animal disease

▪ non-human

Introduction

      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 (zoonosis), 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 (Food and Agriculture Organization)) 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.

General considerations

Historical background

      Historical evidence, like that from currently developing nations, indicates that veterinary medicine originally developed in response to the needs of pastoral and agricultural man along with human medicine. It seems likely that a veterinary profession existed throughout a large area of Africa and Asia from at least 2000 BC. Ancient Egyptian literature includes monographs on both animal and human diseases. Evidence of the parallel development of human and veterinary medicine is found in the writings of Hippocrates on medicine and of Aristotle, who described the symptomatology and therapy of the diseases of animals, including man. Early Greek scholars, noting the similarities of medical problems among the many animal species, taught both human and veterinary medicine. In the late 4th century BC, Alexander the Great designed programs involving the study of animals, and medical writings of the Romans show that some of the most important early observations on the natural history of disease were made by men who wrote chiefly about agriculture, particularly the aspect involving domesticated animals.

      Most of the earliest suggestions of relationships between human health and animal diseases were part of folklore, magic, or religious practice. The Hindu's (Hinduism) concern for the well-being of animals, for example, originated in his belief in reincarnation. From the pre-Christian Era to about 1500, the distinctions between the practices of human and veterinary medicine were not clear-cut; this was especially true in the fields of obstetrics and orthopedics, in which animal doctors in rural areas often delivered babies and set human-bone fractures. It was realized, however, that training in one field was inadequate for practicing in the other, and the two fields were separated.

      Veterinary literature from the civilizations of Greece and Rome contains reference to “herd factors” in disease; contagion within groups of animals kept together, therefore, was recognized, and both quarantine and slaughter were used to control outbreaks of livestock diseases. rinderpest (cattle plague) was the most important livestock disease from the 5th century until control methods were developed. Serious outbreaks of the disease prompted the founding of the first veterinary college (École Nationale Vétérinaire), in Lyon, France, in 1762. Many aspects of animal diseases are best understood in terms of population or herd phenomena; for example, herds of livestock, rather than individual animals, are vaccinated against specific diseases, and housing, nutrition, and breeding practices are related to the likelihood of illness in the herd.

      The work of Pasteur was of fundamental significance to general medicine and to agriculture. Veterinarians became concerned with foods of animal origin after the discovery of microorganisms and their identification with diseases in man and other animals. Efforts were directed toward protecting humans from diseases of animal origin, primarily those transmitted through meat or dairy products. Modern principles of food hygiene, first established for the dairy and meat-packing industries in the 19th and early 20th centuries, have been generally applied to other food-related industries. The veterinary profession, especially in Europe, assumed a major role in early food-hygiene programs.

      Since World War II, the eradication of animal diseases, rather than their control, has become increasingly important, and conducting basic research, combatting zoonoses, and contributing to man's food supply have become indispensable services of veterinary medicine.

Importance

Economic importance

      About 50 percent of the world's population suffers from chronic malnutrition and hunger. Inadequate diet claims many thousands of lives each day. When the lack of adequate food to meet present needs for an estimated world population of more than 4,600,000,000 in the 1980s is coupled with the prediction that the population may increase to 7,000,000,000 by the year 2000, it becomes obvious that animal-food supplies must be increased. One way in which this might be accomplished is by learning to control the diseases that afflict animals throughout the world, especially in the developing nations of Asia and Africa, where the population is expanding most rapidly. Most of the information concerning animal diseases, however, applies to domesticated animals such as pigs, cattle, and sheep, which are relatively unimportant as food sources in these nations. Remarkably little is known of the diseases of the goat, the water buffalo, the camel, the elephant, the yak, the llama, or the alpaca; all are domesticated animals upon which the economies of many developing countries depend. It is in these countries that increased animal production resulting from the development of methods for the control and eradication of diseases affecting these animals is most urgently needed.

      Despite the development of various effective methods of disease control, substantial quantities of meat and milk are lost each year throughout the world. In countries in which animal-disease control is not yet adequately developed, the loss of animal protein from disease is about 30 to 40 percent of the quantity available in certain underdeveloped areas. In addition, such countries also suffer losses resulting from poor husbandry practices.

Role in human disease

       A partial list of zoonosesAnimals have long been recognized as agents of human disease. Man has probably been bitten, stung, kicked, and gored by animals for as long as he has been on earth; in addition, early man sometimes became ill or died after eating the flesh of dead animals. In more recent times, man has discovered that many invertebrate animals are capable of transmitting causative agents of disease from man to man or from other vertebrates to man. Such animals, which act as hosts, agents, and carriers of disease, are important in causing and perpetuating human illness. Because about three-fourths of the important known zoonoses (zoonosis) are associated with domesticated animals, including pets, the term zoonoses was originally defined as a group of diseases that man is able to acquire from domesticated animals. But this definition has been modified to include all human diseases (whether or not they manifest themselves in all hosts as apparent diseases) that are acquired from or transmitted to any other vertebrate animal. Thus, zoonoses are naturally occurring infections and infestations shared by man and other vertebrates. Although the role of domesticated animals in many zoonoses is understood, the role of the numerous species of wild animals with which man is less intimately associated is not well understood. The discovery that diseases such as yellow fever, viral brain infections, plague, and numerous other important diseases involving man or his domesticated animals are fundamentally diseases of wildlife and exist independently of man and his civilization, however, has increased the significance of studying the nature of wildlife diseases. Table 10 (A partial list of zoonoses) contains a partial list of zoonoses, including the causative agents and the animals involved.

Animals in research: the biomedical model

      Although in modern times the practice of veterinary medicine has been separated from that of human medicine, the observations of the physician and the veterinarian continue to add to the common body of medical knowledge. Of the more than 1,200,000 species of animals thus far identified, only a few have been utilized in research, even though it is likely that, for every known human disease, an identical or similar disease exists in at least one other animal species. Veterinary medicine plays an ever-increasing role in the health of man through the use of animals as biomedical models with similar disease counterparts in man. This use of animals as models is important because research on many genetic and chronic diseases of man cannot be carried out using humans.

       A partial list of biomedical models in veterinary medicineHundreds of thousands of mice and monkeys are utilized each year in research laboratories in the U.S. alone. Animal studies are used in the development of new surgical techniques (e.g., organ transplantations), in the testing of new drugs for safety, and in nutritional research. Animals are especially valuable in research involving chronic degenerative diseases because they can be induced experimentally in them with relative ease. The importance of chronic degenerative diseases, such as cancer and cardiovascular diseases, has increased in parallel with the growing number of communicable diseases that have been brought under control. See Table 2 (A partial list of biomedical models in veterinary medicine) for a list of animals with diseases similar to those that occur in man.

      Examples of animal diseases that are quite similar to commonly occurring human diseases include chronic emphysema in the horse; leukemia in cats and cattle; muscular dystrophies in chickens and mice; atherosclerosis in pigs and pigeons; blood-coagulation disorders and nephritis in dogs; gastric ulcers in swine; vascular aneurysms (permanent and abnormal blood-filled area of a blood vessel) in turkeys; diabetes mellitus in Chinese hamsters; milk allergy and gallstones in rabbits; hepatitis in dogs and horses; hydrocephalus (fluid in the head) and skin allergies in many species; epilepsy in dogs and gerbils; hereditary deafness in many small animals; cataracts in the eyes of dogs and mice; and urinary stones in dogs and cattle.

      The study of animals with diseases similar to those that affect man has increased knowledge of the diseases in man; knowledge of nutrition, for example, based largely on the results of animal studies, has improved the health of animals, including man. Animal investigations have been used extensively in the treatment of shock, in open-heart surgery, in organ transplantations, and in the testing of new drugs. Other important contributions to human health undoubtedly will result from new research discoveries involving the study of animal diseases.

Role of ecology

       epidemiology, the study of epidemics, is sometimes defined as the medical aspect of ecology, for it is the study of diseases in animal populations. Hence the epidemiologist is concerned with the interactions of organisms and their environments as related to the presence of disease. The multiple-causality concept of disease embraced by epidemiology involves combinations of environmental factors and host factors, in addition to the determination of the specific causative agent of a given disease. Environmental factors include geographical features, climate, and concentration of certain elements in soil and water. Host factors include age, breed, sex, and the physiological state of an animal as well as the general immunity of a herd resulting from previous contact with a disease. Epidemiology, therefore, is concerned with the determination of the individual animals that are affected by a disease, the environmental circumstances under which it may occur, the causative agents, and the ways in which transmission occurs in nature. The epidemiologist, who utilizes many scientific disciplines (e.g., medicine, zoology, mathematics, anthropology), attempts to determine the types of diseases that exist in a specific geographical area and to control them by modifying the environment.

      Diseases in animal populations are characterized by certain features. Some outbreaks are termed sporadic diseases because they appear only occasionally in individuals within an animal population. Diseases normally present in an area are referred to as endemic, or enzootic, diseases, and they usually reflect a relatively stable relationship between the causative agent and the animals affected by it. Diseases that occasionally occur at higher than normal rates in animal populations are referred to as epidemic, or epizootic, diseases, and they generally represent an unstable relationship between the causative agent and affected animals.

      The effect of diseases on a stable ecological system, which is the result of the dominance of some plants and animals and the subordination or extinction of others, depends on the degree to which the causative agents of diseases and their hosts are part of the system. Epidemic diseases result from an ecological imbalance; endemic diseases often represent a balanced state. Ecological imbalance and, hence, epidemic disease may be either naturally caused or induced by man. A breakdown in sanitation in a city, for example, offers conditions favourable for an increase in the rodent population, with the possibility that diseases such as plague may be introduced into and spread among the human population. In this case, an epidemic would result as much from an alteration in the environment as from the presence of the causative agent Pasteurella pestis, since, in relatively balanced ecological systems, the causative agent exists enzootically in the rodents (i.e., they serve as reservoirs for the disease) and seldom involves man. In a similar manner, an increase in the number of epidemics of viral encephalitis, a brain disease, in man has resulted from the ecological imbalance of mosquitoes and wild birds caused by man's exploitation of lowland for farming. Driven from their natural habitat of reeds and rushes, the wild birds, important natural hosts for the virus that causes the disease, are forced to feed near farms; mosquitoes transmit the virus from birds to cattle and man.

Detection and diagnosis

Reactions of tissue to disease

       Selected infectious and parasitic diseases of animals Examples of noninfectious diseases of animals Some common diseases of dogs Some common diseases of domestic cats Some Common Diseases of Domestic Poultry, Table Some common diseases of fish Some important diseases of some common laboratory animalsAs previously noted, disease may be defined as an injurious deviation from a normal physiological state of an organism sufficient to produce overt signs, or symptoms. The deviation may be either an obvious organic change in the tissue comprising an organ or a functional disturbance whose organic changes are not obvious. The severity of the changes that occur in cells and tissues subjected to injurious agents is dependent upon both the sensitivity of the tissue concerned and the nature and time course of the agent. A mildly injurious agent that is present for short periods of time may either have little effect or stimulate cells to increased activity. Strongly injurious agents in prolonged contact with cells cause characteristic changes in them by interfering with normal cell processes. Most causative agents of disease fall into the latter category. Causative agents and some of the symptoms of many of the diseases mentioned in this section are found in Tables 5 (Selected infectious and parasitic diseases of animals), 6 (Examples of noninfectious diseases of animals), 7 (Some common diseases of dogs), 8 (Some common diseases of domestic cats), 9 (Some Common Diseases of Domestic Poultry, Table), 10 (Some common diseases of fish), and 11 (Some important diseases of some common laboratory animals).

Characteristics of cell and tissue changes

      Changes in cells and tissues as a result of disease include degenerative and infiltrative changes. Degenerative changes are characterized by the deterioration of cells or a tissue from a higher to a lower form, especially to a less functionally active form. When chemical changes occur in the tissue, the process is one of degeneration. When the changes involve the accumulation of materials within the cells comprising tissues, the process is called infiltration. Diseases such as pneumonia, metal poisoning, or septicemia (the persistence of disease-causing bacteria in the bloodstream) may cause the mildest type of degeneration—parenchymatous changes, or cloudy swelling of the cells; the cells first affected are the specialized cells of the liver and the kidney. Serious cellular damage may cause the uptake of water by cells (hydropic degeneration), which lose their structural features as they fill with water. The causes for the accumulation in cells of abnormal amounts of fats (fatty infiltration and degeneration) have not yet been established with certainty but probably involve fat metabolism. Poisons such as phosphorus may cause sudden increases in the accumulation of fats in the liver. An abnormal protein material may accumulate in connective-tissue components of small arteries as a result of chronic pneumonia, chronic bacterial infections, and prolonged antitoxin production (in horses); the condition is known as amyloid degeneration and infiltration. Hyaline degeneration, characterized by tissues that become clear and appear glasslike, usually occurs in connective-tissue components of small blood vessels as a result of conditions that may occur in kidney structures (glomeruli) of animals with nephritis or in lymph glands of animals with tuberculosis. Certain structures (glomeruli) of animals with nephritis result in degeneration.

      The condition in which mucus, a secretion of mucous membranes lining the inside surfaces of organs, is produced in excess and accumulates in greater than normal amounts is referred to as mucoid degeneration. Major causes of this condition include chronic irritation of mucous membranes and certain mucus-producing tumours. Abnormal amounts of glycogen, which is the principal storage carbohydrate of animals, may occur in the liver as a result of certain inherited diseases of animals; the condition is known as glycogen infiltration. The abnormal deposition of calcium salts, which is known as hypercalcification, may occur as a result of several diseases involving the blood vessels and the heart, the urinary system, the gallbladder, and the bonelike tissue called cartilage. Pigments (coloured molecules) from coal dust or asbestos dust may infiltrate the lungs of certain dogs in two types of lung disease: anthracosis and asbestosis. Abnormal amounts of iron-containing coloured molecules (hemosiderin) resulting from the breakdown of hemoglobin, the oxygen-carrying protein of red blood cells, are often deposited in the liver and the spleen after diseases that involve excessive breakdown of red blood cells. A dark-coloured molecule (melanin) occurs abnormally in the livers of certain sheep suffering from Dubin–Johnson syndrome and in certain tumours called melanomas. Uric acid infiltration, which occurs in poultry, is characterized by the deposition of uric acid salts.

       necrosis, the death of cells or tissues, takes place if the blood supply to tissues is restricted; poisons produced by microbes, chemical poisons, and extreme heat or electricity also may cause necrosis. The rotting of the dead tissue is known as gangrene.

      Atrophy of animal tissue involves a process of tissue wasting, in which a decrease occurs in the size or number of functional cells—e.g., in inherited muscular dystrophy of chickens. Hypertrophy—an increase in the size of the cells in a tissue or an organ—occurs in heart muscle during diseases involving the heart valves, in certain pneumonias, and in some diseases of the endocrine glands. Aplasia is the term used when an entire organ is missing from an animal; hypoplasia indicates arrested or incomplete development of an organ, and hyperplasia an increase in the production of the number of cells—e.g., the persistent callus that forms on the elbows of some dogs. Metaplasia is used to describe the change of one cell type into another; it may occur in chronic irritation of tissues and in certain cancerous tumours.

Characteristics of inflammatory reactions

      When tissues are injured, they become inflamed. The inflammation may be acute, in which case the inflammatory processes are active, or chronic, in which case the processes occur slowly and new connective tissue is formed. The reaction of inflamed tissues is a combination of defensive and repair mechanisms. Acute inflammation is characterized by redness, heat, swelling, sensitivity, and impaired function. Several types of acute inflammation are known. Mild acute inflammations of mucous membranes resulting in the production of thin watery material (exudate) are called catarrhal inflammations; parenchymatous inflammations occur in organs undergoing degeneration. If the exudate formed in response to an injury is of a serous nature—that is, resembling blood plasma—the process is called serous inflammation. In fibrinous inflammation, a protein (fibrin) forms on membranes, including those in the lungs. In suppurative inflammation, dead tissue is replaced with pus composed of colourless blood cells (leucocytes) and tissue juices.

      During the inflammatory reaction, the injured tissue is surrounded by an area of rapidly dividing cells. Specialized cells called macrophages enter the tissue and remove blood and tissue debris. Other cells, called neutrophils, ingest disease-causing bacteria and other foreign material. In chronic inflammations, the connective tissue contains fibroblasts, cells that divide and form new connective, or scar, tissue.

Characteristics of circulatory disturbances

      An increase in the rate of blood flow to a body part, which is referred to by the term congestion, or hyperemia, occurs during inflammation; a diminished blood flow to tissues is referred to by the term ischemia, or a local anemia. Examples of hemorrhage, the escape of blood from vessels, include epistaxis, or nosebleeds, in racehorses; hematemesis, or regurgitation of blood, in dogs with uremia; hemoptysis, or blood loss from lungs; hematuria, or blood in urine, of cattle with inflammation of the urinary bladder. edema, a condition that is characterized by abnormal accumulations of fluid in tissues, occurs not only in a tissue during inflammation but also over the entire body if the concentration of blood-serum proteins, especially albumin, is low. A thrombosis, which is a blood clot in a blood vessel, may block or slow circulation of blood to tissues; if blood vessels become blocked, the condition is known as an embolism. The term infarction describes the necrosis that occurs in tissues whose blood supply is blocked by an embolism.

Methods of examination

      Before an unhealthy animal receives treatment, an attempt is made to diagnose (diagnosis) the disease. Both clinical findings, which include symptoms that are obvious to a nonspecialist and clinical signs that can be appreciated only by a veterinarian, and laboratory test results may be necessary to establish the cause of a disease. A clinical examination should indicate if the animal is in good physical condition, is eating adequately, is bright and alert, and is functioning in an apparently normal manner. Many disease processes are either inflammatory or result from tumours (tumour). Malignant tumours (e.g., melanomas in horses, squamous cell carcinomas in small animals) tend to spread rapidly and usually cause death. Other diseases cause the circulatory disturbances or the degenerative and infiltrative changes that are summarized in the preceding section. If a specific diagnosis is not possible, the symptoms of the animal are treated.

      A case record of the information pertaining to an animal (or to a herd of animals) that is suspected of having a disease is begun at the time the animal is taken to a veterinarian (or the veterinarian visits the animal) and is continued through treatment. It includes a description of the animal (age, species, sex, breed); the owner's report; the animal's history; a description of the preliminary examination; clinical findings resulting from an examination of body systems; results of specific laboratory tests; diagnosis regarding a specific cause for the disease (etiology); outlook (prognosis); treatment; case progress; termination; autopsy, if performed; and the utilization of scientific references, if applicable.

      The veterinarian must diagnose a disease on the basis of a variety of examinations and tests, since he obviously cannot interrogate the animal. Methods used in the preparation of a diagnosis include inspection—a visual examination of the animal; palpation—the application of firm pressure with the fingers to tissues to determine characteristics such as abnormal shapes and possible tumours, the presence of pain, and tissue consistency; percussion—the application of a short, sharp blow to a tissue to provoke an audible response from body parts directly beneath; auscultation—the act of listening to sounds that are produced by the body during the performance of functions (e.g., breathing, intestinal movements); smells—the recognition of characteristic odours associated with certain diseases; and miscellaneous diagnostic procedures, such as eye examinations, the collection of urine, and heart, esophageal, and stomach studies.

General inspection

      Deviation of various characteristics from the normal, observation of which constitutes the general inspection of an animal, is a useful aid in diagnosing disease. The general inspection includes examination of appearance; behaviour; body condition; respiratory movements; state of skin, coat, and abdomen; and various common actions.

       Selected infectious and parasitic diseases of animals Some common diseases of dogs Selected infectious and parasitic diseases of animalsThe appearance of an animal may be of diagnostic significance; small size in a pig may result from retardation of growth, which is caused by hog-cholera virus (Table 3 (Selected infectious and parasitic diseases of animals)). Observation of the behaviour of an animal is of value in diagnosing neurological diseases; e.g., muscle spasms occur in lockjaw (tetanus) in dogs, nervousness and convulsions in dogs with distemper (Table 5 (Some common diseases of dogs)), dullness in horses with equine viral encephalitis (Table 3 (Selected infectious and parasitic diseases of animals)), and excitement in animals suffering from lead poisoning. Subtle behavioral changes may not be noticeable. The general condition of the body is of value in diagnosing diseases that cause excessive leanness (emaciation), including certain cancers, or other chronic diseases, such as a deficiency in the output of the adrenal glands or tuberculosis. Defective teeth also may point to malnutrition and result in emaciation.

      The respiratory movements of an animal are important diagnostic criteria; breathing is rapid in young animals, in small animals, and in animals whose body temperature is higher than normal. Specific respiratory movements are characteristic of certain diseases—e.g., certain movements in horses with heaves (emphysema) or the abdominal breathing of animals suffering from painful lung diseases. The appearance of the skin and hair may indicate dehydration by lack of pliability and lustre; or the presence of parasites such as lice, mites, or fleas; or the presence of ringworm infections and allergic reactions by the skin changes they cause. The poisoning of sheep by molybdenum in their hay may be diagnosed by the loss of colour in the wool of black sheep. Distension of the abdomen may indicate bloat in cattle or colic in horses.

      Abnormal activities may have special diagnostic meaning to the veterinarian. Straining during urination is associated with bladder stones; increased frequency of urination is associated with kidney disease (nephritis), bladder infections, and a disease of the pituitary gland (diabetes insipidus). Excessive salivation and grinding of teeth may be caused by an abnormality in the mouth. Coughing is associated with pneumonia. Some diseases cause postural changes: for example, a horse with tetanus may stand in a stiff manner. An abnormal gait in an animal made to move may furnish evidence as to the cause of a disease, as louping ill in sheep.

Clinical examination

      Following the general inspection of an animal thought to have contracted a disease, a more thorough clinical examination is necessary, during which various features of the animal are studied. These include the visible mucous membranes (conjunctiva of the eye (eye disease), nasal mucosa, inside surface of the mouth, and tongue); the eye itself; and such body surfaces as the ears, horns (if present), and limbs. In addition, the pulse rate and the temperature are measured.

       Selected infectious and parasitic diseases of animalsThe veterinarian examines the visible mucous membranes of the eye, nose, and mouth to determine if jaundice, hemorrhages, or anemia are present. The conjunctiva, or lining of the eye, may exhibit pus in pinkeye infections, have a yellow appearance in jaundice, or exhibit small hemorrhages in certain systemic diseases. Examination of the nose may reveal ulcers and vesicles (small sacs containing liquid), as in foot-and-mouth disease, a viral disease of cattle, or vesicular exanthema, a viral disease of swine. Ulceration of the tongue may be apparent in animals suffering from actinobacillosis, a disease of bacterial origin (see Table 3 (Selected infectious and parasitic diseases of animals)).

       Some common diseases of dogsA detailed examination of the eye may show abnormalities of the cornea resulting from such diseases as infectious hepatitis in dogs (Table 5 (Some common diseases of dogs)), bovine catarrhal fever, and equine influenza. Cataract, a condition in which the passage of light through the lens of the eye is obstructed, may result from a disorder of carbohydrate metabolism (diabetes mellitus), infections, or a hereditary defect.

      An elevated temperature, or fever, resulting from the multiplication of disease-causing organisms may be the earliest sign of disease. The increase in temperature activates the body mechanisms that are necessary to fight off foreign substances. Measuring the pulse rate is useful in determining the character of the heartbeat and of the circulatory system.

Tests as diagnostic aids

       Selected infectious and parasitic diseases of animalsIn many cases, the final diagnosis of an animal disease is dependent upon a laboratory test. Some involve measuring the amount of certain chemical constituents of the blood or body fluids, determining the presence of toxins (poisons), or examining the urine and feces. Other tests are designed to identify the causative agents of the disease. The removal and examination of tissue or other material from the body (biopsy) is used to diagnose the nature of abnormalities such as tumours. Specific skin tests are used to confirm the diagnoses of various diseases—e.g., tuberculosis and Johne's disease in cattle and glanders in horses (Table 3 (Selected infectious and parasitic diseases of animals)).

      Confirmation of the presence in the blood of abnormal quantities of certain constituents aids in diagnosing certain diseases. Abnormal levels of protein in the blood are associated with some cancers (cancer) of the bone, such as multiple myeloma in horses and dogs. Animals with diabetes mellitus have a high level of the carbohydrate glucose and the steroid cholesterol in the blood. The combination of an increase in the blood level of cholesterol and a decrease in the level of iodine bound to protein indicates hypothyroidism (underactive thyroid gland). A low level of calcium in the serum component of blood confirms milk fever in lactating dairy cattle. An increase in the activities of certain enzymes (biological catalysts) in the blood indicates liver damage. An increase in the blood level of the bile constituent bilirubin is used as a diagnostic test for hemolytic crisis, a disease in which red blood cells are rapidly destroyed by organisms such as Babesia species in dogs and in cattle and Anaplasma species in cattle.

      The examination of the formed elements of blood, including the oxygen-carrying red blood cells (erythrocytes), the white blood cells (neutrophils, eosinophils, basophils, lymphocytes, and monocytes), and the platelets, which function in blood coagulation, is helpful in diagnosing certain diseases. Examination of the blood cells of cattle may reveal abnormal lymphocytic cells characteristic of leukemia. Low numbers of leucocytes indicate the presence of viral diseases, such as hog cholera and infectious hepatitis in dogs. Neutrophil levels increase in chronic bacterial diseases, such as canine pneumonia and uterine infections in female animals. Elevated monocyte levels occur in chronic granulomatous diseases; e.g., histoplasmosis and tuberculosis. Canine parasitism and allergic skin disorders are characterized by elevated eosinophil levels. Prolonged clotting time may be associated with a deficiency of platelets.

       anemia has many causes. They include hemorrhages from blood loss after injuries; the destruction of red blood cells by the rickettsia Haemobartonella felis in cats; incompatible blood transfusions in dogs; the inadequate production of normal red blood cells, which occurs in iron or cobalt deficiency after exposure to radioactive substances; general malnutrition; and contact with substances that depress the activity of bone marrow.

      Poisonings occur commonly in animals. Some species are more sensitive to certain poisons than others. Swine develop mercury poisoning if they eat too much grain that has been treated with mercury compounds to retard spoilage. Dogs may be poisoned by the arsenic found in pesticides or by strychnine, which is found in rat poison. Many plants are poisonous if eaten, such as bracken fern, which poisons cattle and horses, and ragwort, which contains a substance poisonous to the liver of cattle.

       Examples of noninfectious diseases of animalsExamination of an animal's urine may reveal evidence of kidney diseases or diseases of the entire urinary system or a generalized systemic disease. The presence of protein in the urine of dogs indicates acute kidney disease (nephritis). Although constituents of bile normally are found in the urine of dogs, the quantity increases in dogs with the presence of infectious hepatitis, a disease of the liver. The presence of abnormal amounts of the simple carbohydrate glucose and of ketone bodies (organic compounds involved in metabolism) in an animal's urine is used to diagnose diabetes mellitus, a disease in which the pancreas cannot form adequate quantities of a substance (insulin) important in regulating carbohydrate metabolism. The urine of horses with azoturia (excessive quantities of nitrogen-containing compounds in the urine, Table 4 (Examples of noninfectious diseases of animals)) or muscle breakdown may contain a dark-coloured molecule called myoglobin.

      The presence of eggs or parts of worms in the excrement of animals suspected of suffering from intestinal parasites, such as roundworms, tapeworms, or flatworms, aids in diagnosis. Feces that are light in colour, have a rancid odour, contain fat, and are poorly formed may indicate the existence of a chronic disease of the pancreas. Clay-coloured fatty feces suggest obstruction of the bile duct, which conveys bile to the intestine during digestion.

       Selected infectious and parasitic diseases of animalsThe identification of a disease-causing microorganism within an animal enables the veterinarian to choose the best drug for therapy. Agglutination tests, which utilize serum samples of animals and microorganisms suspected of causing a disease, many times confirm the presence of the following bacterial diseases (Table 3 (Selected infectious and parasitic diseases of animals)): brucellosis in cattle and swine, salmonellosis in swine, leptospirosis in cattle, and actinobacillosis in swine and cattle. Other tests measure the antibodies (specific proteins formed in response to a foreign substance in the body) formed against a disease-causing agent, such as those that cause brucellosis, foot-and-mouth disease, infectious hepatitis in dogs, and fowl pest.

      The modern veterinary diagnostic laboratory performs, in addition to the tests mentioned, tests of cells in the bone marrow; specific-organ-function tests (liver, kidney, pancreas, thyroid, adrenal, and pituitary glands); radioisotope tests, tissue biopsies, and histochemical analyses; and tests concerning blood coagulation and body fluids.

Survey of animal diseases

Infectious (infectious disease) and noninfectious diseases

      Diseases may be either infectious or noninfectious. The term infection, as observed earlier, implies an interaction between two living organisms, called the host and the parasite. Infection is a type of parasitism, which may be defined as the state of existence of one organism (the parasite) at the expense of another (the host). Agents (e.g., certain viruses, bacteria, fungi, protozoans, worms, and arthropods) capable of producing disease are pathogens. The term pathogenicity refers to the ability of a parasite to enter a host and produce disease; the degree of pathogenicity—that is, the ability of an organism to cause infection—is known as virulence. The capacity of a virulent organism to cause infection is influenced both by the characteristics of the organism and by the ability of the host to repel the invasion and to prevent injury. A pathogen may be virulent for one host but not for another. Pneumococcal bacteria, for example, have a low virulence for mice and are not found in them in nature; if introduced experimentally into a mouse, however, the bacteria overwhelm its body defenses and cause death.

       Selected infectious and parasitic diseases of animals Selected infectious and parasitic diseases of animalsMany pathogens (e.g., the bacterium that causes anthrax, Table 3 (Selected infectious and parasitic diseases of animals)) are able to live outside the animal's body until conditions occur that are favourable for entering and infecting it. Pathogens enter the body in various ways—by penetrating the skin or an eye, by being eaten with food, or by being breathed into the lungs. After their entry into a host, pathogens actively multiply and produce disease by interfering with the functions of specific organs or tissues of the host. Table 3 (Selected infectious and parasitic diseases of animals) lists some infectious and parasitic diseases of animals and the causative agents.

      Before a disease becomes established in a host, the barrier known as immunity must be overcome. Defense against infection is provided by a number of chemical and mechanical barriers, such as the skin, mucous membranes and secretions, and components of the blood and other body fluids. Antibodies (antibody), which are proteins formed in response to a specific substance (called an antigen) recognized by the body as foreign, are another important factor in preventing infection. Immunity among animals varies with species, general health, heredity, environment, and previous contact with a specific pathogen.

      As certain bacterial species multiply, they may produce and liberate poisons, called exotoxins, into the tissues; other bacterial (bacteria) pathogens contain toxins, called endotoxins, which produce disease only when liberated at the time of death of the bacterial cell. Some bacteria, such as certain species of Clostridium and Bacillus, have inactive forms called spores, which may remain viable (i.e., capable of developing into active organisms) for many years; spores are highly resistant to environmental conditions such as heat, cold, and chemical compounds called disinfectants, which are able to kill many active bacteria.

       Selected infectious and parasitic diseases of animalsThe term infestation indicates that animals, including spiny-headed worms (Acanthocephala), roundworms (Nematoda), flatworms (Platyhelminthes), and arthropods such as lice, fleas, mites, and ticks, are present in or on the body of a host. An infestation is not necessarily parasitic. Table 3 (Selected infectious and parasitic diseases of animals) includes various infestations.

       Examples of noninfectious diseases of animalsNoninfectious diseases are not caused by virulent pathogens and are not communicable from one animal to another (see Table 4 (Examples of noninfectious diseases of animals)). They may be caused by hereditary factors or by the environment in which an animal lives. Many metabolic diseases (metabolic disease) are caused by an unsuitable alteration, sometimes brought about by man, in an animal's genetic constitution or in its environment. Metabolic diseases usually result from a disturbance in the normal balance of the physiological mechanisms that maintain stability, or homeostasis. Examples of metabolic diseases include overproduction or underproduction of hormones, which control specific body processes; nutritional deficiencies; poisoning from such agents as insecticides, fungicides, herbicides, fluorine, and poisonous plants; and inherited deficiencies in the ability to synthesize active forms of specific enzymes, which are the proteins that control the rates of chemical reactions in the body.

      Excessive inbreeding (i.e., the mating of related animals) among all domesticated animal species has resulted in an increase in the number of metabolic diseases and an increase in the susceptibility of certain animals to infectious diseases.

Zoonoses (zoonosis)

       A partial list of zoonosesAs stated previously, zoonoses are human diseases acquired from or transmitted to any other vertebrate animal. Zoonotic diseases are common in currently developing countries throughout the world and constitute, with starvation, the major threat to human health. More than 150 such diseases are known; some examples are listed in Table 10 (A partial list of zoonoses).

      Zoonoses may be separated into four principal types, depending on the mechanisms of transmission and epidemiology. One type includes the direct zoonoses, such as rabies and brucellosis, which are maintained in nature by one vertebrate species. The transmission cycle of the cyclozoonoses, of which tapeworm infections are an example, requires at least two different vertebrate species. Both vertebrate and invertebrate animals are required as intermediate hosts in the transmission to humans of metazoonoses; arboviral and trypanosomal diseases are good examples of metazoonoses. The cycles of saprozoonoses (for example, histoplasmosis) may require, in addition to vertebrate hosts, specific environmental locations or reservoirs.

      Most animals that serve as reservoirs for zoonoses are domesticated and wild animals with which man commonly associates. People in occupations such as veterinary medicine and public health, therefore, have a greater exposure to zoonoses than do those in occupations less closely concerned with animals.

       A partial list of zoonosesIn addition to the numerous human diseases spread by contact with the parasitic worm helminth and by contact with arthropods (see Table 10 (A partial list of zoonoses)), many diseases are transmitted by the bites and venom of certain animals; poisonous or diseased food animals also transmit diseases. Dog bites may seriously injure tissues and also can transmit bacterial infections and rabies, a disease of viral origin. The bite of a diseased rat may transmit any of several diseases to man, including plague, salmonellosis, leptospirosis, and rat-bite fevers. Cat-scratch disease may be transmitted through cat bites, and the deadly herpes B virus can spread by monkey bites. The bites of venomous snakes (snakebite) and fish account for considerable human discomfort and death. About 200 of the 2,500 known species of snakes can cause human disease. One estimate for snakebite deaths worldwide is 30,000 to 40,000 per year, the vast majority of them in Asia. Poisonous wild animals inadvertently used for food include animals harbouring the anthrax bacillus and those containing the causative agents of salmonellosis, trichinosis, and fish-tapeworm (fish) infection. The flesh of various types of fish is toxic to man. Japanese puffers, for example, contain the poisonous chemical compound tetrodotoxin; scombroid fish harbour Proteus morganii, which causes gastrointestinal diseases; and mullet and surmullet can cause nervous disturbances.

      Approaches to the control of zoonoses differ according to the type under consideration. Because the majority of direct zoonoses and cyclozoonoses and some saprozoonoses are most effectively controlled by techniques involving the animal host, methods used to combat these diseases are almost entirely the responsibility of veterinary medicine. A good example is the elimination of stray dogs, for they are an important factor in the control of zoonoses such as rabies, hydatid disease, and visceral larva migrans. In addition, the control of diseases such as brucellosis and tuberculosis in cattle involves a combination of methods—mass immunization, diagnosis, slaughter of infected animals, environmental disinfection, and quarantine. Several supportive measures for the control of disease are useful in some cases. Air-sanitation measures are helpful in direct zoonoses in which human illness is spread by droplets or dust, and zoonotic infections that are spread through a fluid medium, such as water or milk, sometimes can be controlled. Heat, cold, and irradiation are effective in killing the immature forms of Trichinella spiralis, the causative agent of trichinosis, in meat; and certain antibiotic drugs help to prevent deterioration of food.

      The control of metazoonoses may be directed at the infected vertebrate hosts, at the infected invertebrate host, or at both. Particularly effective in this instance has been the use of chemical insecticides (insecticide) to attack the invertebrate carriers of specific infections, even though several difficulties have been encountered—for example, the inaccessibility of the invertebrate to the chemicals, which occurs with organisms that breed in swiftly flowing waters or in dense vegetation, and the development of insecticide resistance by the organisms. Insecticides are used to destroy the mosquitoes that spread malaria (Anopheles). Mechanical filters placed across irrigation ditches help to prevent the dissemination of the snails that transmit Schistosoma mansoni, a parasitic flatworm.

Disease prevention, control, and eradication

      Prevention is the first line of defense against disease. At least four preventive (preventive medicine) techniques are available for use in the prevention of disease in an animal population. One is the exclusion of causative agents of disease from specific geographic areas, or quarantine. A second preventive tool utilizes control methods such as immunization, environmental control, and chemical agents to protect specific animal populations from endemic diseases, diseases normally present in an area. The third preventive measure concerns the mass education of people about disease prevention. Finally, early diagnosis of illness among members of an animal population is important so that disease manifestations do not become too severe and so that affected animals can be more easily managed and treated.

       quarantine—the restriction of movement of animals suffering from or exposed to infections such as bluetongue and scrapie (in sheep), foot-and-mouth disease (in cattle), and rabies (in dogs)—is one of the oldest tools of preventive medicine. It was applied to domesticated animals as early as Roman times. The establishment of international livestock quarantine in the United States in 1890 provided for the holding of all imported cattle, sheep, and swine at the port of entry for 90, 15, and 15 days, respectively. In this way, such diseases as Nairobi sheep disease, surra, and infections caused by Brucella melitensis were eliminated or excluded from the United States, but international quarantine barriers did not prevent the entry of bluetongue, scrapie, and the tick Rhipicephalus evertsi, which is a carrier for several animal diseases. On the other hand, long-term quarantine of all dogs entering Great Britain has been effective since its initiation in 1919 (the quarantine also includes cats). It is possible that aircraft may pose new problems regarding livestock-disease quarantine since many disease carriers (e.g., insects and viruses) may be accidentally brought by plane into a country.

       Some Common Diseases of Domestic Poultry, TableMass immunization as a preventive technique has the advantage of allowing the resistant animal freedom of movement, unlike environmental control, in which the animal is confined to the controlled area; immunization may, however, provide only short-lived and partial protection. Mass-inoculation techniques against diseases such as Newcastle disease in chickens and distemper in mink and dogs have been successful. Animal diseases have been prevented by methods involving environmental control, including the maintenance of safe water supplies, the hygienic disposal of animal excrement, air sanitation, pest control, and the improvement of animal housing. One specific environmental program, called the portable-calf-pen system, involves routine movement of the pens to avoid a concentration of specific pathogens in them. Other programs involve the utilization of automatic and sanitary watering and feeding equipment and buildings with environmental controls. The use of chemical compounds to prevent illness (chemoprophylaxis) includes a variety of pesticides, which are used to kill insects that transmit diseases, and substances either used internally or applied to the animal's body to prevent the transmission or the development of a disease. An example is the use of sulfonamide drugs in the drinking water of poultry to prevent coccidiosis (see Table 7 (Some Common Diseases of Domestic Poultry, Table)). Environmental-control methods in the poultry industry have resulted in the most efficient means of poultry production developed thus far.

      The early detection of a disease in a population of animals—a herd of cattle, for example—is particularly useful in controlling certain chronic infectious diseases, such as mastitis, brucellosis, and tuberculosis, as well as certain noninfectious diseases such as bloat. Laboratory tests—the agglutination test in pullorum disease, the tuberculin skin test for tuberculosis, the examination of feces for eggs of specific parasites, the physical and chemical tests performed on milk to diagnose bovine mastitis—are used for the early detection of diseases in an animal population.

      Methods of disease control and eradication have been successful in various countries. In the United States, for example, the test-and-slaughter technique, in which simple tests are used to confirm the existence of diseased animals that are then slaughtered, has been of great value in controlling infectious and hereditary diseases, including dourine, a venereal disease in horses, fowl plague, and foot-and-mouth disease in cattle and deer. Bovine tuberculosis has been eliminated from Denmark, Finland, and The Netherlands and reduced to a low level in various other countries, including Great Britain, Japan, the United States, and Canada, by the test-and-slaughter method. Many infectious diseases have been eradicated from Great Britain—sheep pox, rinderpest, pleuropneumonia, glanders, and rabies. Diseases eliminated from Australia by a combination of methods—control of agents that carry disease, the test-and-slaughter technique, the use of chemical agents, and, more recently, biological control—include hog cholera, rinderpest, scrapie, glanders, surra, rabies, and foot-and-mouth disease.

      In biological control, enemies of the agents that transmit the disease, enemies of the reservoir host, or a specific parasite are introduced into the environment. If a natural enemy of the tsetse fly could be found, for example, African sleeping sickness in man and trypanosomiasis in cattle could be controlled in West Africa. Successful biological control of the European-rabbit population in Australia has been accomplished through the use of the myxomatosis virus, which is transmitted by mosquitoes and causes the formation of malignant tumours. Although the Brazilian white rabbit is relatively unaffected by the virus, it causes rapid death in the European rabbit. The elimination of the European rabbit in France by the virus was accompanied by a decrease in tick-borne typhus in people, suggesting that the rabbit may be a significant intermediate host for the causative agent, Rickettsia conorii. Screwworms, an immature form of the fly Cochliomyia hominivorax, have been eradicated in the United States by the release of more than 3,000,000,000 sterilized males.

       Animal diseases usually confined to certain regions of the worldDisease control and elimination programs require many sophisticated techniques, in addition to diagnosis and the slaughter of affected animals. They include: the control of insects known to transmit diseases; the cooperation of animal owners; the development through research of new diagnostic tests for use on large populations; the eradication of animal species from areas in which they are known to transmit disease (Table 13 (Animal diseases usually confined to certain regions of the world)); sterilization of strains of animals known to carry inheritable metabolic diseases; and effective meat inspection.

Charles E. Cornelius Ed.

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 Ed.

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