Chapter XXV: Introduction

Pathogenic Bacteriology treats of the unicellular microörganisms which are responsible for disease conditions, _i.e._, pathological changes in other organisms. Hence not only are bacteria considered, but also other low vegetable forms, as yeasts and molds, likewise protozoa in so far as they may be pathogenic. For this reason the term pathogenic “Microbiology” has been introduced to include all these organisms. It is largely for the reason that the methods devised for the study of bacteria have been applied to the investigation of other microörganisms that the term “bacteriology” was extended to cover the entire field. The general discussion in this chapter is intended to include, therefore, microörganisms of whatever kind pathogenic to animals.

The term pathogenic as applied to an organism must be understood in a purely _relative_ sense, since there is no single organism that can cause disease in all of a certain class, but each is limited to a more or less narrow range. Some form of tuberculosis attacks nearly all vertebrates, but no other classes of animals and no plants. Lockjaw or tetanus attacks most mammals, but not any other vertebrates naturally. Typhoid fever affects human beings; hog cholera, swine, etc. This point is more fully discussed in Chapter XXIII but can not be too greatly insisted upon.

“The greatest enemy to mankind is man.”

Exceptions to this statement do occur and are important and must be considered in efforts to protect completely human beings from disease (tuberculosis from cattle, glanders from horses, poisoning from spoiled canned goods, anthrax from hair, hides, wool, of animals dead of the disease), but the most common human diseases are derived from other human beings directly or indirectly.

Diseases which are due to unicellular pathogenic microörganisms are called _infectious_ diseases, while if such diseases are transmitted under natural conditions from organism to organism they are spoken of as _contagious_ diseases. Most infectious diseases are contagious but not all. Tetanus is a good illustration of a non-contagious infectious disease. There are very few such diseases.

When a unicellular microörganism gains entrance into the body and brings about any pathological changes there, the result is an _infection_. Undoubtedly many pathogenic organisms get into the body but never manifest their presence by causing disease conditions, hence do not cause an infection. It is the pathological conditions which result that constitute the infection, and not the mere _invasion_.

The time that elapses between the entrance of the organism and the appearance of symptoms is called the _period of incubation_ and varies greatly in different diseases.

The term _infestation_ is used to denote pathological conditions due to _multicellular_ parasites. Thus an animal is _infested_ (not infected) with tapeworms, roundworms, lice, mites, etc. Many of these conditions, probably all, are contagious, _i.e._, transmissable naturally from animal to animal. The word _contagious_ has been used in a variety of ways to mean _communicated by direct contact_, communicated by a living something (_contagium_) that might be carried to a distance and finally _communicable_ in any manner, transmissable. The agency of transmission may be very roundabout--as through a _special tick_ in Texas fever, a _mosquito_ in malaria, etc.,--or by direct personal contact, as generally in venereal diseases. After all, though exactness is necessary, it is better to learn all possible about the _means of transmission of diseases_, than quibble as to the terms to be used.

An infectious disease may be _acute_ or _chronic_. An acute infection is one which runs for a relatively short time and is “self-limited,” so-called, _i.e._, the organisms cease to manifest their presence after a time. In some acute infections the time is very short--German measles usually runs five or six days. Typhoid fever may continue eight to ten weeks, sometimes longer, yet it is an acute infectious disease. It is not so much the time as the fact of _self-limitation_ that characterizes acute infections.

In chronic infections there is little or no evidence of limitation of the progress of the disease which may continue for years. Tuberculosis is usually chronic. Leprosy in man is practically always so. Glanders in horses is most commonly chronic; in mules and in man it is more apt to be acute.

Many infections begin acutely and later change to the chronic type. Syphilis in man is a good illustration.

The differences between acute and chronic infections are partly due to the nature of the organism, partly to the number of organisms introduced and the point of their introduction and partly to the resistance of the animal infected.

An infectious disease is said to be _specific_ when one kind of organism is responsible for its manifestations--as diphtheria due to the _Corynebacterium diphtheriæ_, lockjaw due to _Clostridium tetani_, Texas fever due to the _Piroplasma bigeminum_, etc. It is _non-specific_ when it may be due to a variety of organisms, as _enteritis_ (generally), _bronchopneumonia_, _wound infections_.

Henle, as early as 1840, stated certain principles that must be established before a given organism can be accepted as the cause of a specific disease. These were afterward restated by Koch, and have come to be known as “Koch’s postulates.” They may be stated as follows:

1. The given organism must be found in all cases of the disease in question.

2. No other organism must be found in all cases.

3. The organism must, when obtained in pure culture, reproduce the disease in susceptible animals.

4. It must be recovered from such animals in pure culture and this culture likewise reproduce the disease.

These postulates have not been fully met with reference to any disease, but the principles embodied have been applied as far as possible in all those infections which we recognize as specific, and whose causative agent is accepted. In many diseases recognized as infectious and contagious no organism has been found which is regarded as the specific cause. In some of these the organism appears to be too small to be seen with the highest powers of the microscope, hence they are called “_ultramicroscopic_” organisms. Because these agents pass through the finest bacterial filters, they are also frequently called “_filterable_.” The term “_virus_” or “_filterable virus_” is likewise applied to these “ultramicroscopic” and “filterable” agents.

The term _primary infection_ is sometimes applied to the first manifestation of a disease, either specific or non-specific, while _secondary_ refers to later developments. For example, a _secondary_ general infection may follow a _primary_ wound infection, or _primary_ lung tuberculosis be followed by _secondary_ generalized tuberculosis, or _primary_ typhoid fever by a _secondary_ typhoid pneumonia. The terms _primary_ and _secondary_ are also used where the body is invaded by one kind of an organism and later on by another kind; thus a _primary_ measles may be followed by _secondary_ infection of the middle ear, or a _primary_ influenza may be followed by a _secondary_ pneumonia, or a _primary_ scarlet fever by a _secondary_ nephritis (inflammation of the kidney). Where several organisms seem to be associated simultaneously in causing the condition then the term _mixed infection_ is used--in severe diphtheria, streptococci are commonly associated with the _Corynebacterium diphtheriæ_. In many cases of hog-cholera, mixed infections in the lungs and in the intestines are common. Wound infections are usually _mixed_. _Auto-infection_ refers to those conditions in which an organism commonly present in or on the body in a latent or harmless condition gives rise to an infectious process. If the _Bacterium coli_ normal to the intestine escapes into the peritoneal cavity, or passes into the bladder, a severe peritonitis or cystitis, respectively, is apt to result. “Boils” and “pimples” are frequently autoinfections. Such infections are also spoken of as _endogenous_ to distinguish them from those due to the entrance of organisms from without--_exogenous_ infections. _Relapses_ are usually instances of autoinfection.

Those types of _secondary infection_ where the infecting agent is transferred from one disease focus to another or several other points and sets up the infection there are sometimes called _metastases_. Such are the transfer of tubercle bacilli from lung to intestine, spleen, etc., the formation of abscesses in internal organs following a primary surface abscess, the appearance of glanders nodules throughout various organs following pulmonary glanders, etc.

The characteristic of a pathogenic microörganism which indicates its ability to cause disease is called its _virulence_. If slightly virulent, the effect is slight; if highly virulent, the effect is severe, and may be fatal.

On the other hand, the characteristic of the host which indicates its capacity for infection is called _susceptibility_. If slightly susceptible, infection is slight, if highly susceptible, the infection is severe.

Evidently the degree of infection is dependent in large measure on the relation between the _virulence_ of the invading organism and the _susceptibility_ of the host. High virulence and great susceptibility mean a severe infection; low virulence and little susceptibility a slight infection; while high virulence and little susceptibility or low virulence and great susceptibility might mean a moderate infection varying in either direction. Other factors influencing the degree of infection are the number of organisms introduced, the point where they are introduced and various conditions. These will be discussed in another connection (Chapter XXV).

The study of pathogenic bacteriology includes the thorough study of the individual organisms according to the methods already given (Chapters XVIII-XXI) as an aid to diagnosis and subsequent treatment, bacteriological or other, in a given disease. Of far greater importance than the _treatment_, which in most infectious diseases is not specific, is the _prevention_ and _ultimate eradication_ of all infectious diseases. To accomplish these objects involves further a study of the _conditions under which pathogenic organisms exist outside the body_, _the paths of entrance into and elimination from the body_ and those _agencies within the body itself_ which make it _less susceptible to infection or overcome the infective agent after its introduction_. That condition of the body itself which prevents any manifestation of a virulent pathogenic organism after it has been once introduced is spoken of as _immunity_ in the modern sense. Immunity is thus the opposite of susceptibility and may exist in varying degrees.

That scientists are and have been for some years in possession of sufficient knowledge to permit of the prevention and eradication of most, if not all, of our infectious diseases can scarcely be questioned. The practical application of this knowledge presents many difficulties, the chief of which is the absence of a public sufficiently enlightened to permit the expenditure of the necessary funds. Time and educative effort alone can surmount this difficulty. It will probably be years yet, but it will certainly be accomplished.