Animal diseases are important because of the possible economic losses that may occur, the transmission of disease to humans, and the ecological imbalances that may occur. \Tveterinary medicine\t is the study, prevention, and treatment of diseases in both domesticated and wild animals. Zoonoses are diseases communicable from animals to humans, and programs for their control are closely related to human health. SIGNIFICANCE In 1796, Edward \TJenner\t, a British physician, discovered that milkmaids who had been infected with cowpox, a localized skin eruption in cows and man, were protected against \Tsmallpox\t. This showed that animal virus diseases also afflicted humans and that their control was important to human survival. Additional evidence has been found in the fossil remains of the tsetse fly in Colorado. The tsetse, now found in Africa, transmits trypanosomes, which cause \Ttrypanosomiasis\t, or sleeping sickness, and Chagas' disease in humans and animals. This suggests the reason why prehistoric horses became extinct in North America, as well as placing the origin of trypanosomiasis at about 4000 BC. Certainly from this time, and probably since the origin of humans, diseases have been shared by humans and animals. More than 100 known diseases are transmitted from animals to humans, and three-fourths of the rural human population of the world suffers from animal diseases. Rabies alone causes the death of several thousand people each year, and about 10 percent of the world's animals suffer from \Ttuberculosis\t. In many parts of the world the soil is heavily contaminated with spores of \Tanthrax\t, a disease that, according to the Book of Exodus, caused a plague in Egypt about 1900 BC. Not only have animal diseases become a human health hazard, but the presence of animal infection reduces the food supply immeasurably and interferes with the profitable use of animals as beasts of burden. Food produced by sick animals is of inferior quality, the wool of sheep afflicted with keds (wingless parasitic flies) is less valuable, and cows with Foot-and-mouth disease have reduced milk production. Not only the quality but quantity of production is affected; in addition, the loss of dead or sick animals may be expected to be that a farmer must replace 10 percent of his animals annually. The cost of caring for sick animals also rises. The diagnosis, treatment, and eradication of an infection usually causes the price of meat and animal products paid by consumers to increase. Veterinarians with training and experience in disease control and treatment are responsible for protecting the milk and meat supply, both of which are important in preventing protein starvation. Animal diseases are also important ecologically. Few people question the feeding of wild birds or the hazard of an overloaded waste-disposal system; in the case of Salmonella (bacteria that cause food poisoning), however, both bird-feeding and waste disposal complete an infectious cycle that is well documented. Rats, mice, and unconfined birds may come in contact with Salmonella organisms when a disposal system overflows, and animal or bird droppings may accidentally be carried into houses when they adhere to people's shoes. Humans may also be infected through hand contamination. Food may be contaminated when insufficient or inadequate sanitation is practiced. Salmonellosis, in its various forms, is among the major diseases of both wild and domestic animals and of birds. In swine, for example, Salmonella cholerasuis has resulted in extensive losses of pigs, particularly when it has been found in conjunction with hog \Tcholera\t virus. It causes ulcers of the intestines as well as diarrhea, which, in turn, causes the intestines to become inflamed. INFECTIOUS DISEASES Animal diseases fall into two classifications, infectious and noninfectious. Infectious diseases are caused by agents that invade the host and produce an alteration in the state of health. These agents include viruses, true bacteria, protozoalike bacteria, funguslike bacteria, rickettsial organisms, chlamydial organisms, fungi, yeastlike fungi, and mycoplasmal agents. Internal and external parasites may also be included in the infectious group. In addition to these causative agents, stress must be included as a cause or condition for the development of disease. Infectious diseases do not always become established when direct exposure occurs, although the host must be exposed in order to become infected. For exposure to take place, a reservoir of the agent must exist in humans, in animals, or in the free-living state. Transmission by hands, clothing, food, or water, as well as by insects, facilitates possible exposure and thus the opportunity for infection. Often, infection is a factor of exposure: if a large dose is received, a person is more likely to become infected. Equally important is the nature of the organism and the method of establishing the exposure. For example, an animal may lie down in material containing \Ttetanus\t organisms or spores without getting the disease; if the organism is forced into the body in a deep wound, however, the probability of infection is increased. In establishing infection, most organisms bypass primary defenses such as the skin. The organisms may enter through breaks in the skin, such as abrasions, cuts, or insect bites. At other times they enter through the body orifices, such as the mouth, nose, eyes, urinary tract, udder, or genital tract. The easiest mode of infection for most animals is contaminated food, water, or air. For instance, careless assistance in the birth of animals often establishes infection, although contaminated nose leads, calf pails, shovels, carts, and other equipment may also serve to transmit agents of disease. Contamination of premises and equipment results from the release of infected discharges from any part of the body; thus it is essential to isolate infected animals in order to contain infection. Once in the body, infectious agents may cause a rise in temperature and an increase in pulse and respiration rates. Local or generalized infection may ensue. Animals in the early stages of disease may show no evidence of infection. As the infection progresses, however, the animals may stop eating and producing. Numerous other signs may also be evident, though only a competent veterinarian can make a reliable diagnosis. Slow Virus Diseases Slow virus diseases include scrapie, a nervous disorder of sheep and goats. This is an infection imposed by a polypeptide (a part of a protein) closely associated with deoxyribonucleic acid (\TDNA\t) molecule, which together may be considered a partial virus or viruslike agent (see \Tvirus, slow\t). Reported in many parts of the world, Scrapie produces an intense, progressive itching along with a lack of coordination and finally an inability to walk. Scrapie develops slowly and may last months or years before it is fatal. Although contact exposure appears sufficient to transmit the agent, breeding is perhaps the most common means of transfer. No treatment is known for scrapie. A relationship may, however, exist between this disease and kuru, a similar neurological disease known only among a New Guinea tribe. Viral Diseases True viruses produce many of the most common human and animal diseases. These tiny particles of \TDNA\t or \TRNA\t have no independent metabolism, although they use the living cells of the host for growth and reproduction. They are invisible under a light microscope, and few are affected by drugs that do not also harm the host cells. \Trabies\t, or hydrophobia, is a disease that is characterized by nerve irritability, brain damage, inability to move, and, finally, death. The viral agent belongs to the Rhabdo family of viruses. The disease is transmitted primarily by the bite of an infected carnivorous animal. It has been reported in virtually all animals and birds. Skunks, bats, raccoons, foxes, and mongooses are major wildlife hosts. This disease has largely been controlled in certain areas of the world, including the United States; worldwide, however, it remains a serious problem. In the Americas alone the vampire bat is directly responsible for a high loss among dairy cattle. Dogs may carry the virus in their saliva for 7 or 8 days before the onset of signs of infection, and they remain infectious until they die. Thus it is important to have animals vaccinated, to prevent the development of the disease. Pox is an old, established febrile (fever-producing), worldwide, infectious skin disease of animals, birds, and humans. The pox viruses are classified in groups--\Tcowpox\t, monkey pox, rabbit pox, and smallpox are grouped together; goat and sheep pox together; and the bird pox viruses together. Wartlike growths appear on the skin and on lining membranes of the mouth and throat. Any break in the skin or membranes may permit the entrance of the viruses. Insects may transmit pox viruses as they penetrate skin tissues. Specific pox vaccines are available for controlling and preventing infection. Rinderpest, or cattle plague, is a serious febrile Paramyxo virus related to the \Tdistemper\t virus of dogs. It affects cattle and buffalo as an erosive disease of the mouth, stomach, and intestines. The disease is common in Asia and Africa, but is not established in North America. Infection is usually the result of inhalation, but it also spreads by direct and indirect contact. The disease may be controlled by vaccination in cases where animals are not isolated or slaughtered. \Tfoot-and-mouth disease\t is a highly contagious, febrile virus disease chiefly of cloven-hoofed animals. It is transmitted by both direct and indirect contact with contaminated discharges. Although it appears throughout the world, the disease has not gained a foothold in the United States (the last outbreak occurred in 1932), Canada, Great Britain, Australia, or New Zealand. The greatest meat, milk, and productivity loss results from debility and an inability to eat properly. Blisters, or fluid-filled vesicles, appear in the mouth and nostrils and on the feet and teats. Because the Picorna virus occurs in several types and subtypes, specific vaccines are used for the type involved in an outbreak. Canine distemper is a highly contagious, febrile, Paramyxo virus disease of dogs. Feline distemper, caused by a different virus, affects foxes, wolves, mink, ferrets, skunks, raccoons, and related species. Distemper is transmitted primarily by nose or eye discharges. It appears later as a respiratory or lung disease and, in uncontrolled cases, involves the nervous system. Young animals and unvaccinated pets often die even with treatment, although successfully vaccinated animals usually withstand exposure and respond to treatment even when the infection has become established. Infectious \Thepatitis\t of dogs is a highly contagious, febrile Adeno virus disease that occurs worldwide. If not arrested, the disease first takes the form of a liver disorder and later becomes a brain infection. In some dogs it is a respiratory disorder. Infectious hepatitis is transmitted by the discharges of the infected animal, and contaminated urine is often the source of infection. Animals tend to lick or sniff the discharges of others, and this habit aids the transmission. The disease prolongs bleeding time and produces a marked lowering of white blood cells. A common symptom is opaqueness of the cornea in one or both eyes. Puppies may be effectively vaccinated at 9 weeks of age. Yearly vaccinations are usually advisable. Bacterial Diseases \Tleptospirosis\t is an acute or chronic infectious bacterial disease found worldwide in many animals, including cattle, hogs, dogs, rats, and humans. It is caused by Leptospira organisms transmitted when contaminated food or water is taken into the body. It is characterized by anemia, degeneration of the liver and kidney, jaundice, and abortion. The main symptoms in humans are fever, muscle pain, and conjunctivitis. \Ttuberculosis\t is a chronic, infectious bacterial disease of ancient origin, affecting humans and numerous animals. It is caused by Mycobacterium tuberculosis of the human, bovine, or avian type. Tuberculosis is transmitted by ingestion, inhalation, or wound infection and is characterized by a persistent invasion of tissues, loss of flesh, and eventual death in animals. In birds, lesions occur in the liver and spleen; in other animals they occur in the lungs and lymph nodes. \Tanthrax\t is a serious infectious bacterial disease that afflicts sheep, goats, cattle, horses, and humans throughout the world. Animals may die without any indication of trouble before death. The organism may be ingested through contaminated feed or water, or it may be inhaled; the latter occurs less frequently. Even biting insects may instill the spores, which develop whenever the organism is exposed to air. Infected animal products have been found to cause outbreaks. Grazing animals usually get anthrax in infected pastures; most cases are fatal. Common symptoms in animals include high fever and bleeding from body openings. A chronic localized form may occur in dogs and swine. Entering the upper respiratory tract, anthrax results in severe edema of the head and neck and usually results in death. \Tbrucellosis\t is a worldwide bacterial reproductive disease of animals and humans; it is caused by 4 species in the genus Brucella. B. Animals transmit it by ingestion and infrequently by copulation. In humans it produces undulant fever. In cattle infected by B. abortus, it is a characterized by retained placenta, weak young, difficult breeding, infertility, and abortion. Swine often have arthritis and enlarged testes, and horses may have an abscess at the withers. Rickettsial Diseases Q FEVER is a rickettsial disease of cattle, sheep, and goats and an acute respiratory disease of humans. It is caused by Coxiella burneti and is transmitted by inhalation, tick bites, contaminated milk, and direct and indirect contact with infected animals and their excretions. It has been reported in several countries of Europe, as well as in Australia and North America. Chlamydial Diseases \Tpsittacosis\t is an acute or chronic respiratory and systemic disease of wild and domestic birds, which may be transmitted to animals and humans. It is caused by Chlamydia psittaci. Mammalian strains may cause reproductive and arthritic problems. Psittacine birds are often infected. Fungal Diseases Fungal diseases include such diseases as \Taspergillosis\t and \Tringworm\t. The latter is an infectious skin disease of animals, whereas the former is a respiratory tract infection of birds. Aspergillus is, however, known more widely for its aflatoxin. Candida albicans is a yeastlike fungus commonly found in the digestive tract of animals and birds. It may be either a serious disease-producing organism or a secondary invader, and it may cause thrush in human infants. Mycoplasma Diseases \TMycoplasma\t agents generally result in localized respiratory infection, but they may also produce generalized systemic infections. Contagious pleuropneumonia was the first cattle disease found to be caused by this type of organism. Chronic respiratory disease of chickens is another common disease of this type. Parasitic Diseases Parasitic diseases are classified as internal and external. Internal parasites include \Ltapeworm\ls and flukes; roundworms such as intestinal \Lhookworm\ls and roundworms of the heart, lung, stomach, kidney, eye, trachea, muscle, and blood vessels; thorn-headed worms; and protozoa such as coccidia, toxoplasma, sarcocystis, trypanosomes (see \Ttrypanosomiasis\t), \Tleishmania\t, trichomonads (see \Ttrichomoniasis\t), amoebae, and blood organisms, including malarial. External parasites include insects such as bots, ticks, mites, lice, fleas, gnats, flies, mosquitoes, and bedbugs. Parasitic diseases are among the most debilitating of animal diseases. Skin mites cause mange. They are tiny creatures that live in or on the skin and suck blood, causing serious skin problems. \Tcoccidiosis\t, an extremely common animal and bird disease, afflicts the intestinal tract with cellular destruction and in some cases causes severe loss of blood. Roundworms occur in most parts of the body, particularly in the small and large intestines. Depending on the particular type, they produce mild to serious problems. Hookworms suck blood from the intestinal tract. \Ttrichinosis\t from uncooked pork finds its way into the muscles and often results in death. Flukes, or flatworms, appear worldwide and can produce serious and often fatal problems in humans and animals. External parasites can cause extensive debility in all livestock. NONINFECTIOUS DISEASES Noninfectious diseases usually receive less attention, although they are perhaps far more common than infectious problems. The causes or conditions for these diseases include nutritional problems, metabolic disorders, toxicities, burns, cysts, tumors, and pathological and anatomical changes. Nutritional deficiencies occur often in spite of the wealth of information available concerning standard dietary levels for each species. Overdosage with vitamin supplements may occur, although nutritional problems also occur, involving the intake of water, protein, fat, or fiber. Excess amounts of any ingredient in the diet, or of any extraneous additive or accidental contaminant, may be toxic and cause poisoning in animals. Metabolic disorders, such as milk fever of cattle or pullet disease of chickens, result from a disturbance in the body chemistry. These conditions have been noted during periods of stress. Burns may be chemical or thermal and result from mismanagement in most instances. Cysts and tumors occur in various parts of the body. These represent excessive new growths of specific tissues that persist with or without original stimuli. Marek's disease, a virus-induced cancer of chickens, is infectious for other chickens. Mammary tumors of dogs, on the other hand, occur more often in the female, possibly as a result of estrogen, and are not considered infectious. Pathological conditions include such diseases as abscesses, edema, and emphysema, all of which involve changes in physiology and underlying function. Anatomical diseases are those in which a change in anatomical structure and associated function is evident. This includes bloat, which develops when the release of stomach or intestinal gas is interfered with. Fractures or broken bones are similarly classified. DIAGNOSIS AND TREATMENT The responsibility for diagnosis, control, treatment, and eradication of a disease remains in the hands of the competent, informed veterinarian. A farmer or pet owner alert to the signs of disease must decide when and if assistance is needed. With the help of the veterinarian, a diagnosis can be established. This may require bacteriological or virological studies, serological testing, gross autopsy procedures, and histological evaluations. Once a diagnosis is made, treatment and control measures may be initiated, although temporary measures may be taken pending a diagnosis. The control procedures used to eradicate a disease may include \Tquarantine\t, treatment, immunization, and even slaughter. Reportable diseases, which may involve adjacent farms or create human health hazards, may involve state regulatory officials and college extension personnel in alerting the populace to the presence of a disease. \TDavid\t \TTudor\t, M.D. Bibliography: Blood, D.C., et al., Veterinary Medicine, 6th ed. (1983); Brown, C.M., et al., eds., Concise Veterinary Dictionary (1988); Fraser, C.M., ed., Merck Veterinary Manual, 6th ed. (1986); Schwabe, Calvin, Veterinary Medicine and Human Health, 3d ed. (1984); Timony, J.F., et al., Hagan and Bruner's Microbiology and Infectious Diseases of Domestic Animals, 8th ed. (1988); Vargosche, Richard and Steinberg, Peter, The Household Book of Animal Medicine (1980); Ward, Amy, Small Animal Health Care (1984); West, Geoffrey, ed., Black's Veterinary Dictionary, 15th ed. (1985). See also: \Tcat\t; \Tdog\t; \Thorse\t; \Tpig\t; articles on other animals.