| C.—Infective Diseases in which an organism has been found, but has not finally been connected with the disease. | |
| Hydrophobia. | Scarlet Fever. |
| D.—Infective Diseases not yet proved to be due to micro-organisms. | |
| Small-pox. | Mumps. |
| Typhus Fever. | Whooping Cough, &c. |
| Measles | |
| A.—Diseases due to Vegetable Parasites. |
| I.—To Schizomycetes, Bacteria or Fission Fungi. |
| 1. Caused by the Pyogcnclic Micrococci. |
Suppuration and Septicaemia.—It is now recognized that although nitrate of silver, turpentine, castor oil, perchloride of mercury and certain other chemical substances are capable of producing suppuration, the most common causes of this condition are undoubtedly the so-called pus-producing bacteria. Of these perhaps the most important are the staphylococci (cocci arranged like bunches of grapes), streptococci (cocci arranged in chains), and pneumococci, though certain other organisms not usually associated with pus-formation are undoubtedly capable of setting up this condition, e.g. Bacillus pyocyaneus, Bacillus coli communis, and the typhoid bacillus. These organisms (the products of which, by chemical irritation, stimulate the leucocytes to emigration) bring about the death and digestion of the tissues and fluids (which no longer “clot”) with which they come in contact, pus (matter) being thus formed: this accumulates in the tissues, in the serous cavities, or even on mucous surfaces; septicaemia or blood-poisoning, secondary infection of tissues and organs at a distance from the original site of infection, or pyaemia, with the formation of secondary abscesses, may thus be set up.
In septicaemia the pus-forming organisms grow at the seat of introduction, and produce special poisons or toxins, which, absorbed into the blood, give rise to symptoms of fever. From the point of introduction, however, the organisms may be swept away either by the lymph or by the blood, and carried to positions in which they set up further inflammatory or suppurative changes. In the streptococcal inflammations spreading by the lymph channels appears to be specially prevalent. In the blood the organisms, if in small numbers, are usually destroyed by the plasma, which has a powerful bactericidal action; should they escape, however, they are carried without multiplication into the capillaries of the general circulation, of the lung, or of the liver, where, being stopped, they may give rise to a second focus of infection, especially if at the point of impaction the vitality of the tissues is in any way lowered. Unless the blood is very much impoverished, its bactericidal action is usually sufficiently powerful to bring about the destruction of anything but comparatively large masses of pyogenetic organisms. This bactericidal power, however, may be lost; in such case the pus-forming organisms may actually multiply, a general haemic infection resulting. Should microorganisms be conveyed by the veins to the heart, and there be deposited on an injured valve, an infective endocarditis is the result; from such a deposit numerous organisms may be continuously poured into the circulation. Simple thrombi or clots may also become infected with micro-organisms. Fragments of these, washed away, may form septic plugs in the vessels and give rise to abscesses at the points where they become impacted. A distinction must be drawn between sapraemia and septicaemia. In sapraemia the toxic products of saprophytic organisms are absorbed from a gangrenous or necrotic mass, from an ulcerating surface, or from a large surface on which saprophytic organisms are living and feeding on dead tissues: for example, we may have such a condition in the clots that sometimes remain after childbirth on the inner surface of the wall of the womb. So long as no micro-organisms follow the toxins, the condition is purely sapraemic, but should any organisms make their way into and multiply in the blood, the condition becomes one of septicaemia. The term pyaemia is usually associated with the formation of fresh secondary foci of suppuration in distant parts of the body. If the primary abscess occurs in the lungs, the secondary or metastatic abscesses usually occur in the vessels of the general or systemic circulation, and less frequently in other vessels of the lung. When the primary abscess occurs in the systemic area, the secondary abscess occurs first in the lung, and less frequently in the systemic vessels; whilst if the primary abscess be in the portal area (the veins of the digestive tract), the secondary abscesses are usually distributed over the same area, the lungs and systemic vessels being more rarely affected.
Infective Endocarditis.—Acute malignant or ulcerative endocarditis occurs in certain forms of septicaemia or of pyaemia. It is brought about by the Streptococcus pyogenes (see Plate II. fig. 2), the pneumococcus, or the Staphylococcus pyogenes aureus (see Plate I. fig. 4), or, more rarely, by the gonococcus, the typhoid bacillus or the tubercle bacillus, as they gain access to acute or chronic valvular lesions of the heart. The aortic and mitral valves are usually affected, the pulmonary and tricuspid valves much more rarely, though Washbourn states that the infective form occurs on the right side more frequently than does simple endocarditis. A rapid necrosis of the surface of the valve is early followed by a deposition of fibrin and leucocytes on the necrosed tissue; the bacteria, though not present in the circulating blood during life, are found in these vegetations which break down very rapidly; ulcerative lesions are thus formed, and fragments of the septic clot (i.e. the fibrinous vegetations with their enclosed bacteria) are carried in the circulating blood to different parts of the body, and, becoming impacted in the smaller vessels, give rise to septic infarcts and abscesses. The ulceration of the valves, or in the first part of the aorta, may be so extensive that aneurysm, or even perforation, may ensue.
In certain cases of streptococcic endocarditis the use of antistreptococcic serum appears to have been attended with good results. Sir A. Wright found that the introduction of vaccines prepared from the pus-producing organisms after first lowering the opsonic index almost invariably, after a very short interval, causes it to rise. He found, too, that the vaccine is specially efficacious when it is prepared from the organisms associated with the special form of suppuration to be treated. Whenever the opsonic index becomes higher under this treatment the suppurative process gradually subsides: boils, acne, pustules, carbuncles all giving way to the vaccine treatment. The immunity so obtained is attributed to the increased activity of the serum as the result of the presence of an increased amount of opsonins. Further, Bier maintains that a passive congestion and oedema induced by constriction of a part by means of a ligature or by a modification of the old method of cupping without breaking the skin appears to have a similar effect in modifying localized suppurative processes, that is processes set up by pus-producing bacteria. Wright holds that this treatment is always more effective when the opsonic index is high and that the mere accumulation of oedematous fluid in the part is sufficient to raise the opsonic index of that fluid and therefore to bring about a greater phagocytic activity of the leucocytes that are found in such enormous numbers in the neighbourhood of suppurative organisms and their products.
Erysipelas.—In 1883 Fehleisen demonstrated that in all cases of active erysipelatous inflammation a streptococcus or chain of micrococci (similar to those met with in certain forms of suppuration) may be found in the lymph spaces in the skin. The multiplying streptococci found in the lymph spaces form an active poison, which, acting on the blood-vessels, causes them to dilate; it also “attracts” leucocytes, and usually induces proliferation of the endothelial cells lining the lymphatics. These cells—perhaps by using up all available oxygen—interfere with the growth of the streptococcus and act as phagocytes, taking up or devouring the dead or weakened micro-organisms. Both mild and severe phlegmonous cases of erysipelas are the result of the action of this special coccus, alone, or in
