ANIMAL DISEASE

an impairment of the normal state of an animal that interrupts or modifies its vital functions. Concern with diseases that afflict animals dates from the earliest human contacts with animals and is reflected in early views of religion and magic. Diseases of animals remain a concern principally because of the economic losses they cause and the possible transmission of the causative agents to humans. The branch of medicine called veterinary medicine deals with the study, prevention, and treatment of diseases not only in domesticated animals but also in wild animals and in animals used in scientific research. The prevention, control, and eradication of diseases of economically important animals are agricultural concerns. Programs for the control of diseases communicable from animals to man, called zoonoses, especially those in pets and in wildlife, are closely related to human health. Further, the diseases of animals are of increasing importance, for a primary public-health problem throughout the world is animal-protein deficiency in the diet of humans. Indeed, both the United Nations Food and Agricultural Organization (FAO) and the World Health Organization (WHO) have been attempting to solve the problem of protein deficits in a world whose human population is rapidly expanding. Additional reading Lawrie Reznek, The Nature of Disease (1987), written for the general reader, discusses the nature of disease from several perspectives, including medical, legal, political, philosophical, and economic. David O. Slauson, Barry J. Cooper, and Maja M. Suter, Mechanisms of Disease: A Textbook of Comparative General Pathology, 2nd ed. (1990), written for the veterinary student but a great resource for pathologists and biomedical researchers, provides a fundamental overview of the mechanisms of diseases, often at the molecular level. Max Samter (ed.), Immunological Diseases, 4th ed., 2 vol. (1988), covers the collagen diseases. F.M. Burnet, The Natural History of Infectious Disease, 3rd ed. (1962), offers a unique view of infectious disease as an ecological and evolutionary phenomenon. Books for the general reader include June Goodfield, Quest for the Killers (1985), exploring efforts to conquer several epidemic diseases; Andrew Scott, Pirates of the Cell: The Story of Viruses from Molecule to Microbe, rev. ed. (1987); and Peter Radetsky, The Invisible Invaders: The Story of the Emerging Age of Viruses (1991). William Burrows Dante G. Scarpelli Calvin W. Schwabe, Veterinary Medicine and Human Health, 3rd ed. (1984), provides a comprehensive reference on medical public health. Also of interest is Paul R. Schnurrenberger, Robert S. Sharman, and Gilbert H. Wise, Attacking Animal Diseases: Concepts and Strategies for Control and Eradication (1987). J.F. Smithcors, Evolution of the Veterinary Art: A Narrative Account to 1850 (1957), comprehensively treats veterinary medical history and the history of the knowledge of animal diseases; it is brought up to date by D.H.V. Stalheim, The Winning of Animal Health: 100 Years of Veterinary Medicine (1994). Lise Wilkinson, Animals and Disease: An Introduction to the History of Comparative Medicine (1992), studies the interrelation of animal and human diseases. O.M. Radostits, D.C. Blood, and C.C. Gay, Veterinary Medicine, 8th ed. (1994), focuses on livestock. Carlton L. Gyles and Charles O. Thoen (eds.), Pathogenesis of Bacterial Infections in Animals (1986); John W. Davis, Lars H. Karstad, and Daniel O. Trainer (eds.), Infectious Diseases of Wild Mammals, 2nd ed. (1981); and Ival Arthur Merchant and Ralph David Barner, An Outline of the Infectious Diseases of Domestic Animals, 3rd ed. (1964), are textbooks about animal diseases. George F. Boddie, Diagnostic Methods in Veterinary Medicine, 6th ed. (1969); Jiro J. Kaneko (ed.), Clinical Biochemistry of Domestic Animals, 4th ed. (1989); G.R. Carter and John R. Cole, Jr. (eds.), Diagnostic Procedures in Veterinary Bacteriology and Mycology, 5th ed. (1990); and Charles M. Fraser et al. (eds.), The Merck Veterinary Manual: A Handbook of Diagnosis, Therapy, and Disease Prevention and Control for the Veterinarian, 7th ed. (1991), are general references on physical and laboratory diagnostic techniques. Charles E. Cornelius The Editors of the Encyclopdia Britannica Evolution and paleontology All the adaptations in the living world have been produced by natural selection. This selection acts continuously, on many levels and time scales. Thus, an animal may become well adapted to an ecological niche that then disappears, forcing the animal either to evolve rapidly to fill another or, more likely, to become extinct. Another animal, adapted to a more permanent niche, survives. There is also long-term selection on the ability to adapt, as well as on current adaptation, for environments change, in both their physical and biotic components. Mass extinctions of the past testify to major changes, some perhaps catastrophic, the causes of which are still debated. These mass extinctions tended to eliminate more active and specialized groups, partly setting broad-scale evolution back and selecting for the inactive and resistant. Evolution proceeds by the incremental acquisition of adaptations. It may be impossible for a lineage to evolve into a more effective way of life, because its present adaptations would have to be lost first. An adaptive zone is the niche of a (perhaps large) group of species; in general, the more different and basic the overall adaptive zone, the higher the rank of the taxonomic group. When an adaptive transition has occurred, a new group has arisen. It is possible to turn this process around and to infer the course of evolution from its results. Species that share a derived character are likely to have had a common ancestor with that character, although there are many exceptions. Incorporating as much evidence as possible, morphological and molecular, makes the inference more likely. Fossils help too, letting scientists know what actually did live at each time. Only hard skeletons are ordinarily preserved intact or as fossils, though, so that groups without them have a sparse record or none. Simpler skeletons are sometimes ambiguous as to what animal they came from, and many groups have existed that have no close relatives today. There are nevertheless several dozen faunas through the geologic record that preserve soft-bodied animals and thereby help fill in the historical record of animals. Appearance of animals Animals first appeared in the Vendian, soft-bodied forms that left traces of their bodies in shallow-water sediments. The best-known are coelenterates of various sorts, including some that were more irregular than any today, and there are several groups with unclear affinities. At least some of the latter groups probably left no descendants. Most of the Vendian animals were thin, with each cell able to diffuse nutrients from the water, and many may have photosynthesized with symbiotic algae. No sponges are known to have existed in the Vendian, but they probably had already arisen from choanoflagellate protists. The first known mass extinction ended the Vendian. In the Cambrian Period (570 to 505 million years ago) began the great evolutionary radiation that produced most of the known phyla. Evolution occurred rapidly then, as it ordinarily does when adaptive zones are more or less empty and evolutionarily accessible. More soft-bodied faunas show that there were a number of sorts of animals that have no apparent relation to known phyla. It is unclear how many of these are aberrant members of known phyla and how many are more basically different. Although natural selection adapts the parts of animals to function and develop harmoniously with one another, at such an early time much of this internal coadaptation may not yet have occurred, making it easier to change in major ways. There were many groups of arthropods and echinoderms that also have unclear specific affinities with their longer-lasting relatives. Priapulid worms, a minute component of the modern free-living biota, were abundant and diverse. Coeloms evolved and many animals burrowed, and burrowing has increased throughout the Phanerozoic (from roughly 570 million years ago to the present). The Cambrian was also the time when hard skeletons originated in many groups and predators began to prowl the ocean floor. The probability that a taxonomic family of animals would become extinct in a million years was highest in the Cambrian and declined exponentially until a mass extinction occurred in the late Permian Period (about 260 to 245 million years ago). It then declined again exponentially thereafter. This pattern is due entirely to the decline and extinction of whole groups that are more susceptible to extinction; within each group the probability of extinction stays about constant except during mass extinctions. The probability that a family will give rise to a new family usually has declined exponentially both within groups and overall, however, so that most groups tend to decline in large-scale diversity over time. There has been nevertheless an overall increase through the Phanerozoic in the number of taxa at levels from species to family as new groups like mammals and teleost fishes have originated and as others, like clams and insects, have gradually diversified. Extinction is the common fate of a lineage, while the survivors multiply disproportionately. Sponges are first definitely known in the Cambrian, including a short-lived major group, the Archaeocyatha. They have not evolved much since then. Some of their larvae became sexually mature without growing up and gave rise to the coelenterates and perhaps the placozoans. Most groups of coelenterates also appeared early and evolved slowly. All corals of the Paleozoic Era (570 to 245 million years ago) belong to groups that are restricted to that era. After the Permian extinction, a group of sea anemones evolved a skeleton and diversified into modern corals. In the Cretaceous Period (144 to 66.4 million years ago), some clams became corallike, even with symbiotic algae, and for a while outcompeted the corals on reefs. The fossil record is uninformative for flatworms and pseudocoelomates. The interrelations of these groups have also not yet been studied adequately by modern comparative methods. They probably form an adaptive radiation distinct from that of the coelomates, however. Some anatomic evidence suggests that the pseudocoelomates were all derived from gnathostomulid-like Platyhelminthes. The Introverta seem related to the rotifers, and the gastrotrichs to the nematodes. The Mesozoa may be an unnatural group, with its classes being simplified descendants of different phyla, while the Nemertea are probably derived from turbellarian Platyhelminthes. Coelomates appear to have had a single origin, probably from ancestral turbellarian Platyhelminthes. They were already diverse in the early Cambrian, and the hydroskeletal function of the coelom in small animals suggests that the ancestor was a burrowing worm. Segmentation arose very early in the group and is retained in its probably primitive form by most annelids. Leeches arose from freshwater oligochaetes, and oligochaetes probably from ancestral polychaetes. Annelid fossils merely show that they have been around since the Cambrian. Arthropods have been the most diverse phylum since the Cambrian. Trilobites and crustaceans dominated then, with the former declining in abundance through the Paleozoic and the latter expanding into great adaptive diversity. Chelicerates also arose in the early Paleozoic and later radiated widely on land. Myriapods are a terrestrial group and gave rise to insects about 400 million years ago (during the Devonian). Insects were already diverse in the Carboniferous (360 to 286 million years ago), and modern orders have gradually originated and then replaced many of the earlier ones. The interrelations among the four major arthropod groups are unclear, as is the position of the Onychophora. The latter, a relative of annelids and known as early as the Cambrian, may be ancestral to myriapods, in which case the Arthropoda must be divided into two phyla. The position of the weakly segmented Tardigrada is even less clear, as they show special similarities to both the Onychophora and the Gastrotricha. Segmentation has been reduced or lost in many groups. The Pogonophora retain segmentation mostly at their hind end. Another annelid relative known since the Vendian, this group has gutless members that get nutrients only from symbiotic bacteria and what is dissolved or suspended in the water. The Apometamera show traces of segmentation in the Echiura but none in the overall more-derived Sipuncula. Apometamera are also annelid relatives but may be even closer to mollusks. They lack useful fossils, unless the tube-forming Hyolitha, which may alternatively be annelids or a separate phylum and which lived throughout the Paleozoic, belong here. Some living mollusks retain traces of segments, and the Machaeridia, an early Paleozoic group probably at the base of this phylum, were highly segmented. The Mollusca can be said to have originated when the radula did. The primitive Aplacophora lack a shell and are unknown as fossils, while their relatives the chitons come from the Cambrian. The Monoplacophora are mostly Paleozoic and gave rise to snails, cephalopods, and the Paleozoic class Rostroconchia of semibivalves. From the latter originated the clams (which have lost the radula) and the scaphopods, always a sparse group. Clams and snails have gradually expanded, the latter especially since the Cretaceous, when one group evolved a movable proboscis. Cephalopod evolution has been more rapid and complex, with nautiloids dominant in the early Paleozoic and ammonoids from then to their final extinction at the end of the Mesozoic Era (i.e., 66.4 million years ago), after having nearly disappeared three times before. Octopuses and squids grow too rapidly to form an external shell, but one group with an internal shell is known to have thrived in the Mesozoic. Three phyla of annelid relatives feed by a lophophore and are probably related to each other. They too have nearly lost segmentation. Phoronids lack a useful fossil record and probably have always been sparse. Brachiopods and the colonoid bryozoans, on the contrary, were the predominant filter feeders of the Paleozoic Era. Most brachiopods succumbed to the Permian extinction, and the phylum has never recovered. A group of bryozoans, though, has managed to diversify since the middle Cretaceous. Chaetognaths, abundant but with few species, lack a useful fossil record (unless some Cambrian teeth came from them) but appear related in some way to the remaining phyla. Echinoderms had remarkable structural diversity in the early Paleozoic, with no less than 20 classes usually recognized then: some asymmetric, stalked, helical, mobile, or cemented down, with multiple origins of adaptively similar forms. Most were both rare and with few species, but blastoids were abundant in the later Paleozoic, and crinoids were a major group throughout that era. Blastoids became extinct in the Permian, and crinoids nearly so. Most later crinoids are free-swimming rather than stalked like their ancestors. An expansion of powerful general predators (crabs and fishes) in the Jurassic Period (208 to 144 million years ago) reduced the numbers of crinoids and some other groups. Hemichordates are another group now inconspicuous but diverse in the Paleozoic. Most of the latter are called graptolites, colonoids abundant in the Ordovician Period (505 to 438 million years ago) and Silurian Period (438 to 408 million years ago). Hemichordates are very primitive deuterostomes related to both echinoderms and chordates. Of the latter, tunicates lack useful fossils, but a Cambrian cephalochordate shows the early existence of human ancestors. Small pelagic animals called Conodonta, with phosphatic teeth and segmented muscles but no hard skeleton, are most likely cephalochordates but may have been very primitive fishes. They also appeared in the Cambrian and were among the most abundant animals to the end of the Triassic Period (i.e., 208 million years ago). They may have filtered through their spiny teeth rather than through their gills as their ancestors did. Form and function To stay alive, grow, and reproduce, an animal must find food, water, and oxygen, and it must eliminate the waste products of metabolism. The organ systems typical of all but the simplest of animals range from those highly specialized for one function to those participating in many. The more basic functional systems are treated below from a broadly comparative basis. Support and movement A skeleton can support an animal, act as an antagonist to muscle contraction, or, most commonly, do both. Because muscles can only contract, they require some other structure to stretch them to their noncontracted (relaxed) state. Another set of muscles or the skeleton itself can act as an antagonist to muscle contraction. Only elastic skeletons can act without an antagonist; all antagonistic muscles act through a skeleton, which can be either rigid, flexible, or hydrostatic.