Trichinella is also a biological renovator, and in some ways it’s more impressive than Plasmodium: it’s a multicellular animal that can live inside a single cell. When this worm hatches from an egg in its host’s gut, it drills through the intestinal wall and travels the body through the circulatory system. It follows the flow into the fine capillaries, where it leaves the bloodstream and works its way into the muscles. It crawls along the long muscle fibers and then penetrates one of the long, spindle-shaped cells that make them up. In the 1840s, when scientists first recognized Trichinella’s cysts lodged in muscles, they thought the tissue had degenerated and that the parasite slept inside, simply waiting to reach its final host. At first, the invaded muscle cell does seem to atrophy. The proteins that serve as the scaffolding of the cell and make it rigid fade away. The muscle’s own DNA loses its power to make new proteins, and within a few days after the worm has entered, the muscle changes from wiry to smooth and disorganized.
But the parasite is only tearing down the cell so that it can rebuild it. Trichinella doesn’t disable its host’s genes—in fact, they start copying themselves until they’ve quadrupled. But this abundance of genes now follows Trichinella’s commands, making proteins that will turn the cell into a proper home for the parasite. Scientists once thought this kind of genetic control was limited to viruses, which use their host’s DNA to make more copies of themselves. Trichinella, they now realize, is a viral animal.
Trichinella turns the muscle cell into a parasite placenta. By making the muscle cell loose and flexible, the parasite makes room on its surface for new receptors for taking in food. The parasite also forces the cell’s DNA to churn out collagen, which forms a tough capsule around the cell. It makes the cell produce a signal molecule known as vascular endothelial growth factor. This molecule normally sends a signal to blood vessels to grow new branches in order to help heal wounds or nurture growing tissues. Trichinella uses the signal for its own purposes: to weave a mesh of capillaries around it, using the collagen capsule as their mold. Through the vessels comes a nourishing flow of blood, allowing the parasite to grow and swell inside its muscle cell, which bulges and groans as the worm rocks back and forth and probes its little home.
Parasites can also reconstruct the interiors of plants as drastically as they can those of animals. It may come as a surprise that plants actually have parasites at all, but they’re positively overrun with them. Bacteria and viruses live happily in plants, sharing them with animals, fungi, and protozoa. (Trypanosomatids, close relatives of the parasites that give us sleeping sickness, can live inside palm trees.) Plants are even hosts to parasitic plants that drive their roots into their hosts. Parasitic plants come into this life lacking at least some of the skills that a plant needs to live on its own. Bird’s beak, which lives in salt marshes, is a part-time parasite that has to steal fresh water from pickleweed and other plants that can get rid of the salt; they can handle their own photosynthesis and get their own soil nutrients. Mistletoe can photosynthesize, but it can’t draw its own water and minerals from the soil. Broomrape can do nothing for itself.
There are also millions of species of insects and other animals that live on plants, but before 1980, few ecologists thought of them as parasites. They were considered herbivores, essentially little spineless goats. But Peter Price, an ecologist at Northern Arizona University, pointed out that there’s a fundamental difference between these animals and herbivores. Herbivores are to plants as predators are to prey: an animal that can eat any number of species. A coyote will be happy with a bat, a rabbit, or a cat, while a sheep is equally easy about the plants it eats, entering a field and devouring the clover, the timothy, the Queen Anne’s lace. Some insects, like woolly bear caterpillars, graze like sheep, taking small bites from individual plants of different species and moving on. But many insects are limited to only one plant, at least for one stage of their life. A caterpillar that goes from egg to pupa on a single milkweed plant is no different from a tapeworm, which can live as an adult only in the intestines of a human. And many plant-eating insects spend their entire lives on a single plant, shaping their lives to that of their host.
One of the most powerful demonstrations of Price’s argument is nematodes that live in plant roots. These parasites are spectacular pests, destroying 12 percent of all the cash crops in the world. One particular kind—root-knot nematodes of the genus Meloidogyne—are also an uncanny botanical reflection of Trichinella. Each nematode hatches from an egg in the soil and crawls to the tip of a root. It carries a hollow spike in its mouth, which it stabs into the root. Its saliva makes the outer cells burst, freeing up a space through which the nematode can slip. It nudges its way between the cells inside the root until it reaches the root’s core.
The nematode then pierces a few cells around it, injecting a peculiar poison into them. The cells start making copies of their DNA, and the extra gene starts making a flurry of proteins. Genes switch on in these root cells that would never normally become active. The job of a root cell is to pull in water and nutrients from the soil and pump them into a plant’s circulatory system, a network of tubes and cavities that carries the food to the rest of the plant. But under the spell of the nematode, a root cell starts working backward. It begins to suck in food from the plant. Its cell walls become leaky enough to let the food flow in easily, and it sprouts fingery ingrowths, where it can store the food. The nematode spits molecules into the altered cell, which form themselves into a sort of intercellular straw, which it uses to suck up the food being pumped in from the rest of the plant. As the cell swells with food, it threatens to burst the entire root open. To protect it, the nematode makes the surrounding cells multiply and form a sturdy root knot to withstand the pressure. Just as Trichinella speaks the genetic language of mammals, root nematodes have learned the language of plants.
Parasites live in a warped version of the outer world, a place with its own rules of navigation, of finding food and making a home. While a badger digs itself a den or a bird weaves itself a nest, parasites often act as architects, casting a biochemical spell to make flesh and blood change into the form they desire, a heap of planks swirling together into a house. And inside their hosts, parasites also have their own bizarre inner ecology.
Ecologists study how the millions of species on Earth share the world, but rather than take on the whole planet at once, they generally focus on a single ecosystem, be it a prairie, a tidal flat, or a sand dune. Even within those limits, they are frustrated by loose frontiers, by the way seeds blow in from miles away or wolves lope in from the other side of a mountain. As a result, ecologists have done some of their most important work on islands, which may be colonized only a few times over the course of millions of years. Islands are nature’s own isolated laboratories. On them, ecologists have figured out how the size of a given habitat determines how many species can survive on it. And they’ve taken that knowledge back to the mainland, showing how a fragmented ecosystem becomes its own archipelago, where extinctions can strike.