the National Parasite Collection in a disused guinea pig barn in Maryland. There seems to be no limit to how rapidly the bugs can spread, how they can jump among hosts and how they can complicate their life cycles to avoid eradication. Against their evolutionary life story, our human history seems little more than a burst of opportunity for them. Blood flukes have passed between snails and rats at watering holes on the African savanna for millions of years. Zimmer shows that when our hominid ancestors wandered out from the forests and stopped at watering holes, the flukes were only too happy to diversify their portfolio and infest a new host.


Zimmer concludes by documenting recent attempts to reduce parasitism on domesticated animals and crops by giving the parasites a taste of their own medicine. Introducing parasites of parasites has also been used to cut populations of invading or ‘weed’ species of plants and animals that are destroying or out-competing native species. This is a brilliant and accessible presentation of the new science of evolutionary parasitology. It’s also a thoughtful treatment of the global strategies and policies that scientists, health workers and governments will have to consider in order to manage parasites in the future.


For a 43 minute podcast introduction to parasitism, listen to the BBC Radio 4 ‘In Our Time’ programme on the subject. Chaired by Melvyn Bragg, its contributors are Steve Jones (Emeritus Professor of Genetics at University College, London), Wendy Gibson (Professor of Protozoology at the University of Bristol) and Kayla King (Associate Professor in the Department of Zoology at the University of Oxford). Available at this link


Enquire at your local library. See if this important science title is in stock by consulting the online catalogue at



320 pages in Simon & Schuster

First published 2001

ISBN  978-0743200110



Carl Zimmer



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