Diseases of Animals. I INTRODUCTION Diseases of Animals, disorders that influence an animal's health and ability to function. Animal diseases are of great concern to humans for several reasons. Diseases can reduce the productivity of animals used to produce food, such as hens and dairy cows. Animals that are raised as food, such as pigs and beef cattle, that become ill may affect the economic well-being of many industries. Some animal diseases can be transmitted to humans, and control of these types of diseases, known as zoonoses, is vital to public health. In the wild, animal populations reduced by disease can upset the ecological balance of an area. And, in the case of pets, prevention and treatment of animal diseases helps pets live long and healthy lives, enhancing the companionship shared by a pet and its human owner. Animal diseases are characterized as infectious and noninfectious. Infectious diseases are caused by an agent, such as bacteria or a virus, that penetrates the body's natural defense mechanisms, while noninfectious diseases are caused by factors such as diet, environment, injury, and heredity. Sometimes the cause of a disease is unknown. An animal may also experience one disease or a combination of diseases at any one time. To identify a disease, a veterinarian (a doctor who treats animals) first determines the animal's signalment--its species, breed, age, and sex. This information helps to identify a disease because some diseases are more prevalent in certain species, or a disease may preferentially affect one sex or age group. The veterinarian then gathers a complete history of the animal and its problem. This history includes the symptoms the animal is displaying and when they first appeared, as well as whether the animal has been exposed to something new in its surroundings or to other animals. The veterinarian gives the animal a thorough physical examination, which may include measuring its body temperature, listening to its heart, checking its pulse, and feeling its abdomen and lymph nodes. The veterinarian then creates a list of possible diseases that may be making the animal sick. The list may be narrowed by running diagnostic tests such as X rays, electrocardiograms, blood analyses, and bacterial or fungal cultures. Once the disease is identified, the doctor develops a treatment plan for the animal (see Veterinary Medicine). II INFECTIOUS DISEASES Many microscopic organisms naturally and peacefully exist in enormous quantities within animal bodies. For example, the multichambered stomach of a cow contains bacteria that help the animal digest its food. But many other microscopic organisms, known as pathogens, cause diseases in animals. Pathogens include bacteria, viruses, fungi, prions--newly identified mutated proteins--and parasites. Pathogens are easily spread: an animal may consume food or drink something that has been contaminated with infected fecal material, for example. If the ground is contaminated by Salmonella bacteria, for instance, infection can travel from barn to barn on the soles of a farmer's boots. Or an animal may be exposed while walking across contaminated ground. Some diseases are transmitted by biting insects; others are spread by sexual contact. In addition to reducing the productivity of livestock, some infectious diseases pose a danger to humans. More than 100 zoonoses are recognized. Most cases are transmitted from animals that have close contact with humans, such as pets, farm animals, or rats. Examples of zoonoses include toxocariasis, a disease caused by a parasitic worm transmitted by infective eggs within canine feces; psittacosis, a respiratory disease caused by the bacteria-like Chlamydia psittaci and transmitted from infected birds; hantavirus pulmonary syndrome, spread by contact with rodent feces and urine; and rabies, a viral infection transmitted in the saliva of infected animals, typically foxes, bats, and raccoons, that causes damage to the brain and spinal cord. As the human population grows and expands into wilderness territories, humans are coming into closer contact with other animals that carry pathogens dangerous to humans. Some of these pathogens are carried by insects, as in the case of yellow fever, spread from monkeys to humans via mosquito bites. Some hemorrhagic fevers, such as that caused by the Ebola virus, are recognized as zoonoses, but the exact transmission route from animal to human is still unknown. A Bacterial Diseases Salmonellosis is any disease caused by the Salmonella bacteria, characterized by septicemia and severe diarrhea. In its many forms, it is one of the major diseases of wild and domestic mammals, birds, and reptiles, as well as humans. Salmonella bacteria usually enter the body through the mouth, most commonly along with food or water contaminated by infected feces. Transmission also may occur through direct contact with an infected animal. In addition, salmonella bacteria can be spread by contact with objects, such as bowls and cutting boards, that have been contaminated by infected animal products, such as eggs or meat. Anthrax is one of the oldest and most destructive diseases recorded in history. Caused by the bacterium Bacillus anthracis, anthrax can affect virtually all warm-blooded animals and humans. The onset of anthrax may be sudden and death may occur before symptoms are observed. In other cases, typical symptoms include restlessness, lethargy, appetite loss, fever, rapid breathing, and unsteady gait. The disease is contracted from contaminated soil, feed, or water. It can also spread when the skin is penetrated by insect bites or by objects contaminated with anthrax spores. Leptospirosis, caused by spiral Leptospira bacteria, affects cattle, dogs, pigs, sheep, goats, and humans. Ponds, lakes, and other bodies of water are common sources of leptospirosis, and rodents may carry the infection. This infection causes kidney disease and destruction of red blood cells with potential anemia; it may also cause abortion. Brucellosis also causes abortion, as well as swelling of the reproductive organs in males. Caused by the Brucella bacterium, it occurs primarily in cattle, pigs, sheep, dogs, and goats, and may be transmitted to humans (see Undulant Fever). Tuberculosis (TB) is a chronic disease of animals and humans, caused by bacteria of the genus Mycobacterium and transmitted by inhalation of droplets from an infected animal's cough or sneeze, or by wound infection. TB infection causes lesions called tubercles to develop in certain tissues, such as the lung or liver. Symptoms include fever, emaciation, and progressive loss of strength. Kennel cough is a respiratory disease of dogs that is caused by the bacterium Bordetella bronchiseptica, with or without the aid of various viruses. Symptoms include a harsh, dry cough, appetite loss, discharge from the nose or eyes, and lethargy. It typically spreads when dogs are grouped together, such as at dog shows or boarding kennels. B Viral Diseases Viruses are unable to grow and reproduce outside of the living cells from other hosts. Viruses attach and invade a cell and replicate, and then the newly created viruses destroy the host cell and seek out other cells to continue replication. Feline leukemia is caused by the feline leukemia virus. Often fatal, it can seriously impair the immune system and, in some cases, cause the growth of life-threatening tumors. Spread from direct contact with an infected cat, symptoms of the disease include lethargy, weight loss, anemia, and fever. A cat may not appear ill until years after exposure. Foot-and-mouth disease is caused by a virus found in the saliva of cattle, pigs, and other hoofed animals. Highly contagious, it is spread from direct contact with an infected animal. It may also spread in milk or in garbage that contains contaminated meat. Typical symptoms include blisters that appear on the mouth and feet; animals may become lame when their hooves degenerate. Canine distemper is a highly contagious disease caused by the paramyxovirus, which is transmitted in discharges from the nose and eyes. Symptoms begin with fever, malaise, and nasal and ocular discharges and may progress to convulsions and other nervous system disorders. Parvoviruses affect dogs and in some cases cattle, pigs, and humans. Usually fatal if left untreated, canine parvovirus causes inflammation of the intestines, producing diarrhea, vomiting, fever, and loss of appetite. C Fungal Diseases A fungal infection typically develops slowly and recurs more frequently than a bacterial infection. Histoplasmosis, characterized by a chronic cough and diarrhea, is contracted by inhaling the Histoplasma capsulatum fungus, which grows in soil. In the Central United States histoplasmosis is the most widespread fungal disease diagnosed in dogs, although it also affects other animals. Ringworm, a common skin disease of many species, causes circular patches of hair loss and scaly, reddened skin. It readily spreads by direct contact with an infected animal. Yeast, another type of fungus, grows in warm and moist places, such as the ear canals of dogs. It may cause otitis externa, an infection of the outer ear. The yeast Candida albicans is commonly found in the intestinal tract of birds and other animals. It may be the primary cause of disease, or it may be a secondary invader in an animal already sick with another infection. D Parasitic Infections Diseases caused by parasites are widespread in domestic animals and wildlife. Parasites may be internal or external. Internal parasites include Coccidia, a microscopic protozoal (single-celled) organism that causes diarrhea and extreme weight loss in many young animals. Other internal parasites include the roundworm, tapeworm, and fluke. Larval roundworms can cause considerable damage to lungs and other organs in some animals. For instance, Capillaria worms may attack the lining of the digestive tract of chickens and turkeys; they parasitize the respiratory and urinary tracts of dogs. Adults of the heartworm Dirofilaria immitis, another roundworm, live in the heart of dogs and produce microscopic larval stages, which swim in the blood. Symptoms of heartworm disease include coughing, fatigue, and weight loss. If left untreated, an animal may experience heart failure. Tapeworms may have very damaging larval stages. In echinococcosis, the larval tapeworms may form large cysts in liver, lungs, and other organs of humans and animals. Flukes may directly damage the liver, lungs, or intestines, or they may act as carriers of other disease agents, as in the case of salmon poisoning of dogs in which the fluke, encysted in the body of a salmon, carries a virulent rickettsial agent. External parasites live or feed on the surface of the animal's body. This group includes bloodsucking insects, such as mosquitoes, gnats, some flies, fleas, and some lice. Some insects are bloodsuckers in larval stages, such as ear maggots of hawk nestlings. Others, including some larval flies and some lice, eat tissue. Great damage to the meat and hides of cattle is caused by larval flies such as the ox warble, which migrates through the tissues and, after boring breathing holes through the skin, leaves the body to reproduce. Bloodsucking flies can transmit parasitic blood protozoans and some viruses. Lice are of two types, those with chewing mouthparts and those with sucking mouthparts. Lice cause irritation, carry disease agents, and may cause anemia. Fleas are all bloodsuckers, and may transmit larval tapeworms, roundworms, and other disease agents. The sticktight flea may kill young birds by excessive bloodsucking. Mites may be external bloodsuckers, such as the red mite of birds (it can also affect humans and other animals), or they may be internal parasites, such as the Sternostoma mites of the lungs and air passages of canaries and other birds. Ticks, larger than mites, feed on blood and can carry serious infectious agents such as the bacteria that cause Q Fever and Lyme disease, which can be transmitted to humans. E Prion Diseases Newly identified protein particles called prions have been found in the brains of animals that have died from diseases such as scrapie and bovine spongiform encephalopathy, more commonly known as mad cow disease. How prions act is unclear, but scientists theorize that prions attach to normal proteins in the brain. Once attached, the prions cause the normal proteins to change into an abnormal shape, leading to progressive destruction of brain cells and death. Prion diseases are thought to spread by means of feed supplements derived from infected animals. In recent years, public health officials have been concerned about the possibility that prion diseases may be transmitted to humans. This happens when humans eat contaminated beef or organs, causing them to contract such rare neurological diseases as Creutzfeldt-Jakob disease. F Prevention and Treatment Controlling the spread of infectious animal diseases begins with isolating, or quarantining, animals with threatening infections, such as salmonella, to prevent further transmission. Many bacterial diseases can be treated with various antibiotics, such as penicillin and streptomycin. But as with all disease, prevention is more important than treatment, and a major activity for veterinarians is immunization of animals. Immunization commonly involves an injection of a weakened or killed pathogen for a specific disease that encourages the immune system to fight off infection. Many infectious diseases, including rabies, canine distemper, feline leukemia, anthrax, and brucellosis, can be prevented by immunization. In the case of severe outbreaks of infectious disease, public health officials may work with animal owners to destroy large groups of animals. This was the case in the early 1990s, when an outbreak of bovine spongiform encephalopathy triggered the slaughter of many beef cattle in Britain. Transmission of animal diseases to humans is a constant concern of public health officials. To protect people from disease, veterinarians inspect food animals for wholesomeness; quarantine and examine animals brought into the United States from other countries; test animals for the presence of disease; and actively work to prevent and eradicate diseases that threaten human health. III NONINFECTIOUS DISEASES Even if it were possible, a world without pathogens would not be disease-free. Many animal diseases are caused by noninfectious factors such as an animal's environment, genetics, and nutrition. Heatstroke, for example, occurs when an animal is forced to endure high temperatures without access to water, adequate ventilation, or suitable shade. A common scenario involves an animal that has been locked inside a car without air-conditioning during hot weather. Conversely, extreme cold can lead to hypothermia or frostbite. Other environmental hazards include the vast array of products humans use to eliminate pests and weeds from homes, farms, and gardens. For example, rodenticide, poison used to kill rats and mice, can cause fatal internal hemorrhaging in any animal that ingests this toxic substance. Improper use of flea powders, sprays, dips, and collars can also cause illness. Automobile antifreeze is another well-known poison. Its sweet taste appeals to some animals, such as cats and dogs, but consuming only a small amount can result in death. Many plant species are also toxic to animals. Some, such as pokeweed and yew, commonly grow in pastures and yards. Poor feeding practices can lead to diseases such as nutritional secondary hyperparathyroidism, a condition involving the muscles and bones of dogs that is associated with an all-meat diet. Large, rapidly growing puppies that consume too many calories and too much calcium can develop hypertrophic osteodystrophy, a disease resulting in lameness. Cats need sufficient amounts of an essential amino acid called taurine in their diets. Without it, they may develop eye problems. Not enough iodine intake can cause a goiter, or enlargement of the thyroid gland, in cows, horses, and other animals. Trauma is a leading cause of injury and premature death in animals, especially pets that are allowed to roam free outdoors. Many animals are hit by cars or bitten by other animals. Farm animals may be attacked by predators, or they may harm themselves on sharp fencing or discarded nails. Untreated wounds can become infected and cause permanent damage. Hip dysplasia, a painful and debilitating skeletal condition, is a noninfectious disease caused in part by heredity. Certain defects of the heart or palate, the roof of the mouth, may also be inherited. Some animals are genetically predisposed to diseases such as generalized demodectic mange, a skin disease caused by mites and characterized by hair loss and scaling around the eyelids, mouth, and front legs. An animal's immune system is designed to detect and eliminate invading organisms. Occasionally, however, it behaves as though the animal's own body were the attacker, and it destroys healthy tissue. Diseases caused by this response, known as autoimmune diseases, include pemphigus foliaceous, a skin disease of dogs, cats, and horses; and rheumatoid arthritis, a severe type of arthritis that involves inflammation of the joints. In the autoimmune disease hemolytic anemia, the animal's own red blood cells are destroyed by its immune system. Cancer exists in all animals. It is classified as either benign--that is, relatively noninvasive and unlikely to return after treatment; or as malignant--that is, aggressive and likely to spread. Any organ or system can be affected, either directly or through metastasis--when cancer cells from one part of the body spread to other areas of the body. Some forms of cancer are more widespread in animals of a particular breed, age, or sex, and even individuals of a specific color. For example, cancer of the mammary gland occurs more often in female animals, while melanoma, or skin cancer, is the most frequent tumor of elderly gray horses, and lymphosarcomas, tumors of the lymph nodes, are the most common type of specific tumor in cats. The study of cancer, known as oncology, is a growing field in veterinary medicine. Contributed By: Elizabeth M. Bodner Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.