Beasts of the Earth: Animals, Humans, and Disease

Introduction

= x i Introduction This book is about human infectious diseases and the microbes that cause them. As the recent resurgence of AIDS, tuberculosis, and influenza has shown, infections are still major causes of illness and death in our society. We are also discovering that infectious agents may play a role in many chronic ailments such as cancer, heart disease, and schizophrenia. The microbes that cause infectious diseases are thus very much part of our daily lives. The vast majority of these microbes has been, and continues to be, transmitted to humans from other animals. A few diseases, heirloom infections such as those caused by herpes and hepatitis viruses, first infected our primate ancestors, and their microbes descended through early hominids to Homo sapiens. A large number of diseases, such as measles and tuberculosis, resulted from microbes transmitted to humans following the domestication of animals. Many other diseases, such as AIDS, SARS, mad cow disease, monkeypox, and bird flu have been transmitted from animals to humans in recent years as the relationship between animals and humans has changed. Each change in this relationship is accompanied by a risk of additional animal microbes being transmitted to humans. We have become accustomed to thinking of animals as our friends, not as sources of human disease. Through the influence of Aesop, the Brothers Grimm, Beatrix Potter, and Walt Disney, it has become increasingly difficult to appreciate that the animal represented by Donald Duck was the origin of an influenza pandemic that killed twenty million people, that Mickey Mouse may be spreading deadly hantavirus, that Clarabelle Cow is the source of prions that cause mad cow disease, and that Pluto may be carrying leishmaniasis. We do not associate Bambi with Lyme disease, Big Bird with West Nile virus, Rocky Raccoon with rabies, or Garfield with toxoplasmosis. Even Barney, the dinosaur beloved of small children, is almost certainly a carrier of Salmonella bacteria, as all reptiles are. In the past two hundred years, we have entered another period in which R3186.indb xi R3186.indb xi 11/3/04 6:44:09 AM 11/3/04 6:44:09 AM the relationship between animals and humans is changing in important ways. The changes include our personal relationship to animals as pets and ways of processing animals for our food supply. The present period of changing animal -human relationships may represent the most profound alteration in this relationship since animals were domesticated ten thousand years ago. One of the consequences of this changing relationship has been the emergence of new diseases that are transmitted to humans from animals. Concurrent with the changing animal-human relationship have been changes in technology and in ways humans interact with each other. Changing sexual mores, the use of injections, increased urbanization, and increased access to air travel and other forms of transportation may all promote the spread of microbes from person to person. These changes magnify the effects of transmission of microbes from animals to humans, making it more likely that an infection of a single individual will become perpetuated as a chain of human infections. It is important to appreciate the complexity of the animal-human relationship . On the one hand, animals have fulfilled many important human material and, as we have discovered more recently, psychological needs. On the other hand, animals have also been the source of many of the most important human diseases. These are the two sides of the beasts of the earth. x i i = B e a s t s o f t h e E a r t h R3186.indb xii R3186.indb xii 11/3/04 6:44:09 AM 11/3/04 6:44:09 AM = = Beasts OF THE Earth R3186.indb xiii R3186.indb xiii 11/3/04 6:44:09 AM 11/3/04 6:44:09 AM R3186.indb xiv R3186.indb xiv 11/3/04 6:44:09 AM 11/3/04 6:44:09 AM ...