It’s an elegant way to multiply, but not a very efficient one. The flow of blood in the veins where the blood flukes live travels away from the intestines and up to the liver. As a result, it washes away half of the eggs before they can burrow out. They end up in the liver instead, where they form granulomas. But in the liver, the granulomas can do no good for the parasite, and they can end up killing the host. Parasitologists suspect that the blood flukes may actually keep their damage to their host under control by limiting their own numbers. Like their eggs, adult blood flukes also make the body produce TNF-α. The molecule doesn’t do much harm to the adults, but it is lethal to tender young larvae that have just invaded a person but haven’t had a chance to build their defenses. As a result, a person who already harbors blood flukes is far less likely to be infected with a new batch. Apparently, the blood flukes help the immune system attack latecomers of their own species to keep the host from getting overcrowded.
What’s most impressive about a blood fluke is not how many people it cripples or kills, but how it manages to thrive in the vast majority of its hosts while causing them only a little trouble. They are, in fact, selfish guardians.
Only vertebrates have the sort of immune system I’ve been describing up to this point, with its ever-adapting B and T cells. Invertebrate animals—everything from starfish to lobsters to earthworms to dragonflies to jellyfish—branched away from our own ancestors over 700 million years ago and evolved powerful defenses of their own. Insects, for example, bury intruders in a blanket of cells that ooze out poisons. Eventually the cells form a suffocating seal around the parasite. The parasites that specialize in invertebrates have adapted to their peculiar immune systems, with subterfuges as cunning as anything they use on humans.
One of the best-studied cases is that of the parasitic wasp Cotesia congregata. This mosquito-sized wasp uses the tobacco hornworm for its host, a tubby green caterpillar with black hooks on its feet and an orange prong sticking up from its back end like a horn. Scientists have studied this host and parasite so closely because the hornworm is a champion pest, devouring not just tobacco but tomatoes and other vegetables. It is also so big that scientists can simply mash it onto a slide to see what’s going on inside.
The attack of a Cotesia wasp is so fast you’re unlikely to catch it. It lands on a hornworm, crawls up its flank a short way, and stabs its egg-laying syringe into the host. The hornworm may squirm a bit to fight off the wasp, but to no avail. The wasp’s eggs hatch inside the hornworm as cigar-shaped larvae. They sip their host’s blood while breathing through silvery balloons of tissue on their back ends. The tobacco hornworm has a vibrant immune system, and yet the wasp young go about their business unmolested. But it’s not the larvae themselves that stop the immune system. For that, they need a gift from their mother.
The mother wasp injects the eggs as part of a soupy mix. The eggs depend on the soup for their survivaclass="underline" if you take out the eggs, clean off the soup, and then put them directly into a caterpillar, the host’s immune system rages full tilt and mummifies the eggs. The parasite survives thanks to millions of viruses swimming in the soup. These viruses are not much like the ones that we’re familiar with—the sort that cause a cold, for example. A cold virus wanders from host to host, invading the cells in the lining of the nose and throat, and then commandeering the cell’s own proteins in order to make new copies of the virus. Other viruses, like HIV, go so far as to stitch their genes into the DNA of their host cell and make copies of themselves from there. A few go even further: their hosts are born with the virus’s DNA already embedded in their own genes and transmit it to their children.
The viruses of parasitic wasps are stranger still. The wasps are born with the virus’s genetic code scattered across many of their chromosomes. In males the instructions stay in this scattered form. But as soon as a female begins to take its adult form in her pupa, the virus awakens. In certain cells of her ovary, the pieces of the virus’s genome are cut out of the wasp DNA and sewn together, like chapters assembled into a complete viral book. These genes then direct the formation of actual viruses—strands of DNA encased in a protein shell, in other words—and these viruses begin to load up inside the nucleus of the ovary cell. When the nucleus is filled to capacity, the entire cell bursts open, and millions of the viruses float free in the wasp’s ovary.
But they don’t make a female wasp sick. The wasp actually uses them as a weapon against the tobacco hornworm. When it injects the viruses into a caterpillar along with its eggs, the viruses start invading the host’s cells in a matter of minutes. They commandeer the host’s DNA, forcing the cells to make strange new proteins normally never seen inside a hornworm, which flood the body cavity of the caterpillar. These proteins destroy the hornworm’s immune system. The cells start sticking to one another instead of to the parasites, and then they burst open. The host is left as immunologically helpless as a person with full-blown AIDS (which is also caused by a virus that blows apart immune cells). Thanks to the virus, the wasp eggs can hatch and begin to grow without any harrassment by their host.
But unlike a person infected with AIDS, the hornworm recovers from the wasp virus after a few days. By then, the wasp larvae seem to be able to handle the immune system on their own, without help from mother. They may fool their host in ways similar to the ways blood flukes fool us, by borrowing the insect’s own proteins or by mimicking them.
It may seem perverse for a virus to do the dirty work for another organism, even going so far as wiping out a host’s immune system only to be wiped out itself. But within every egg that the virus protects, there are instructions for making new viruses that will survive if some viruses attack the host. At the same time, though, it may be wrong to think of a virus as a separate organism with its own evolutionary ends. The truth may be even more perverse, for the virus’s DNA resembles some of the wasp’s own genes. The resemblance may actually be hereditary: the virus may descend from a fragment of wasp DNA that mutated into a form that escaped from the normal way genes are copied and stored. It may not be strictly correct to call the viruses viruses at all—they may represent a new way that wasps package their own DNA. (One scientist has suggested calling the viruses genetic secretions.) If that’s the case, then parasitic wasps are managing to insert their own genes into another animal’s cells to make it a better place for the wasps’ to live.
These wasps may seem as if they belong on another planet, but they actually demonstrate a universal quality to parasites here on Earth: parasites find ways to battle immune systems, tailored precisely to the peculiarities of their host. Whether they end up killing or sparing their hosts depends on how they can best make more of themselves.
4
A Precise Horror
You still don’t know what you’re dealing with, do you? Perfect organism. Its structural perfection is matched only by its hostility … I admire its purity; unclouded by conscience, remorse, or delusions of morality.