In the developed world at least, most people probably don’t think about parasites much, because they aren’t plagued by them to anything like the degree people were just a few generations ago (or still are in the developing world). Occasionally something will happen to remind us of their existence and general ickiness–a local school has a head lice problem, New York City has a bedbug outbreak–but mostly they’re not a part of our day to day lives.
Now partly that depends on how you want to define “parasite.” If it’s just any living thing that spends at least part of its life cycle living off of or inside of some other living thing, then that would include various bacteria, fungi, or even viruses. And those certainly do still plague us.
But most people probably don’t think of the AIDS virus, for example, when they think of a parasite. So we could define parasite more narrowly to limit it to more substantial living things like insects, or at least protozoa, to bring it more into line with common usage.
Author Carl Zimmer acknowledges both senses of the word in Parasite Rex, but mostly sticks to talking about cases that fall within the narrower definition.
It turns out that parasites, in that sense, are extremely prolific, and can be at least as complex or advanced in an evolutionary sense as non-parasite species. Though it’s hard to know for sure since so many animal species have not yet been catalogued, there are likely considerably more parasite species than non-parasite species in the world. As far as their complexity, the bulk of this book consists of descriptions of some of the intriguing little parasitic monsters that natural selection has given us, which are certainly quite complex.
They’re durable too, by the way. Just like we talk about how cockroaches would survive a nuclear holocaust that wipes out humans and most other species, various parasite species have survived catastrophic Earth events and mass extinctions. When dinosaurs or whatever are erased, plenty of parasite species succeed in adapting to new hosts instead, and life goes on.
Living off of other creatures turns out to be in some ways an easier gig than trying to make it in the outside world. There are challenges to it–animals, including humans, have immune systems precisely to defend against such intruders–but parasites have generally been quite good at evolving to be able to deal with those challenges.
At least as well as prey animals evolve to be able to deal with predators, I’d say, and probably better. Indeed, for much of the book I was wondering why there seems to be no so-called “arms race” in this context. From all the author’s talk about successful parasites, it seems like parasite versus host is a very one-sided affair compared to the way predators and prey continually evolve roughly equally efficiently in response to each other.
Later in the book there are indeed a few examples of ways natural selection has made hosts better able to fend off parasites, but one still gets the impression that it’s far from an even competition, that most of the time parasites have the edge and are able to have their way with hosts.
Zimmer takes an obvious delight in telling about some of the more bizarre creatures he’s learned about in his research.
Take parasitic wasps, for instance. And this isn’t some rare exotic species in New Guinea or something; apparently there are a huge number of species of wasp that pursue this particular parasitic lifestyle.
These wasps sting the back of a caterpillar or other suitable host, though it’s not really a conventional stinger but more of a hollow tube that the wasp uses to squirt eggs into the body of the caterpillar. When the eggs hatch, the wasp larvae eat the caterpillar from the inside.
It’s like the single creepiest scene in the Stephen King movie The Mist, where the spider-like alien creature has still-living soldiers strung up in its web, and we see a mass of tiny spider-like things burst out of their bellies from the inside, evidently having come from eggs laid inside the horrified men.
The most successful parasites have evolved to keep the host alive as long as they need it and no longer. So in deciding what parts of it to eat, what trouble to cause, etc., they preserve its ability to eat, and to flee from or fight off predators and such, but anything not essential to the host’s survival is fair game. For instance, it doesn’t help them for the host to waste its time and energy mating and giving birth and all that, so they often eat the genitalia.
Some parasites also release chemicals to influence the brain of the host in their favor. Once they’ve eaten the genitalia, for instance, they might lay their own eggs roughly where the host itself would have laid its eggs had it not been neutered, and then they influence the host’s maternal instinct to kick in so that it will protect these eggs and treat them as its own. In some cases they’re even able to influence a male host they’ve neutered to behave like a female and similarly protect the parasite eggs.
Or say the parasite needs to be in a certain insect for part of its life cycle, but then needs to be swallowed by a cow for the next stage. When it’s ready to make the switch, it alters the brain chemistry of the insect so that it compulsively seeks out the highest point on a blade of grass where cows graze and waits there passively to be killed.
Guinea worms are particularly interesting and gruesome. After they do their thing and grow to a ridiculous length inside a human body, and it’s time to move on, they make their way to the leg, and burrow up through the skin to make their escape. (The opening in the leg carries with it a painful, burning sensation, designed to encourage the host to submerge it in water, since the next stage of the worm’s life is to release its larvae in water.) It takes them a long time to very gradually ease their way out, and you have little choice but to wait for them to do so. If you try to yank them out or kill them, all that happens is now you have a few inches or feet of dead worm inside your leg, which is an invitation to all kinds of delightful infections and such.
The treatment, such as it is, is to place a stick along the hole in your leg to entice the worm to use that to wrap itself around to facilitate its pulling its way out of your leg. At least that way it may get out a little faster.
This treatment goes back to ancient times, and may be the origin of the Rod of Asclepius from Greek mythology, the staff that is associated with the medical profession. (This is misidentified as a caduceus in the book, a slightly different symbol associated with the messenger god Hermes.) Most people naturally assume that the animal entwined around the staff in the Rod of Asclepius symbol is some sort of serpent or snake, but it’s possible it originally represented a guinea worm being eased out of a body.
For the most part the book is highly interesting and readable. At times for me the descriptions of the parasites threatened to get just a little repetitious as the novelty and shock value gradually wore off, but then the author delves into related matters about parasites that kept me interested.
For one of multiple examples, he includes a discussion of using parasites beneficially, as a sort of biological pesticide.
Let’s say some insect pest is ravaging the crops in a certain area because it has no natural enemies there. If we can identify a parasite that would be interested in infesting it–e.g., a parasitic wasp that likes to lay its eggs inside that kind of insect–in theory we could then introduce that species of parasite into the area in sufficient numbers to significantly reduce the population of that insect.
The author first relates an impressive success story of that kind of biological pesticide strategy, but then it turns out that most of the time it doesn’t work. The unintended consequences of that kind of thing are typically too hard to anticipate and control, and you can end up inadvertently doing a lot more harm than good. So where he leaves it is basically that this is an approach that has occasionally worked out very well, and certainly has promise for the future, but that for the most part it’s not such a good idea to implement yet. That is, it’s worthy of further study, and of keeping an open mind toward, but at present it’s not something to use in any but quite rare cases.
The book is now over a decade old, so it’s quite possible that situation has changed. But that’s where it stood back then.
In thinking about this book, one of the things I found running through my mind is just how completely mindless and unintentional natural selection is, and indeed almost all living creatures are.
It’s natural when talking and writing about these things–and the author does it in the book as I do it here–to describe living things and their evolution as if there were some sort of goal-seeking behavior to it all. But–with the exception of humans and maybe to a very limited extent a few other of the higher species of apes and such–there simply isn’t. That kind of descriptive language is metaphorical, or it’s a way to make the creatures a little more entertaining or easy to relate to by rhetorically anthropomorphizing them. You have to remind yourself that none of that stuff is literally true.
For example, a parasite doesn’t really release a certain chemical into the host’s bloodstream so that it will go to the brain and alter the brain’s chemistry to make it behave in a way that is convenient for the parasite. Attributing intent like that is kind of a fun way to describe it, as if the parasite were a little person like us, but it doesn’t literally operate according to purposes like that the way we (sometimes) do.
More literally what’s going on is the overwhelming majority of the parasites release certain chemicals or whatever for no rhyme or reason whatsoever except that that’s just what their bodies have always done. Then some fraction of 1% of them have some mutation–again for no purpose whatsoever but just because mathematically a tiny proportion of any species when it reproduces will have random mutations–which causes them to release a slightly different chemical or none at all or do something else entirely. In the rare instances where such a mutation results in the parasite being more likely to reproduce successfully, the proportion of the species that has that mutation will gradually increase, while the proportion of the species that has some other mutation or that is like the majority will gradually decrease and possibly die out entirely. Over time most or all of members of the parasite species release a chemical that causes the ant they’re inside to seek out higher ground where cattle are grazing, but it’s not like the parasite has the foggiest idea any of that is happening.
It’s not like a wasp is thinking “Hmm, if I stick my eggs in this species of caterpillar rather than that, more of them will hatch and develop to adulthood,” or a protozoan is intentionally eating this host’s testicles rather than its liver because it knows the host will stay alive longer that way and serve as a convenient place for it to live.
Living things, and certainly processes like evolution, don’t have intentions and reasons and such for what they do (with the aforementioned rare exceptions, like humans, and even them not all that often). A protozoan doesn’t literally “hide” from white blood cells the way a human soldier camouflages himself to be harder for an enemy to spot, any more than a fire “consumes” wood the way a fat guy consumes cheeseburgers. We might use similar language, but when we do we should be aware that we’re dealing in metaphors.
These parasites just do whatever completely mindless thing they do, and depending on the consequences of doing so they either survive or they don’t. They’re not doing something clever, or goal-oriented, or anything like that, any more than an inanimate object like a pebble is intentionally behaving in a way to end up in a certain place in a pile of pebbles. Depending on its size and shape and the timing of the wind or earthquake or whatever that moves it, it might end up at the bottom or the top of the pile, but you wouldn’t want to admire the pebble’s intelligence and resourcefulness in getting to where it gets, as if it were capable of giving a crap about its location or choosing means to the end of getting there.
In some ways I think this kind of book encourages a certain humility in people, a tendency to recognize that these creatures we share the Earth with are complex like us and have cleverly worked out their own way of living and surviving in their niche the way we have in ours. I don’t react that way, though. This kind of material doesn’t make me feel a kinship with other living things. What it brings home to me is that in the ways that matter most, the overwhelming majority of living things are utterly unlike me, just as rocks and ammonia and rain clouds are. The dividing line between living and non-living is a pretty insignificant one to me. What matters are things like rationality and intelligence.
To react in the more common way is to forget that the anthropomorphizing language we use in talking about living things like parasites isn’t literal, but is metaphorical, or shorthand, or cutesy. They aren’t little people; they’re mindless, purposeless objects.