This blog contains material I wrote and posted on between the years 2005 and 2011 only. It does not contain any new material. For newer writing, please check my main blog (Bill the Butcher).

Saturday, 24 November 2012

Parasite Rex: Review

Author:Carl Zimmer
A few mornings ago, from my bathroom window, I was watching a large white spider scurry along the garden wall. It caught my attention because it seemed to be rather a strange place and time for a large white spider to be running along; eight am on a sunlit morning, when it would be highly visible to birds and lizards; and besides, I’d rarely seen a spider run so fast. The spider jumped off the wall and began running across the garden path, just below my window; and then I noticed that it was being followed by a tiny insect, brilliant blue-green in colour and running even faster than the spider. As i watched, it jumped on the spider, knocked it over, and within seconds chewed off all its legs. That done, it flew away, carrying what was left of the spider (still much bigger than itself). Only the eight white legs were left behind.

That blue-green insect must have been one of the thousands of species of parasitic wasps, and its larvae are now probably eating that limbless, paralysed spider alive. A horrible thing, isn’t it?

Come to think of it, isn’t all parasitism a horrible thing?

Well, not according to the book I’m reviewing here. Carl Zimmer, a science writer and ex-Discover Magazine senior editor, has taken a look into the world of parasites which he apparently finds just as fascinating as I do (and long-time readers of mine know I love so-called “horrible” things like those fascinating parasites). Also, since I already have a fair amount of knowledge of this stuff, it’s even more entertaining and interesting to read Zimmer’s handling of the topic and his conclusions than if it were a field about which I knew nothing at all.

OK, so what is it about, anyway? Zimmer, in this book, subtitled “Inside The Bizarre World Of Nature’s Most Dangerous Creatures”, walks us through the blizzard of parasites that affect the human body and of other animals and plants, starting from a Sudanese boy called Justin who all but died of sleeping sickness. Zimmer describes most organisms as walking packets of parasites, which in turn are packets of parasites of their own. He describes parasites fascinating and weird and terrible; he talks to scientists who spend their lives studying them. He describes how parasites used to be thought of as no more than degenerates, creatures lost in sloth and apathy, and how modern science has shown how sophisticated a lifestyle parasitism really is.

Parasitism, Zimmer demonstrates, is really a constant battle, on many fronts. First, it’s a battle to get into a new host; then, it’s a battle to establish itself within the new host, fighting the body’s defences, the immune system and the digestive juices and all the other weapons we have against foreign invaders. And then it’s a battle to find and locate a mate and yet another battle to find a way to send one’s offspring safely on their way so that they can parasitise more hosts. And, in creatures like flukes and others which have not one but a succession of intermediate hosts, every single stage has its own sequence of distinct battles.

One of the most fascinating aspects of parasitism, which Zimmer documents in some detail, is how so many parasites actually succeed in modifying the behaviour of their hosts so that those hosts actually become little more than zombies controlled by the parasites. For instance, many species of parasitic flukes use water snails as an intermediate host, but can’t reach their final hosts (birds or animals like sheep or cows) unless the host snail is eaten by them. To do this they make the snails foolhardy and reckless, coming out of the water and climbing onto grasses and so on where they can be easily eaten. Another species of parasite, whose final host is a cat, does the same to rats. And as I said some time ago in a blog post, there’s a kind of parasitic wasp that can make the caterpillar in whose body it lays its eggs stand guard over the wasp cocoons until they hatch. There’s even evidence that a parasite called Toxoplasma gondi, which affects billions of people but does virtually no harm (it’s a cat parasite that only accidentally reaches humans) can affect human behaviour and reduce natural inhibitions. This

Zimmer shows how, in fact, parasites aren’t stowaways in an ecosystem but a vital part of them; in fact, there’s good reason to believe that parasites are essential to the health of any ecosystem in many ways. They can keep populations of some species in check (and have been used as biological pest controls) and help others to survive; for instance, by making infected killifish expose themselves to be eaten by water birds (their permanent host) a species of worm increases the food supply of the birds and helps them maintain higher populations.

Also, we tropical people have all suffered from parasites at one time or another; I’ve myself had roundworm, pinworm, and on one celebrated occasion I went three days with a hard-shelled tick stuck to my throat, drinking blood (I was on a jungle camping trip and only discovered it after I could strip down afterwards). Zimmer claims that parasites, in fact, keep our immune systems up to scratch; people with a manageable parasite count never suffer from such autoimmune disorders as Crohn’s Disease (the most reliable cure for which seems to be artificially infecting sufferers with intestinal worms). That’s as may be; but the average tropical person doesn’t have the “yuck” reaction to parasites that someone from more salubrious lands might have.

Another point about parasites needs to be made; they are THE apex predators of the living kingdoms. If you think about a food pyramid, it starts from the most basic bacteria along with algae and primitive plants that take water and nutrients from the environment, and oxygen from the air, and make food with the help of sunlight, which they use to sustain their bodies. These bodies will be in turn consumed, either directly or in the form of nutrients created when their owners die and decay, by more complex organisms, plants and animals. A rabbit eats grass; the rabbit dies and its carcass is eaten by a pig which is eaten by a wolf, for instance. In the wolf’s stomach is a tapeworm, which is the ultimate beneficiary of the chain. Fine so far?

The significance of this point is the fact that if the soil in which the grass grows is polluted, then the grass will have taken up some of the pollutants and will pass them on to the rabbit. A pig that has eaten several dead rabbits will gather rather more of the pollutant in its tissues over time. A wolf that eats pigs over years will get a larger dose yet of the pollutants. And the tapeworm? Well, it’s going to be blitzed by so many pollutants that it’s unlikely to survive. If you find a wolf population without tapeworms, the ecosystem is in trouble.

So, parasites are actually an indicator of the health of an environment, is the ultimate conclusion. Parasites are respectable, honourable, even essential, and we’d better recognise that.

I have two quibbles with the book, due to which I’m only giving it 4 stars. First, it needs diagrams. There are a few black-and-white plates in the centre but apart from exploiting the “yuck” factor they have absolutely nothing to contribute to the narrative. Most of the biological explanations are comprehensible to me but I recognise that it’s because of my medical training; as an average reader I’d get utterly lost in Zimmer’s completely unillustrated explanation of the workings of the immune system, for instance. A few basic diagrams, such as from a zoology textbook, would have been a much better idea than a photo of a human brain with a botfly larva in it.

The second problem, and a major one, is that Zimmer almost completely neglects ectoparasites. He goes into dissertations on parasites that live inside the bodies of others; but if at all he mentions leeches, fleas, lice, mites and ticks (like my neck-sucker) it’s only in passing. The omission is not huge; it’s colossal, enormous, gigantic. Some endoparasites like Plasmodium cause millions of deaths by malaria, but let’s not forget the Black Death (bubonic plague) that wiped out much of Europe a few hundred years ago. What caused it? The bacterium Yersinia pestis, which is spread by the rat flea.

(There’s a third but entirely personal grouse, and it won’t really come as a surprise to those who know me. I know Zimmer is an American and so he wrote in American English with the American “system” of weights and measures. But the issue I read was published in Britain by Random House UK, and the least I’d have expected them to do – just as American editions of the works of British authors invariably Americanise the spellings – would be to convert American English into English. It’s incredibly irritating to read “color” instead of “colour”, “meter” in place of “metre”, “aluminum” instead of “aluminium” or “three hundred fifty” instead of “three hundred and fifty.” I wouldn’t have minded so much if it was an American edition, because then I’d have expected it.)

Overall, I’d recommend this book, but only if you’re interested in the subject. The blurb on the back recommends that it’s a book to read if you wish to “stun your friends at social gatherings”, but that’s the LAST reason anyone should have to read a book – this or any other.

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