The foregoing experiments show that in cats a definite lobar pneumonia may be caused by Bacillus mucosus capsulatus. Judging both from the clinical course and from the pathological findings, this form of pulmonary infection differs from the usual pneumococcus types of pneumonia and closely resembles the so called Friedländer's bacillus or Bacillus pneumoniœ in man. In all instances in which a lobar pneumonia was found after the injection of the bacillus, a similar organism was recovered from the lung, and in no case was this associated with other organisms. The course of the disease in cats is very short, the animals developing early symptoms of profound toxemia. In 87 per cent of the animals showing a lobar pneumonia positive blood cultures were obtained. The pathological findings, judging from the early stages of the disease, are subject to considerable variation. In some instances the process may suggest a pseudolobar or confluent lobular distribution. In these cases the lung has a mottled, marble-like appearance. In the majority of cases, however, the process gave a more homogeneous appearance, suggesting a diffuse and uniform distribution. Foci of hemorrhage were not uncommon in both. Such areas cause the mottled appearance sometimes found. In all instances the consolidated lung presents a greater infiltration of tissue than is usually seen in other types of experimental pneumonia. Although the exudate as seen on the cut surface may be abundant and especially viscid in character, this is not present in most cases. The cut surface of the consolidated lung does not present a granular appearance. The histological findings are also subject to considerable variation. In most instances the infundibular and alveolar spaces are completely filled with an exudate made up chiefly of polymorphonuclear cells. Associated with these are the capsulated bacilli, large vacuolated mononuclear phagocytic cells, and red blood cells, and occasionally small amounts of fibrin. The organisms may vary greatly in numbers. Some sections show spaces almost completely filled with bacilli. The contrast between spaces containing an exudate consisting chiefly of polymorphonuclear forms and an adjoining one filled with organisms is often striking (Fig. 4). The bacilli found are both intra- and extracellular.
The large vacuolated cells are numerous in this type of pneumonia. They apparently are the first cells to become phagocytic. Often they are seen to contain as many as 10 to 15 capsulated bacilli, while polymorphonuclear cells in the same exudate contain no organisms. The histogenesis of these cells seems to be somewhat clearer from the study of these early stages of pneumonia. In many instances one sees swollen, partially desquamated epithelial cells along the alveolar wall. These closely resemble the large vacuolated forms. Various types of these vacuolated mononuclear cells were observed. These may well represent stages of development from the desquamating epithelial cell to the large vacuolated form. Although similar cells may arise elsewhere, we have been led to regard them in our studies as epithelial in origin (Fig. 5).
The number of red blood cells and the amount of fibrin present in the exudate vary greatly. Small foci consisting of alveolar spaces filled with erythrocytes are not uncommon. The fibrin is very much less abundant than in most types of pneumonia.
From the above experiments it is seen that a lobar pneumonia in cats can be produced at least by two methods, either by intrabronchial insufflation of the organism or by direct injection into the veins, provided that in the latter case an irritant is introduced into the lungs. In each case there is little doubt but that a local injury of the lung parenchyma was produced. Without this injury (that is, by intravenous injection of the organism alone), no pulmonary lesion was obtained.
Further studies with both these methods must be undertaken to ascertain more exactly the sequence of the pathological process. It seems probable that they are identical in each case.
The results obtained from the second method employed to produce a lobar pneumonia offer suggestive evidence in support of a hematogenous causation of this disease in at least certain instances. It is not proposed to discuss the aerogenous versus hernatogenous theories at this time. Kidd5 has recently reviewed the subject and states that the aerogenous theory for the causation of pneumonia is most widely held. This view has gained credence especially since the work of Meltzer and Lamar. In spite of this, Kidd emphasizes the fact that based on our knowledge of pulmonary infections in man and upon theoretical grounds and upon certain experimental facts, the hernatogenous theory seems more plausible.
No definite conclusions can be drawn from the last series of experiments. From this limited study it seems probable that lobar pneumonic processes are produced less easily after intravenous injection of various cocci and insufflation of irritating substances than by-similar treatment with Friedländer's bacillus.
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