Fearsome Fauna: A Field Guide to the Creatures That Live in You

Fearsome Fauna: A Field Guide to the Creatures That Live in You

by Roger M. Knutson

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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

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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.

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Editorial Reviews

Shari Roan
Although we know some fourth-grade boys who would relish this disgusting stuff, most of us would be appalled to learn about the creatures that drink our blood, eat our cells and infest our muscles.... The last chapter deals with tips to avoid the loathsome beasts. It's not reassuring. Here's tip No. 5: Be lucky.
Los Angeles Times
Science News
Amusing and informative, if not totally gross.
Discover Magazine
For all the appalling facts he assembles, Knutson nevertheless tries to evoke empathy for parasites. After all, they live in some of the most disgusting habitats in the world, including our lower intestine.
From the Publisher
"You might want to spend the rest of the day scrubbing yourself under a hot shower after you read Furtive Fauna."—Cleveland Plain Dealer

"This off-beat guide reads like a science textbook that comedian Jerry Seinfeld and "The Far Side" cartoonist Gary Larson got ahold of before it went to press."—Santa Barbara News-Press

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Holt, Henry & Company, Inc.
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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 there 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 the internal parasites, our fearsome fauna, live and work for most of their lives, also 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 one of our parasites will actually burrow through or dissolve away something that we think of as us; our cornea, our muscles, our kidney, or even our brain. A few of them are capable of hollowing out spaces in some of our solid parts...


The easy answer to this question is pretty much wherever they want to...

The most common sites for our endoparasites in order of their popularity are: our gut or alimentary canal, our circulatory or blood and lymph tubes, our liver, our respiratory system or lungs and tracheae, and our coelom, which is a fancy name for the spaces between and among our internal organs. Some wander through our skin and related tissues. A few of the very smallest (and often the most troublesome) live right inside some of our favorite cells. Malarial parasites live in our blood cells and a few others are able to live inside muscle cells. Some that we don't want to think about much live in or on the cells of our eyes or our brain.

Since the most successful parasites cause their hosts the least difficulty, we should expect successful parasites to live in places where their food supply is assured and where we are able to repair any minor damage they might cause. Just as our skin makes new cells constantly to replace those worn away by the slings and arrows of time, the lining of our gut produces new cells in abundance, making it possible for a parasite like a hookworm to feed continuously and have a new supply of food growing all the time, just like new grass in a pasture. The cells of our blood are renewed in unbelievable numbers each day. One hundred billion new red blood cells (potential home to malaria parasites among others) are made by each of us each day. Our liver, the third most popular diet of our parasites, has cells capable of considerable and continuous regeneration. If the liver can make new cells fast enough, a parasite can eat some of them every day and still have a continuous food source. The sorts of cells making up our body that are least capable of renewal (brain or muscle) are home to few parasites, although any that get there may cause death or serious disease almost immediately. Parasites are not smart enough to go only to those places with renewable food supplies; but any parasite that tried to feed on the less replaceable parts of us survived less well and left fewer offspring for the next generation. Natural selection makes less harmful parasites.

We provide a multitude of potential habitats: some with abundant oxygen and some without, some acidic and some alkaline, some large and spacious (if a parasite is small enough), and some that have to be squirmed through. We are a wondrous continent to our tiniest residents, who would certainly profit from not causing their home any serious harm.


The love that some of us profess for all living things often does not extend to those creatures that might use us for food. Next time you are having a discussion about the value of all life with a friend from the 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 the furry sorts of creatures, those with feathers, and in a few cases even those with scales. It is barely possible to feel something akin to sorrow for the nightcrawler about to be impaled on a hook in pursuit of a fish we hope will be impaled on the same hook. By contrast, how many feel any pangs when an appropriate medication causes the death of nearly all of the intestinal parasites of our favorite dog? How much less are we likely to be in any way upset if medical advances make possible the worldwide eradication of malaria parasites or the complete 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 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...

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 non-reproducible genetic history. They have been doing their own trial and error research on how to survive best in their somewhat unusual habitat (us) 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 that 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...


Copyright(c) 1999 by Roger M. Knutson. 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.

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Meet 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.

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