Diseases of Animals

I

Introduction

Diseases of Animals, disorders affecting animals, mainly farm and pet animals, but also laboratory animals, zoo animals, and wildlife. These diseases are of concern to humans because of their importance economically and as regards public health.

Animal diseases may be classified, according to the causative agent, as bacterial diseases, fungal diseases, viral diseases, parasitic diseases, hereditary diseases, and diseases caused by environmental factors. Frequently, diseases may be brought on by a multitude of causes. A relatively mild viral infection, for example, if favoured by hereditary susceptibility, may then weaken the body's resistance to bacterial invasion.

II

Bacterial Diseases

Bacteria cause disease in several ways. Some produce powerful poisons or toxins; for example, the botulinus bacillus, the tetanus bacillus, and the gas gangrene bacillus. Other bacteria cause local or general death of body tissues, block the flow of blood, or cause severe irritation. Salmonelloses, or any disease caused by Salmonella bacteria, are widespread. Pullorum disease, caused by S. pullorum, threatened the chicken and turkey industry until brought under control by elimination of infected birds through blood testing. Almost 2,000 other kinds of Salmonella are known, each of which may cause disease in humans and animals. The bacterium S. typhimurium is responsible for about half the so-called food-poisoning cases in humans, and for many losses of poultry and other animals.

Leptospirosis, due to spiral bacteria of the genus Leptospira, causes losses in cattle, dogs, and humans. Ponds, lakes, and other bodies of water are common sources of leptospirosis, and rodents may carry the infection.

Tuberculosis may be caused by bacteria of the genus Mycobacterium. Monkeys and other primates in zoos must be protected by glass walls from exposure to the bacteria from tubercular humans. Humans must likewise be protected from tubercular cattle by periodic testing of milk cows and by examination of meat animals at slaughter.

Anthrax, caused by Bacillus anthracis, affects humans and domestic animals. Resistant spores that are carried in the hair or hides of animals or in floodwaters explain the sudden appearance of this bacterial disease.

Pasteurellosis, or any infection caused by bacterium of the genus Pasteurella, such as fowl cholera caused by P. multocida, is troublesome, affecting wildlife, domestic poultry, rabbits, and other animals.

Tiny, soft-walled bacteria of the genus Mycoplasma cause a variety of diseases in animals and humans, including pleuropneumonia of cattle, infectious sinusitis of turkeys, and chronic respiratory disease of chickens.

Diseases that were traditionally thought to be viral in nature, such as psittacosis, or parrot fever, are now believed to be caused by bacteria of the genus Chlamydia. Some serious diseases that occur in both humans and animals are in this group.

III

Fungal Diseases

Fungi cause many serious diseases of animals. Aspergillus fungi may cause necrosis of the lungs, the nervous system, and other organs. These fungi may also produce toxic products in feed components, causing mycotoxicosis in the animal ingesting such feed. A yeast-like fungus, Candida albicans, may cause death in turkeys, ptarmigan, hummingbirds, and other animals. Dermatophytic fungi affect the skin of animals and humans. Dust-borne fungi, such as Coccidioides immitis and Histoplasma capsulatum, produce lung disease or generalized disease in animals and humans.

IV

Viral Diseases

Viral agents are multitudinous, causing equine infectious anaemia, Newcastle disease, pig cholera, fowl pox, rabies, canine distemper, encephalitis, and a host of other diseases. Several viral agents cause tumour formation in poultry, known as the leukosis complex, resulting in serious economic loss. Influenza viruses cause serious problems in pigs, horses, and birds.

Some viruses spread from mother to offspring through the placenta or through the egg, and some have very resistant forms that can survive in dust. Other viruses require intimate contact to be contagious. Still others are spread by the bite of arthropods.

Viruses are not always limited to one species of animals or to one kind of tissue or organ. On the other hand, the severity of some diseases may be much greater in one species or one kind of tissue.