The word made Sukhdeo smile, and at that moment his goatee looked particularly devilish. “It was the high point of my career.”
Once a parasite manages to find the place in its host where it will live, it can’t just sit back and enjoy life. For one thing, it needs a way to stay put in its new home. As an adult, a liver fluke is adapted only to life in the liver; put it in the heart or the lung and it will die. For every place that parasites have to live, evolution has produced a way for them to stay there. For example, there are parasitic copepods (a kind of crustacean) that live all over the bodies of fish. There are copepods that live in the eye of the Greenland shark. There are copepods that live on the scales of Mako sharks, and others that live on their gill arches. There are copepods that live inside the noses of blue sharks. There are copepods that ram themselves through the side of a swordfish and clamp onto its heart.
Each of these copepods looks so different from the other species that it’s hard for anyone except an expert to see that they all evolved from a common ancestor. Far from degenerating, these copepods have developed into bizarre forms in order to hold tight in their chosen niches. If these copepods should lose their grip, they would float away to a certain death. Every shark has its own special geometry of its scales, and copepods that live on the scales clasp their legs around them perfectly, like a lock and key. The copepod that lives in the Greenland shark has turned one of its legs into a mushroom-shaped anchor that it rams into the jelly of the eye.
Even for tapeworms, snug in the intestine, staying in place takes major effort. As they feed, tapeworms grow at a spectacular rate, increasing their size by a factor of as much as 1.8 million in two weeks. They can’t eat the way most animals do, because they have no mouth or gut. Their digestion doesn’t happen on the inside of their body but rather on the outside, their skin consisting of millions of delicate, blood-filled, fingery projections that can absorb food. The intestines of their host are also lined with almost identical projections. You could say that a tapeworm isn’t really missing a digestive tract—it’s an intestine turned inside out.
Tapeworms live in surging tides of half-digested food, blood, and bile, driven by the intestine’s endless peristalsis. If they do nothing, peristalsis will carry tapeworms out of their host altogether. Some species of tapeworms clamp themselves to the intestines with hooks and suckers on their heads, but others are perpetually slithering to where the food is. When we eat, peristalsis immediately ripples through our intestines, and these unanchored tapeworms respond by swimming upstream. They reach the incoming food and keep swimming until they hit the highest concentration. At that point, they soak up their meal through their skin, but as they eat, the food is carried downstream, and for a while the tapeworms let themselves be carried along with their movable feast. All the while, the tapeworms keep track of how far they’ve drifted by sensing how their host’s peristalsis changes. If they move too far downstream, they stop eating and swim back up. As tapeworms grow to their spectacular lengths, this swimming upstream can get to be complicated. The trouble is that peristalsis may make the intestines ripple quickly in one place and not at all farther up. Somehow tapeworms can detect these differences. They respond by making some parts of their body swim fast and some slowly.
The intestines are also home to hookworms, parasites that play a far riskier game whenever they eat. Hookworms start their lives in damp soil, where they hatch from eggs and grow into tiny larvae. They can travel into a human body by two routes: one simple, one tortuous. If a person swallows a larva, it will travel straight down to the intestines. But hookworms, like blood flukes, can penetrate the skin and burrow into a capillary. They swim through the veins to the heart and the lungs. When their host coughs, the larvae are carried up into their throat and can head down the esophagus.
Once it gets into the intestines, the hookworm grows into an adult, about half an inch long. Unlike tapeworms, the hookworm has a mouth—a powerful one ringed with daggerlike teeth and attached to a powerful, muscle-lined esophagus. And unlike tapeworms, it’s not interested in the half-digested food flowing through the intestines but in the intestines themselves. It drives it mouth into the lining of the intestines, ripping up the flesh. Parasitologists are still debating whether hookworms then drink their host’s blood or sop up the torn-up intestinal tissue. In either case, they release their grip after a while and swim to a new patch of tissue to feed.
But when the hookworm tears up some intestine and puts it in its mouth, the blood starts to clot. Whenever a blood vessel is torn, it picks up molecules from the cells in the surrounding tissue. Some of these new molecules combine with compounds floating in the blood itself. These chemicals trigger a cascade of reactions with other factors in the blood, which ultimately activate special cells known as platelets. The platelets swarm to the wound and clump together, while the cascade also creates a mesh of fibers around them, forming a hard clot that stops the bleeding. For a hookworm, clotting can mean starvation as the blood vessels in its mouth turn hard.
The parasite responds with a sophistication biotechnologists can only ape. It releases molecules of its own that are precisely shaped to combine with different factors in the clotting cascade. By neutralizing them, the hookworm keeps the platelets from clumping and allows the blood to keep flowing into its mouth. Once a hookworm finishes feeding at one place, the vessels can recover and clot while the parasite moves on to a fresh bit of intestines. If the hookworm were to use some crude blood-thinner that flooded the intestines, it would turn its hosts into hemophiliacs who would quickly bleed to death and take away the hookworm’s meal. A biotechnology company has isolated these molecules and is now trying to turn them into anticlotting drugs.
For some parasites, reaching their new home in the body is not enough. Before they can eat and multiply, they build new houses for themselves, using their host’s tissue as lumber.
Plasmodium, the parasite that causes malaria, enters the bloodstream through a mosquito bite and lives for a week or so in a liver cell. It then breaks out and gets back into the bloodstream. It rolls and yaws its way in search of its next home, a red blood cell. It’s here in the red blood cell that Plasmodium can feed on hemoglobin, the molecule that holds on to the oxygen that the red blood cells carry from the lungs. Devouring most of the hemoglobin in a cell, Plasmodium can gain enough energy to divide into sixteen new versions of itself, a flock of new parasites bursting out of the cell after two days, all searching the blood for new cells to invade.
Red blood cells are in many ways an awful place to live. Strictly speaking, they’re not even cells at all; they’re corpuscles. All true cells carry genes in a nucleus and duplicate their DNA in order to become two new cells. Red blood cells originate from cells deep inside our bones. These stem cells, as they’re known, divide and take the form of the various components of the blood, such as white blood cells, platelets, and red blood cells. But while other cells get their proper rations of DNA and proteins, red blood cells get no DNA at all. Their job is simple. In the lungs they store oxygen in molecules of hemoglobin. Because oxygen is a powerful atom that can easily react—and damage other molecules—the hemoglobin actually surrounds it by its four chains. Once the red blood cell leaves the lungs and travels through the body, it eventually sets the oxygen free to help the body burn its fuel to produce energy. The cells are simply crates pushed through the circulatory system by a beating heart. If you put white blood cells under a microscope, they reach out lobes to drag themselves across the slide. Red blood cells just sit on the glass.