Hypochondriacs beware-- would you believe the nastiest creatures in the known universe live inside our bodies? Not content to just find a home and produce offspring in our internal space, parasites will drink our blood, eat our cells, and infest our muscles. There is very little that can be said in their favor, with perhaps one exception-- they are truly fascinating!
Fearsome Fauna is a wickedly amusing and startlingly informative look into the secret world of these fascinating creatures. Perhaps the greatest biological success story of all time (there are more kinds of parasites than insects), parasites have found homes in the vast majority of people on earth and have learned to live in their environment without destroying it (usually). For readers who would like to meet these hardworking beasts-- or learn how to avoid them-- Fearsome Fauna tells you everything you always wanted to know about parasites but were too disgusted or terrified to ask.
|Publisher:||Holt, Henry & Company, Inc.|
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About the Author
Roger M. Knutson is professor emeritus of biology at Luther College in Iowa. He is author of the now-classic Flattened Fauna: A Field Guide to Common Animals of Roads, Streets, and Highways and Furtive Fauna: A Field Guide to the Creatures Who Live on You.
Read an Excerpt
Is There Room for All Those Parasites?
A proper Victorian lady is supposed to have thought of herself as being of solid wood from the neck down. Whatever problems that vision of the human body might have caused or prevented in a previous century, I suspect that it is closer to the way most of us think of the inside of ourselves than teachers of anatomy would like to believe. Sure, we know that some people have internal parts removed or even replaced, and we know the names of a number of things that are in there somewhere and maybe even approximately where they are located. What we are much less likely to know is that there is a remarkable amount of room in our bodies that is not occupied by anything remotely close to solid and that there are inner spaces of real significance.
Why are these interior spaces of such importance? They provide most of the places where internal parasites, our fearsome fauna, live and work for most of their lives — also for most of ours. There are few genuinely vacant spaces in this body of ours, but there is a large amount of fluid-filled space in our arteries and veins and in the spaces between cells and organs. And there exists almost equally large amounts of potential space that we are continually filling with food or air. Occasionally a parasite will actually burrow through or dissolve away something we think of as solid: our cornea, our muscles, our kidney, or even our brain. A few parasites are capable of hollowing out spaces in some of our solid parts. But mostly they live contentedly in the spaces we provide for them. Like a fish in the ocean or a bird in the air, they find a suitable habitat inside us and thrive in it. If we have trouble thinking of ourselves as having internal spaces rather than being mostly solid, we will certainly not be able to mentally accommodate the multitude of things that swim and grow and reproduce in those spaces. We do literally accommodate them, though, so we might as well get used to the idea that, often without drastic cost to ourselves, we provide parasites with a good and comfortable home: food, shelter, and a quiet place to copulate.
Fluid-filled space makes up 20 to 25% of our total body volume, which means that somewhere close to 5 or 6 gallons of us forms an internal ocean where a creature of appropriate size can swim mostly unimpeded. And many of these spaces are separated from the next space by only the thinnest and most easily breached barrier. The entirely fluid portion of our blood is almost a gallon, all of it in rapid circulation to all the nooks and crannies of our body. It can carry with it not only blood cells, dissolved food, and oxygen but any sort of creature small enough to swim or float along. A nearly parallel system of lymph vessels returns blood fluids to general circulation. Unlike in the external ocean, where the search for food and sustenance is the prime motivation for life of all sorts, our internal ocean provides a banquet of food and oxygen. It is comfortable enough to support all our own microscopic cells and anything else that finds its way into that cozy habitat — and that can survive our body's attempts to get rid of it.
The potential spaces or variable spaces are mostly associated with our gut or our lungs. While we may think of our stomach as a sort of saclike structure somewhere near the middle of us, the sides of it are actually close enough to each other to rub just a bit, unless food is inside it. We are likely to feel that rubbing sensation as hunger and do what we can to expand that nearly flat space. The rest of the digestive apparatus, many yards of cylindrical intestine, is equally collapsed if there are no contents. But for most of our lives the intestine is mostly filled with food in various stages of being turned into refuse. Intestinal contents are destined to be more than just refuse. They also become us. Barring long-term starvation, our 30 feet of gut always has something in it. Some of the most unsightly parasites live and thrive in that disgusting muck. Unsightliness is almost as much in the eye of the beholder as is beauty, but tapeworms are nobody's pinups, and hookworms are the stuff of nightmares.
Parasites that live in our gut and its branches are really not inside our body but outside. Mathematical topologists (those who study the shape and surfaces of things) tell us that we and most of our fellow animals are shaped like an extraordinarily long doughnut with a very complicated hole running through the middle of it. Just like the doughnut skin, our skin covering is continuous from what covers our elbow to what lines our gut. Just as a jelly bean placed in the center of a doughnut hole is not inside the doughnut, a parasite living in our gut and its branches is not really inside our tissues. When some creature nibbles on the inner lining of our gut, it is much like a mosquito or black fly nibbling on our outer skin; it's just less visible. The mechanisms of our body that detect and reject anything that is not us don't work particularly well in our stomach or intestine. Large volumes of bacteria and other microorganisms that would be rejected in a few days if they were under our skin live comfortably all our life in the complicated habitat of our stomach and intestines, just as bacteria, fungi, and some larger creatures live and feed on our skin.
Our lungs are, like the gut, capable of dramatic expansion and contraction, though they hold much less food potential for a parasite. And they too are really part of the outside rather than the inside: kind of a complex hole pushed into the doughnut that the topologists proclaim we are. Further, in much of the lung's space for much of the time, the winds must be nearly tornadic for tiny young parasites. For them, living in a lung must be comparable to trying to live outside on the top of a 20-story building on the Chicago lakefront. For most parasites, the lungs are only a small part of an adolescent walkabout or a temporary home on the way to more comfortable and long-term quarters. Our bloodstream carries them there, and some use it as a way of getting into our gut. We cough them up and swallow them with the rest of the phlegm.
How Should We Feel About Our Parasites?
The love some of us profess for all living things often does not extend to creatures that use us for food. Next time you are having a discussion about the value of all life with members of Friends of Animals, ask them how they feel about tapeworms or hookworms or the little amoebas associated with dysentery. Most of us can find some point of identification with furry creatures, those with feathers, and even some with scales. It is barely possible to feel something akin to sorrow for the night crawler about to be impaled on a hook in pursuit of a fish we hope will be impaled on the same hook.
By contrast, who feels any pangs when an appropriate medication causes the death of nearly all the intestinal parasites of their favorite dog? How much less are we likely to be upset if medical advances make possible the worldwide eradication of malaria parasites or the demise of all the world's hookworms or Guinea worms or tapeworms or pinworms or whipworms or blood flukes or liver flukes? All of those creatures and many more can live in our internal ocean and its tributaries and will indeed take up residence if we happen to have been in the wrong campsite at the wrong time. The specific problem is that for most of the people in the world, the wrong place is the place where they live and work and play. The lesson here is that we should be a bit guarded about reverence for all life, unless we are willing and able to include in that revered category some of the creatures that find us to be appropriate food and lodging.
Ugliness and even unpleasantness are almost always matters of opinion, but what might be called biological value is not. Do our endoparasites have any biological value? Does the rain forest have any biological value? Like all creatures, each of our endoparasites has a unique and nonreproducible genetic history. They have been doing their own trial-and-error research on how to survive best in their somewhat unusual habitat for millions of years, and they have the same potential to provide us with valuable medicines and biochemical products as have the creatures of the rain forest or any other complex habitat. Recall just how many thousands of different kinds of parasites there are and then multiply the number by the hundreds of unique chemicals each of them must make to protect itself from its host and live effectively under difficult circumstances. The number of powerful and potentially useful chemicals made by endoparasites is enormous, and we have hardly begun to examine them. Don't wish all the parasites dead until you are certain we won't find a cancer cure among them.
Parasites have managed to solve some ecological problems as well. They survive and often reproduce effectively in their host environment without causing much harm to that environment. They have learned to live without destroying their habitat, a lesson humans have not yet learned. Possibly the study of parasitic life styles could help us learn how to live in our more complex world without causing it excessive harm.
Origin and Evolution of Our Parasites
The development of a close and continuous parasitic relationship is the outcome of thousands of generations of polite and harmless accommodation by both parasite and host. Since we humans have been on Earth in a form we like to recognize for less than a million years, many of our parasites are creatures we have acquired with only minor modifications from other, longer-term inhabitants of Earth. Some parasites are so undiscriminating that they live pretty well in almost any creature with an appropriate internal ocean or internal digestive tube, while others exhibit a faithfulness we might envy. It is always in the best interests of a parasite to cause its host as little trouble as possible. From the parasite's point of view, being completely unobtrusive is a continuing goal. The process of natural selection produces not only hosts that are better able to resist a parasitic attack, but also parasites that are less likely to cause serious harm to their hosts. The outcome of this process is a parasitic relationship that often allows parasite and host to live in a healthy state of harmony with one another.
Most of us are likely to underestimate the number of parasites in the world and may think of their way of life as unusual, extraordinary, abnormal, or even unnatural. The briefest analysis of the situation suggests otherwise. Most kinds of organisms on Earth harbor many kinds of parasites, and many of them can live in only a few different hosts. That means there must be more kinds of parasitic organisms on earth than nonparasitic ones and that what we might regard as a deviant life style is really the more "normal" one. Just as normal climatic temperatures are temperatures most commonly encountered, normal life styles are those of parasites, and all other patterns of making a living are abnormal. Similarly, those of us who maintain that harboring parasites is abnormal have to face the fact that the vast majority of humans on earth house and feed one or more kinds of internal parasites, often without knowing the parasites are there.
If we define biological success as a large variety of different life forms (insects, for example, are very successful), then parasites must be the greatest biological success story on Earth. There are more kinds of parasites on Earth than there are kinds of insects. When we suggest that success is a great thing, we are praising the parasitic pattern of making a living. If a world where there are more parasites than nonparasites and where parasites can be considered the most successful kinds of organisms works pretty well, maybe we should reconsider some of our political or even aesthetic positions.
Many of our own parasites live in our internal ocean, a fact with an ancestral reason. It is an echo of our own very long-ago heritage; we evolved from organisms that lived in the ocean and needed salt water around them, and our individual cells are still bathed in the slightly salty water of our blood and plasma. Our parasites live in something like that primordial ocean. Their nearest relatives, the ones who are not parasites of land-dwelling animals, live in the actual outdoor ocean or a smaller watery habitat. We are, to most worms and amoebas, just a small, isolated patch of ocean with abundant food and a tropical internal climate. We are islands of ocean in a sea of air. Getting from one tiny isolated patch of ocean to the next is often a serious problem with all that dry air in between. Luckily for us, none of our internal parasites has developed the capacity to fly on its own, although many of them do hitch rides on mosquitoes and flies and a few lay eggs that can be carried by air. Though most cannot fly, they are great swimmers (remember that ocean history). Recall just how well they swim and thrive in water the next time you are tempted to swim where you don't know much about the water. Know your water if you are going to swim in it or drink it. Praise the health departments that concern themselves with providing water that doesn't have very many troublesome inhabitants other than ourselves and some game fish, but always suspect that some form of parasitic swimmer is there.
Beyond the common oceanlike heritage, some of the inhabitants of our digestive tract have a slightly different evolutionary history. Being a free-swimming ocean creature may be preadaptive for living in blood or tissue fluids, our ocean equivalent, but our gut is a mucky place with little oxygen and a lot of partly digested organic material. It is also often alkaline or very acidic and generally viscous and murky, not a good place for an ordinary swimmer. Those of our internal companions who live in that unpleasant place are not likely to have had their original home in the slightly salty waters of the open ocean. Their closest relatives outside us are creatures that live or lived in the mucky bottom mud of tidal flats or in other slimy sediments and have some capacity to burrow and feed while surrounded by the same sort of low-oxygen, chemically and physically challenging medium that we produce in the churning mire of our intestine. Living in the dregs of a shallow sea has been preadaptive for living in our intestine, just as living in the open ocean was preadaptive for living in our blood or tissue fluids.
If we are islands of ocean in a sea of air, then we might expect the creatures that live in and with us to share some of the characteristics of other island organisms. One of the peculiarities of plants and animals that live on isolated islands is the presence of larger body sizes than are found in related species on the mainland. Recall the Komodo dragon lizard or the Galapagos tortoises. If we as organisms are biologically like tiny, isolated islands, then might we expect some of our inhabitants (the endoparasites) to be larger than their relatives living in the outside world? Almost all nematodes are about the size of bits of fine silk thread a tiny fraction of an inch long (nematode means "threadworm"). But some of our common parasitic inhabitants are nematodes that are 1 or more feet long, nightmare creatures with no more brains than their tinier cousins (maybe even less) but with an appetite for blood and tissue and the food we have already eaten commensurate with their size. Whales can harbor a parasitic nematode that approaches 30 feet in length. There are no nonparasitic tapeworms, but the ones that live in us and other animals are notoriously large (up to 30 feet or more). Some of the flatworms (flukes) that inhabit our liver and bile duct or our lymph vessels are considerably larger than most of their relatives who manage to survive in the outside world.
It is difficult for any creature to get to an isolated island, but once there it is favored by a very noncompetetive environment, few predators, and usually an abundance of food. It just has to keep from being swept away by the waves or the winds, and large size may be an advantage. The same advantage seems to accrue to parasites. When John Donne said, "No man is an island," he was not thinking biogeography. We are islands, and the creatures that share our intimate geography have evolved in the same patterns found on larger isolated islands.
Excerpted from "Fearsome Fauna"
Copyright © 1999 W. H. Freeman and Company.
Excerpted by permission of W. H. Freeman and Company.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
Table of Contents
1 - Is There Room for All Those Parasites?,
2 - How Should We Feel About Our Parasites?,
3 - Origin and Evolution of Our Parasites,
4 - Are They Parasites or Are They Diseases?,
5 - What Kinds of Creatures Become Parasites?,
6 - Where Do Our Internal Parasites Live?,
7 - Digestive Tract Inhabitants,
8 - Blood-Dwelling Parasites,
9 - Parasites of the Lymphatic System,
10 - Liver Livers,
11 - Worms That Live in Our Muscles,
12 - Connective Tissue Dwellers,
13 - Things That Live in Our Eyes and Brain,
14 - All Over Our Body,
15 - How to Keep Parasites at Bay,