An Experimental Basis for Environmental Medicine

AN EXPERIMENTAL BASIS FOR ENVIRONMENTAL MEDICINE J. H. WHITLOCK* Foreword Parasitic disease has both structure and function, and the most complex structures are to be found where parasite (in the broad sense) and host have become mutually adapted to each other, forming an endemic focus wherein the host population carries with it the seeds of its own partial destruction. By now we know enough to assert as sound theory that these endemic diseases function as actual vehicles of natural selection . The incidence of these diseases in natural and man-made populations suggests that each one removes a rather broad spectrum of unadapted phenotypes from the breeding population. Since the functions of each disease are probably quite interchangeable, the elimination of each endemic disease will have only limited influence on mortality statistics . We have to find ways of dealing effectively with these diseases as a class, or as a set, which will require the development of unusual concepts and techniques. If this proves possible, we should be able to increase the productivity of the ecosystem without damaging it, because it should let us increase livestock production with substantially less waste. Of more importance, however, is the concept that if we can find out how to increase man's options in coping with these diseases as they afflict humans , both in traditional and advanced societies, we shall make a great step forward in reducing much of the misery of the human condition. This paper attempts to design a biological model capable of testing the hypotheses arising from the theory of natural selection by disease, setting such constraints as seem currently useful to optimize the rapidity *Professor of parasitology, New York State Veterinary College, Cornell University, Ithaca, New York 14850. I acknowledge my indebtedness to the following colleagues at Cornell University who made helpful criticisms at one or another stage of the composition: Professors Bruce Wallace, Robert J. Young, Randall K. Cole, Robert F. Kahrs, R. Brooke Thomas, and Instructor Lydden Polley. I am indebted to the Cornell Center for Environmental Quality Management and the Biometrics Unit of the New York State College of Agriculture and Life Sciences for support and for providing the interdisciplinary medium that made it possible to think about biomedical problems in a related manner. Perspectives in Biology and Medicine · Summer 1974 | 455 with which significant experiments can be carried out. For a variety of reasons, some of which are herewith discussed, we may not have a great deal of time left. Introduction The concern of medically trained people for the well-being of the individual patient tends to make them forget that as far as populations are concerned, "states of health or disease are the expression of the success or failure experienced by the organism in its efforts to respond adaptively to environmental challenges" [I]. Yet, Darwin [2] clearly suggested that umbilical myiasis prevented feral horses, cattle, and dogs from becoming a common problem in Paraguay. J. B. S. Haldane [3] carried the basic idea still further and suggested that various types of diseases had markedly different types of impact on natural selection. Since prédation of man and domestic animals has become relatively rare, and the sick are not weeded out, diseases tend to develop to their fullblown stage with measurable symptoms and lesions under civilization or domestication. In contrast, it is rare to find sick animals in the wilderness . There are notable exceptions, such as the alewife die-offs in the Great Lakes and rabies in foxes, but the predator-scavenger mechanisms in most relatively natural ecosystems are extremely efficient. Even a slight injury or illness can displace an antelope from the middle of the herd to the tail and thus achieve a lethal priority on the lion's list. The basic point is that illness and death in relatively natural ecosystems are often a directed process against certain phenotypes (i.e., geneticenvironmental interactions). On the other extreme, oat smuts, chestnut blights, foot and mouth disease, and influenza provide examples of diseases whose occurrence is indistinguishable from a random process. The random process of infection of the American chestnut with the blight which was introduced from the Orient about 1880 has practically eliminated the species. Strains...