February 2004

This months choice for Book of the Month is Carl Zimmer's "Parasite Rex." This book, as you might be able to tell by the name, explores the world of parasites. To most people, parasites are frightening, disgusting, and repulsive creatures; Zimmer does his best to show you how exquisitely beautiful, sophisticated and unique they are. I believe his intent is satisfied rather well, after having read this book. I picked up this book only after reading Zimmer's other books, "Evolution: The Triumph of an Idea" and "At the Waters Edge." Both of these were very well-written, concise, and informative, so it was a reasonable guess that this one would be too. I had no interest in parasites prior to reading this book, but Zimmer wrote about them in such a tantalizing way, that they now hold a new amount of respect (dare I say admiration?) within my frame of mind. Let me show you what I'm talking about.

In his prologue, Zimmer sets the reader up in an exotic location in Africa. He's met up with a boy in a hospital, and the staff that are working on him to prevent him from dying of a parasitic infection. He then goes into a brief description of the type of parasite thats infecting the boy and others in his region: trypanosomes, associated with sleeping sickness. He uses this case to paint a picture of how parasites are pervasive in our every day lives; not just in remote parts of Africa, but in our every day life. Love 'em or hate 'em, parasites are an important asset to life.

Nature's Criminals

This first chapter talks about the history of human contact with parasites, and how we've pointedly put them in a bad light. Parasites have been a part of human existence since before the written word, and through the course of human history, have had an impact on human society. The first recognizable parasites that early historians wrote about were the ones that could be seen without the aid of a microscope, or a similar device. These parasites include ringworms, roundworms, and tapeworms. These parasites have obvious health-related implications, as most people already know. The sicknesses and conditions that these parasites can cause didn't help put them in a favorable light, to say the least. But not all parasites are macroscopic; many, if not most, are microscopic. In 1673, a Dutchman named Anton van Leeuwenhoek developed a form of microscope, and examined just about everything underneath his new tool. Later scientists would use a form of his microscope to discover the world of microscopic parasites. Zimmer spends the rest of the time in this chapter developing for us how parasites were perceived throughout history, up until the present.

Terra Incognita

This next chapter deals with the study of the complex life cycles of parasites. They have some of the most complex life cycles known among the animal kingdom; some are so complex, they aren't even completely documented or understood. The problem here is that some species of parasites have offspring that look nothing like the adult, and live in a completely different medium. Zimmer uses Schistosoma mansoni, a type of parasitic blood fluke. Its larval stage is spent inside of a snail that might live in a pond. If it detects human skin nearby, it darts towards it, and plunges into the skin. Of course, it is so small, the human wouldn't even feel it. It makes its way through the skin with the help of special chemicals that soften the skin, and it heads for a capillary. Using this capillary, the fluke makes its way into the liver, and waits there to reproduce. During this time, it increases its size dramatically. This is a rather simple life cycle compared to some other parasites. The parasite life cycle is so interesting, and important, that whole careers can be spent just studying one species. People can earn Masters Degrees, or PhD's just studying these guys. I won't spoil the fun of this chapter by telling about some of the other parasites mentioned, but they are very disgusting at times, and very interesting.

The Thirty Years' War

This chapter sets up how the human body fights against parasitic infection. Zimmer starts off with a man from Australia, who went into a local hospital to find out why he was so tired. Close inspection of his stool showed that he had a massive infection of the blood fluke Schistosoma mansoni. The problem is, he was in Australia, and Schistosoma mansoni shouldn't be: its native to Africa and Latin America. The doctor treating him asked about his history, and he found out that at one point in his life, he had lived in Africa for 6 years. The weird thing was, it was 40 years ago. Why was this man showing symptoms of schistosomiasis (the name for the infection of Schistosoma mansoni) 40 years after being in Africa, the only place he could have contracted it? The answer: humans have an immune system. One interesting aspect of biology and parsitology is in examining how the human body reacts to a parasitic invasion. Parasites don't just barge in to a human body and take over. They have to avoid being completely destroyed by our immune system. This means that they have to evolve different mechanisms. But, as these parasites evolve defens against our immune system, our immune system responds in tandem to fight off the infection. Its an endless battle, with no real winner.

A Precise Horror

This chapter almost plays as if it were from a movie. Zimmer talks about one of the scary (to us anyway) aspects of parastism: where the parasite controls its host. Quite appropriately, he starts with a nice quote from the movie Alien, in which an alien parasite kills off human hosts. Although that depiction is obiously fantasy, there are parasites which can control the actions of its host. Zimmer's example at the beginning of this chapter is a species of Sacculina. This parasite is a type of barnacle that lives on crabs. In its larval stage, a female Sacculina will seek out a crab host, finding a particular spot on the crabs shell. She will then insert her body into the shell of the cra, and send out "roots" which extract nutrients from the crab. Through the life cycle of this parasite, it will physically alter the crab it lives off of. It prevents the crab from moulting, disabling the regenerative ability if the crab loses a claw, and essentially making the crab reproductively defunct. Sacculina even gets the crab to take care of its offspring. Uninfected female crabs, according to Zimmer, will take care of their offspring in a larval pouch, located on the underside of its body. When it comes time for them to hatch, she will perch herself on a high rock, to catch the current, and release her eggs. Sacculina will have the crab do the same thing; it will even get male crabs to act in this manner. Zimmer introduces several other parasites that can affect the behavior of its hosts, and again I don't want to ruin the surprises he has in store.

The Great Step Inward

Zimmer uses this chapter to talk about how parasites get to do what they do. He spends some time talking about some of the biological relationships of some specific parasites, and then talks about different forms of parasites, such as cuckoos. He covers a lot of ground in this chapter, and a brief summary is rather difficult to construct. Suffice it to say, it's an important chapter.

Evolution from Within

This chapter sums up all of the previous ones to make one of the most interesting claims that this book has to offer: parasites can drive evolution. Not only can they drive evolution, but they may even be at least partially responsible for the formation of sexual reproduction. This concept is a form of the Red Queen hypothesis, which states that evolution between two closely associated organisms (such as prey and predator, or host and parasite) takes place so that there is an equilibrium. In other words, like the Red Queen in the Alice in Wonderland story, you have to keep running, so you can stay in place. The concept that Zimmer puts forth here isn't his own, its n idea originally put forth by William Hamilton from Oxford University. The simplified version of the idea is this: take a species that reproduces asexually and/or sexually. This happens in snails and slugs, for example. Without any predatory or parasitic pressure, they can simply make clones of themselves, with very little genetic variation. But as soon as a source of pressure is introduced, sexual reproduction seems to work better. Sexual reproduction can offer offspring genetic material from two sources, whereas cloning only has material from a single source. Having material from two sources can mean that you might be able to get some form of resistance to parasitism, or to disease. Parasites are very specific to their hosts, they tend not to change species that they've adapted to. In any population, there will be variants. Some variants will develop some form of resistance to a parasite, making them less susceptible to infection. This means that the parasites have to focus on the members that don't have the same sort of resistance; they become a target. If the species cloned, then their offspring would have pretty much the same genetic material, so they would either be resistant to parasites, or susceptible to them. However, if a member that is susceptible to parasitization reproduces with one that isn't, the offspring might likely have resistance to the parasitism as well. Over time, there will be an increase in members that are resistant to parasitism, especially given that sometimes parasitized individuals may not be able to reproduce at all. In response to this, the parasites themselves have to adapt so that they can survive in this population, so they will find ways to fight against the resistance that has been developed. In other words, they have to run just to stay in the same place.

The Two-Legged Host

In this chapter, Zimmer spends time talking about parasite influence on human life and evolution. Fortunately, because of our heritage, we came into the scene with an already well-developed immune system to fend off parasites. We know that parasites can influence living things in grandiose ways; they can determine who gets to reproduce and who doesn't, and where species can live and how they live. But, are humans subject to these same things? One idea that Zimmer introduced earlier is the role that parasites can play on behavior. But, they do more than just control hosts; lacking parasites makes you look "sexier" to a potential mate. If you live in an area where there are insects that need an open wound to enter your body, you might avoid fighting rivals in your group to avoid cuts and scrapes. We see this in animals, but does it apply to us? It sure does, both biologically, and culturally.

How to Live in a Parasitic World

This final chapter deals with how to apply what we know about parasites to improve life for humans. In addition to that, how we can apply our knowledge of them to help correct some of the bad things we've done to environments. Zimmer starts off talking about the European green crab (Carcinus maenas). You might remember this character from before; its what Sacculina parastizes. This crab, as its name implies is native to Europe, but has been introduced to many parts of the world, by means of human sailing ships. The larvae of this crab can live in the ballast tanks of ships, and are easily transported into new environments, where they can decimate local populations of shellfish. While all this is very unfortunate, especially to the shellfish lover, what does this have to do with parasites? Well, one of the few places that hadn't been affected by the European green crab was the west coast of the United States. In 1991, however, one had been caught near San Francisco. This could have huge implications for the native shelfish of this area, and soon for those all along the western coast. What can be done to prevent them from killing the shellfish? Parasites! If you recall, Sacculina, when it infects the crab, makes it sterile. Sterile crabs can't reproduce. If crabs don't reproduce, they don't spread. Whats more, they are less of a threat to the local population of shellfish. This is just one example of knowing how parasites operate can affect the biological world around us. Parasites can potentially be one of our greatest assests as pest control is concerned, as indicated in this example. By exploiting the nature of parasites, we can help repair some of the biological damage we've caused in the world. While it won't bring back extinct organisms, it can help protect those on the verge of becoming so.

Summary

This book absolutely captivated me. I had no previous interest in parasites, but reading this book opened my eyes into their wonderful, exotic, exciting world. Zimmer has pulled off another winning book, which is both entertaining, and informative. If you haven't already considered reading this book, I don't know what else will work. Go out and enjoy!