A. The Skin.—If the skin is healthy there is no opportunity for bacteria to penetrate it. It is protected not only by the stratified epithelium, but also in various animals, by coats of hair, wool, feathers, etc. The secretion pressure of the healthy sweat and oil glands acts as an effective bar even to motile bacteria. Nevertheless a very slight injury only is sufficient to give normal surface parasites and other pathogenics, accidentally or purposely brought in contact with it, an opportunity for more rapid growth and even entrance for general infection. Certain diseases due to higher fungi are characteristically “skin diseases” and rarely become general—various forms of favus, trichophyton infections, etc. A few disease organisms, tetanus, malignant edema, usually get in through the skin; others, black-leg, anthrax, quite commonly; and those diseases transmitted by biting and blood-sucking insects, piroplasmoses, trypanosomiases, spirilloses, scarcely in any other way. Defective secretion in the skin glands from other causes, may permit lodgment and growth of bacteria in them or in the hair follicles. “Pimples” and boils in man and local abscesses occasionally in animals are illustrations. Sharp-edged and freely bleeding wounds are less liable to be infected than contusions, ragged wounds, burns, etc. The flowing blood washes out the wound and the clotting seals it, while there is less material to be repaired by the leukocytes and they are free to care for invading organisms (phagocytosis). Pathogenic organisms, especially pus cocci, frequently gain lodgment in the milk glands and cause local (mastitis) or general infection.
B. Mucosæ directly continuous with the skin and lined with stratified epithelium are commonly well protected thereby and by the secretions.
(a) The external auditory meatus is rarely the seat even of local infection. The tympanic cavity is normally sterile, though it may become infected by extension through the Eustachian tube from the pharynx (otitis media).
(b) The conjunctiva is frequently the seat of localized, very rarely the point of entrance for a generalized infection, except after severe injury. Those diseases whose path of entrance is generally assumed to be the respiratory tract (see “Lungs” below) might also be admitted through the eye. Material containing such organisms might get on the conjunctiva and be washed down through the lachrymal canal into the nose. Experiment has shown that bacteria may pass in this way in a few minutes. In case masks are worn to avoid infection from patients suffering with these diseases, the eyes should therefore be protected as well as the nose and mouth.
(c) The nasal cavity on account of its anatomical structure retains pathogenic organisms which give rise to local infections more frequently than other mucosæ of its character. These may extend from here to middle ear, neighboring sinuses, or along the lymph spaces of the olfactory nerve into the cranial cavity (meningitis). Acute coryza (“colds” in man) is characteristic. Glanders, occasionally, is primary in the nose, as is probably roup in chickens, leprosy in man. The meningococcus and the virus of poliomyelitis pass from the nose into the cranial cavity without local lesions in the former.
(d) The mouth cavity is ordinarily protected by its epithelium and secretions, though the injured mucosa is a common source of actinomycosis infection, as well as thrush. In foot-and-mouth disease no visible lesions seem necessary to permit the localization of the unknown infective agent.
(e) The tonsils afford a ready point of entrance for ever-present micrococci and streptococci whenever occasion offers (follicular tonsillitis, “quinsy”), and articular rheumatism is not an uncommon sequel. The diphtheria bacillus characteristically seeks these structures for its development. Tubercle and anthrax organisms occasionally enter here.
(f) The pharynx is the seat of localized infection as in micrococcal, streptococcal and diphtherial “sore throat” in human beings, but both it and the esophagus are rarely infected in animals except as the result of injury.
(g) The external genitalia are the usual points of entrance for the venereal organisms in man (gonococcus, Treponema pallidum, and Ducrey’s bacillus). The bacillus of contagious abortion and probably the trypanosome of dourine are commonly introduced through these channels in animals.
C. Lungs.—The varied types of pneumonia due to many different organisms (tubercle, glanders, influenza, plague bacilli, pneumococcus, streptococcus, micrococcus and many others) show how frequently these organs are the seat of a localized infection, which may or may not be general. Whether the lungs are the actual point of entrance in these cases is a question which is much discussed at the present time, particularly with reference to tuberculosis. The mucous secretion of the respiratory tract tends to catch incoming bacteria and other small particles and the ciliary movement along bronchial tubes and trachea tends to carry such material out. “Foreign body pneumonia” shows clinically, and many observers have shown experimentally that microörganisms may reach the alveoli even though the exchange of air between them and the bronchioles and larger bronchi takes place ordinarily only by diffusion. The presence of carbon particles in the walls of the alveoli in older animals and human beings and in those that breathe dusty air for long periods indicates strongly, though it does not prove absolutely, that these came in with inspired air. On the other hand, experiment has shown that tubercle bacilli introduced into the intestine may appear in the lungs and cause disease there and not in the intestine. It is probably safe to assume that in those diseases which are transmitted most readily through close association though not necessarily actual contact, the commonest path is through the respiratory tract, which may or may not show lesions (smallpox, scarlet fever, measles, chicken-pox, whooping-cough, pneumonic plague in man, lobar and bronchopneumonias and influenza in man and animals, some cases of glanders and tuberculosis). On the other hand, the fact that the Bacterium typhosum and Bacterium coli may cause pneumonia when they evidently have reached the lung from the intestinal tract, and the experimental evidence of lung tuberculosis above mentioned show that this route cannot be excluded in inflammations of the lung.
D. Alimentary Tract.—The alimentary tract affords the ordinary path of entrance for the causal microbes of many of the diseases of animals and man, since they are carried into the body most commonly and most abundantly in the food and drink.
(a) The stomach is rarely the seat of local infection, even in ruminants, except as the result of trauma. The character of the epithelium in the rumen, reticulum and omasum in ruminants, the hydrochloric acid in the abomasum and in the stomachs of animals generally are usually sufficient protection. Occasionally anthrax “pustules” develop in the gastric mucosa. (The author saw nine such pustules in a case of anthrax in a man.)
(b) The intestines are frequently the seat of localized infections, as various “choleras” and “dysenteries” in men and many animals, anthrax, tuberculosis, Johne’s disease. Here doubtless enter the organisms causing “hemorrhagic septicemias” in many classes of animals, and numerous others. These various organisms must have passed through the stomach and the question at once arises, why did the HCl not destroy them? It must be remembered that the acid is present only during stomach digestion, and that liquids taken on an “empty stomach” pass through rapidly and any organisms present are not subjected to the action of the acid. Also spores generally resist the acid. Other organisms may pass through the stomach within masses of undigested food. The fact that digestion is going on in the stomach of ruminants practically all the time may explain the relative freedom of adult animals of this class from “choleras” and “dysenteries.”
In the preceding chapters statements have been made that “bacteria enter” at various places or they “pass through” different mucous membranes, skin, etc. Strictly speaking such statements are incorrect—bacteria do not “enter” or “pass through” of themselves. It is true that some of the intestinal organisms are motile, but most of the bacteria which are pathogenic are non-motile. Even the motile ones can not make their way against fluids secreted or excreted on free surfaces. Bacteria cannot pass by diffusion through membranes since they are finite particles and not in solution.
In the case of penetrating wounds bacteria may be carried mechanically into the tissues, but this is exceptional in most infections. Also after gaining lodgment they may gradually grow through by destroying tissue as they grow, but this is a minor factor. Evidently, there must be some mechanism by which they are carried through. The known mechanisms for this in the body are ameboid cells, especially the phagocytes. It is most probable that these are the chief agents in getting bacteria into the tissues through various free surfaces. The phagocytes engulf bacteria, carry them into the tissues and either destroy them, are destroyed by them, or may disgorge or excrete them free in the tissues or in the blood.
Dissemination of organisms within the tissues occurs either through the lymph channels or the bloodvessels or both. If through the lymph vessels only it is usually much more restricted in extent, or much more slowly disseminated, while blood dissemination is characterized by the number of organs involved simultaneously.
I. Directly from the point, of injury. This is true in infected wounds open to the surface, skin glanders (farcy), black-leg, surface anthrax, exanthemata in man and animals (scarlet fever (?), measles (?), smallpox; hog erysipelas, foot-and-mouth disease): also in case of disease of mucous membranes continuous with the skin—from nasal discharges (glanders), saliva (foot-and-mouth disease), material coughed or sneezed out (tuberculosis, influenza, pneumonias), urethral and vaginal discharges (gonorrhea and syphilis in man, contagious abortion and dourine in animals), intestinal discharges (typhoid fever, “choleras,” “dysenteries,” anthrax, tuberculosis, Johne’s disease). Material from nose, mouth and lungs may be swallowed and the organisms passed out through the intestines.
II. Indirectly through the secretions and the excretions where the internal organs are involved. The saliva of rabid animals contains the ultramicroscopic virus of rabies (the sympathetic ganglia within the salivary glands, and pancreas also, are affected in this disease as well as the cells of the central nervous system). The gall-bladder in man is known to harbor colon and typhoid bacilli, as that of hog-cholera hogs does the virus of this disease. It may harbor analogous organisms in other animals, though such knowledge is scanty. The kidneys have been shown experimentally to excrete certain organisms introduced into the circulation within a few minutes (micrococci, colon and typhoid bacilli, anthrax). Typhoid bacilli occur in the urine of typhoid-fever patients in about 25 per cent. of all cases and the urine of hogs with hog cholera is highly virulent. Most observers are of the opinion, however, that under natural conditions the kidneys do not excrete bacteria unless they themselves are infected.
The milk both of tuberculous cattle and tuberculous women has been shown to contain tubercle bacilli even when the mammary glands are not involved. Doubtless such bacteria are carried through the walls of the secreting tubules or of the smaller ducts by phagocytes and are then set free in the milk.
It is readily apparent that certain disease organisms tend to locate themselves in definite regions and the question arises, Is this due to any specific relationship between organism and tissue or not? Diphtheria in man usually attacks the tonsils first, gonorrhea and syphilis the external genitals, tuberculosis the lung, “choleras” the small intestine, “dysenteries” the large intestine, influenza the lungs. In these cases the explanation is probably that the points attacked are the places where the organism is most commonly carried, with no specific relationship, since all of these organisms (Asiatic cholera excepted) also produce lesions in other parts of the body when they reach them. On the other hand, the virus of hydrophobia attacks nerve cells, leprosy frequently singles out nerves, glanders bacilli introduced into the abdominal cavity of a young male guinea-pig cause an inflammation of the testicle, malarial parasites and piroplasms attack the red blood corpuscles, etc. In fact, most pathogenic protozoa are specific in their localization either in certain tissue cells or in the blood or lymph. In these cases there is apparently a real chemical relationship, as there is also between the toxins of bacteria and certain tissue cells (tetanus toxin and nerve cells). Whether “chemotherapy” will ever profit from a knowledge of such chemical relationships remains to be developed. It appears that a search for these specific chemical substances with the object of combining poisons with them so that the organisms might in this way be destroyed, would be a profitable line of research.