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.

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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.