Animal Disease | The Canadian Encyclopedia


Animal Disease

The tissues and body fluids of animals are subject to the same types of abnormal structural and functional changes as are those of humans. Causes and circumstances may differ but the disease processes are very similar.

Disease, Animal

The tissues and body fluids of animals are subject to the same types of abnormal structural and functional changes as are those of humans. Causes and circumstances may differ but the disease processes are very similar. An inflammation or a cancer in a dog or a cow could be identical in appearance and development to that found in a human.

The science of anatomy studies normal tissue structure in other animals, as in humans, the science of physiology, normal function; that of pathology, disease processes (abnormal structure and function). Veterinarians diagnose, treat and prevent diseases in animals.

Types of Disease Processes in Animals

Individual cells or groups of cells which are sick and function abnormally are called degenerate because of their reduced ability to function. They have abnormal organelles (specialized cell parts) and functional abnormalities and can often be identified by their appearance under the microscope. Dead cells and tissues have a characteristic appearance called necrosis which is a common abnormality in injured tissue.

Diseases in Various Species

Many animal diseases are related to intensive husbandry resulting in crowded, poorly ventilated conditions often combined with the mixing of susceptible animals from several sources. The major economically important diseases of cattle are neonatal diarrhea and viruses (eg, corona virus, rotavirus). Enzootic pneumonia (ie, always present at a low level) is a serious problem in calves 2-6 months old. The causes are viruses and viruslike micro-organisms, together with secondary bacterial infection, often leading to a persistent bronchopneumonia. Affected calves have a fever, grow poorly, cough and breathe rapidly. Before the discovery of antibiotics, many calves died of pulmonary abscesses but now most survive if treated properly.

Feedlot cattle very often have a more severe, often fatal pneumonia shortly after arrival at the feedlot (see Beef Cattle Farming). Transportation stress, mixing of groups and crowding predispose the animals to bacterial pneumonia, known as "shipping fever," which is so acute that it is difficult to detect affected animals soon enough to treat them. Mastitis is a major problem in dairy cows because chronic inflammation of the mammary gland reduces milk yield (see Dairy Farming).

Infertility, a major problem in cattle, arises through abortion or failure to conceive at the proper time (see Animal Breeding). Abortion is caused by many different infectious agents. It is economically important for a cow to calve annually; therefore, the cow must come in estrus and be bred at the proper time. Detection of estrus is a major problem to many farmers. Some of the great plagues which affected cattle of past centuries (eg, foot-and-mouth, rinderpest, contagious bovine pleuropneumonia) still exist in Africa and Asia and periodically spread to Europe and the Americas. The risk of such diseases results in strict importation regulations for meat, animal products and animals.

Hog cholera and African swine fever are highly infectious, fatal diseases causing lesions in many tissues. Both could be considered in the plague category. The most prevalent diseases of pigs now are diarrhea and pneumonia. That specific diarrheas tend to occur in different age groups helps in diagnosis. Most are fatal or may cause pigs to be stunted and therefore uneconomical.

The main pneumonia is caused by mycoplasma, a bacterialike organism. Diarrheas and pneumonia often lead producers to place antibiotics or similar compounds in the feed at subtreatment levels until marketing (see Hog Farming). Bacteria often become resistant to antibiotics given at preventive rather than treatment levels.

Sheep and goats have many diseases in common, of which parasites are very important. Lungworms are common in both and often lead to chronic lung disease. Constant prevention and treatment efforts are required to overcome them. Several kinds of gastrointestinal parasitic worms occur, especially in sheep. Stomach worms, Haemonchus, suck blood from the stomach lining, often causing fatal acute anemia. White muscle disease caused by vitamin E deficiency can be prevented by adequate diet supplements. The name comes from the pale colour of degenerate muscle lesions in the heart and limbs of affected animals (see Goat Farming; Sheep Farming).

Domestic turkeys and chickens are raised in very crowded conditions (often thousands in one pen); therefore, infections can spread very rapidly (see Poultry Farming). Many vaccines, often given in the drinking water, are routinely used; drugs are included in the feed to prevent bacterial and parasitic diseases. Coccidiosis is a very common protozoan parasite causing a potentially fatal enteritis. The parasite is easily treated or prevented by drugs but keeps changing its sensitivity to drugs and requires constant checking.

Respiratory diseases (particularly of the air sacs and sinuses) are also significant and require constant control measures. A widespread poultry disease is a cancer of lymphocytes in the blood and blood-forming tissues caused by a highly infectious virus. Some of the viral strains pass by contact, others, through eggs. A vaccine is now available for prevention. Intense research is ongoing to determine whether other cancers, possibly those of humans, are viral in origin.

The requirement of veterinary services for pets, particularly dogs and cats, has expanded rapidly and is now highly sophisticated, requiring hospital care and facilities very similar to human hospitals. Dogs have many diseases resembling those of humans and are often studied as models of human diseases. "Comparative medicine" refers to such combined studies and includes many diseases of other animals, particularly certain strains of rats, mice and primates.

Dogs have the highest rates of cancer among animals. These originate in many tissues but those of the skin and blood-forming organs are most numerous. Biopsy examination of surgically removed lumps is a common procedure. The main infectious diseases of dogs are distemper and infectious canine hepatitis, but these are now well controlled by vaccination. The frequently fatal acute parvoviral enteritis, a rather new disease in dogs, apparently arose as a variant of a nonpathogenic strain and became rampant across North America in the late 1970s and early 1980s.

The most devastating disease in cats, infectious feline enteritis, or panleukopenia, has many similarities to canine viral enteritis and is often fatal. It is also fatal to members of the mink family. Cats have a high incidence of lymphosarcoma, a cancer of white blood cells. It is caused by a highly infectious virus, but causes cancer in a relatively few animals which appear to have a defect in their immune response. The disease is being studied to see if the mechanisms involved relate to those of any human cancers.

The use of rabbits, rats, mice, guinea pigs and primates for research on human disease or for testing drugs and vaccines has resulted in rapid expansion of knowledge of their naturally occurring diseases, which are similar to others discussed above (eg, enteritis, pneumonia, parasitic diseases). Many specific models of human disease have been found in these animals and are studied in detail (see Animal Issues).

Wildlife species of all types have diseases, some occurring in epidemics. These diseases are no different from those occurring in domestic species. Less is known about most wildlife diseases because of problems in securing sufficient numbers for study. Particular attention is given to species that carry diseases which could affect humans or domestic animals (eg, rabies). Efforts are being made to vaccinate wildlife species for rabies by means of oral vaccines in food left in their natural environment. Fortunately, there are veterinarians who specialize in diseases of wildlife (see Wildlife Conservation and Management).

Captive animals have many disease problems related to captivity, ie, to strange or unnatural climates, mixing with unnatural neighbours, eating unnatural foods, etc. They may contract diseases to which they have no resistance from humans or from animals native to other continents. Zoo veterinarians have a tremendous responsibility in caring for expensive animals. Many problems occur as a result of restraint while the animals are being moved or treated. Injuries are common. There are now enough zoo veterinarians to allow for sharing of knowledge and experience.

The requirement that animals slaughtered for meat be inspected under veterinary supervision is designed, in part, to protect the public from the many diseases transmissible from animals to humans (see Food Legislation; FOOD SAFETY). These diseases include listeriosis, anthrax, salmonellosis (typhoid fever), brucellosis (contagious abortion), tuberculosis, leptospirosis, some pox viruses, rabies, equine encephalitis, etc. Pasteurization of milk, meat inspection and generally improved personal hygiene have effected major improvements in control of these transmissible diseases.

Because of the importance of animal agriculture to the Canadian economy, veterinary medicine in Canada is pioneering new technology and treatments for diseases of domestic food-producing animals. Guelph researchers are concentrating on reproductive biotechnology including sexing and splitting embryos, in-vitro fertilization, recombinant cloning of toxins to produce anti-toxins, and radiation therapy for cancer in animals.

At the Western College of Veterinary Medicine in Saskatoon, calf diarrhea and respiratory disease of cattle, in particular shipping fever, are the subjects of research. At the Université de Montréal at St-Hyacinthe, reproductive physiology is being researched. In PEI, at the Atlantic Veterinary College, epidemiology and health management systems for farm animals are being researched.


Blood flows through tissues, providing nutriment and oxygen. Blood flow may be abnormal because of tissue lesions or abnormalities in the general circulation. If increased amounts of arterial blood fill capillary beds, the lesion, called hyperemia, is bright red. Such lesions are commonly found in inflammation. If venous drainage is impaired through local or generalized obstruction to flow, congestion results, appearing dark red. If excess fluid collects in tissues outside blood vessels, the lesion is called edema and tissue is swollen. This lesion may be caused by local injury or by a generalized inefficiency in circulation (eg, a failing heart).

If red blood cells are outside blood vessels, the lesion is a hemorrhage. If blood constituents clot within a vessel, the lesion is a thrombus; the process is thrombosis. Thrombi occur because of injury to vessel linings, turbulent flow or slow flow and often occur on the heart valves of animals during a generalized bacterial infection. An infarct, necrosis of a segment of tissue through obstruction of blood flow (thrombosis), often occurs in organs having limited collateral circulation (eg, heart, kidney).


Inflammation, the body's response to injury, involves tissue and blood cells. Injured tissues release chemicals which initiate inflammation and bring white blood cells to the site to overcome an invading agent or remove injured tissue. The white blood cells (leukocytes, including neutrophils and macrophages) are able to eat up (phagocytose) invading agents and debris, although many die in the process. The fluids and cells which accumulate are called exudate. Inflammatory exudates differ according to the cause and the tissue affected. Inflammations which occur rapidly are acute; those occurring gradually, subacute; those which are prolonged, chronic.

Some exudates are mainly fluid (eg, a "running nose," serous exudate); some induce mucus (eg, in bronchi, catarrhal exudate); some induce fibrin formation in blood leaked from damaged vessels (fibrinous exudate). Pus results from a predominance of neutrophils in the exudate (purulent exudate) and, if confined in tissue, is called an abscess. A prolonged battle with a stubborn agent causes macrophages or, sometimes, giant cells to predominate and connective tissue to form, in an effort to confine the lesion. A scar may be the long-term evidence of an inflammation.

Growth Disturbances

Cells may decrease in size and number in a tissue, a process called atrophy. Hypertrophy is enlargement of existing cells; hyperplasia is an increase in number. Defects in tissue and organ formation are called anomalies. Parts may be absent (agenesis), incomplete (hypoplasia) or present but too small. Cancer is a growth disturbance which allows groups of cells to proliferate, sometimes in an uncontrolled manner. It occurs in all tissues and organs in humans, in wild and domestic animals and lower species, and in individuals of all ages, including the unborn.

Causes include irradiation, viruses, chemicals, genetic abnormalities, etc. Carcinogenic agents cause groups of cells to lose their growth-control mechanisms and to proliferate abnormally. Cancers may expand as a solitary mass, infiltrate a tissue or organ, spread in the blood to other organs or implant on internal body surfaces. Tumours are common in animals, particularly poultry, cats, cattle, mice, rats and dogs. Many are known to be caused by viruses and are used to study mechanisms of cancer development in humans.

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