DOGS AND CATS

How Diseases Invade and Are Fought

By Johnny D. Hoskins and John D. Rhoades.

Disease may be defined as an alteration of the state of the body, or of some of its organs, which interrupts or disturbs the proper performance of bodily functions. Functional disturbance soon is manifested by physical signs which the animal detects by its sensations and which usually can be detected by humans.

Disease may be of external or internal origin. Little is known about the fundamental causes of the intrinsic diseases. These include metabolic and endocrine disturbances, degeneration of organs from age, tumors, and possibly autoimmunity.

It is probable that many of these disorders are initiated by extrinsic causes as yet unrecognized. The external causes of disease may be living agents such as bacteria, protozoa, or viruses, or they may be nonliving agents such as injury, heat, cold, chemical poisons, or food deficiencies.

When living agents enter an animal body and set up a disturbance of function in any part, infection is said to have occurred. An infectious disease is one caused by the presence in or on an animal of a foreign living organism, which creates a disturbance leading to the development of signs of illness.

Spread of Infections

Most infections are caused by living organisms that have escaped from the same or another species. This occurs when a human develops rabies from a dog bite, or when a lapdog contracts tuberculosis from its consumptive master.

An infection may be obtained indirectly, as when typhoid fever is contracted by people from contaminated drinking water. Some infections originate from organisms that normally live a free existence in nature for example, the bacillus of tetanus.

Fates of Invaders.

Several possible fates await organisms that cause infections. Some organisms are destroyed by the infected animal's tissue. Infections are not accomplished without resistance on the part of the host, because the host-parasite relationship is not a natural one.

Capacity of the host to destroy invading agents is so great that a large majority of the foreign agents that manage to reach living tissues and fluids of the body are rapidly and completely destroyed. In other cases the resistance is not sufficient to prevent growth and multiplication in the tissues, but the infection does not become extensive, and after a brief time the invading organisms are destroyed.

Sometimes the agent persists and makes slow headway against the resistance of the host, in which case the infection is called chronic.

In a few infections, resistance of the host is overwhelmed so quickly that the organism multiplies in all parts and early death of the host ensues. These cases are known as acute.

Eliminating the Agent

Some organisms usually are eliminated in the secretions or excretions of the host. The diseased animal usually eliminates, in a manner that varies with the disease, the organism that causes it. In chronic infections the host usually eliminates large numbers of the infecting agent.

Sometimes this agent is removed through pus, as when an abscess bursts or is lanced; sometimes through droplets that are discharged when the individual is suffering from one of the respiratory infections, as canine distemper, tuberculosis, or human diphtheria; sometimes in the intestinal discharges (feces) as in the various forms of intestinal parvovirus infections of animals and in human intestinal infections; and sometimes in the urine, as in leptospirosis and in typhoid fever of humans.

In some diseases that become extensive and even fatal, the causative organism may be eliminated in small numbers or not at all, as in some cases of tuberculosis.

The more chronic the disease becomes, the less likelihood there is that the animal will continue to retain all the infecting organisms.

In some diseases the mechanism by which the infection escapes from one animal to another is peculiar, as in rabies, where the seat of the infection is the nervous system and the means of escape is through the salivary glands.