The primary lesions are infected wounds in various parts of the body, most frequently in the extremities, from which, owing to the invasion with virulent necrosis bacilli, necrosis and gangrene and collateral phlegmons develop. Metastases are set in the lungs, and these pulmonary lesions go over to pulmonary and parietal pleura, occasionally to the pericardial pleura. Gangrene and cavern formation sets in in those foci which communicate with the lumen of the bronchial tree. The presence of an increased amount of fluid in thoracic cavity and pericardial sac, even when no inflammatory changes are noted, is probably as much part of the local a defensive reaction in the thoracic cavity as of the general cachectic hydrops. In a few eases a broncho-pneumonia is added to these lesions. The degenerative changes in various internal organs (fatty infiltration of myocard, liver, and kidneys), oligaemia, and emaciation are probably the consequences of a toxaemia, which as far as we can judge would represent the immediate cause of death. In bringing about a cachectic condition the toxaemia was doubtless assisted by the reduced intake of food.
Metastatic pulmonary foci of pure necrobacillosis, i.e. foci in which microscopically only the typical filaments are discovered, have a characteristic histological structure. There is a central necrotic focus which differs from the foci found in the liver, etc., of cattle, in that considerable portions are crowded with nuclei and nuclear detritus. The nuclei, which belong to desquamated alveolar epithelial cells and wandering cells, are deformed and arranged in such a way that a picture results which has a superficial resemblance with a spindle cell sarcoma. A thin demarcation zone with mainly polymorphonuclear cellulation separates the necrotic centre from the surrounding lung tissue, which represents a reaction zone with serofibrinous and slight cellular exudation, and with the appearance of large swollen fibroblasts in the interstitium. Bacterial filaments are found in debris in the centre, well preserved and numerous in the periphery of the necrotic focus, and especially numerous, forming thick dense layers, underneath the compact outermost membrane-like layer of the tunica elastica pleurae.
An organism closely resembling the B. necrophorus was isolated from foci in the lungs of horses dying from necrotic pneumonia. This organism morphologically was identical with B. necrophorus, but appeared to be more difficult to cultivate. It was not proved that this organism was the cause of the disease with which it was associated, and transmission experiments in horses were not successful, the case, of horse 15752 not having been definitely found to have been a true case of necrotic pneumonia. In small animals the results obtained from inoculation of infected material agree with those obtained with B. necrophorus, with the distinction that guinea-pigs were susceptible as well and showed well-marked lesions. It is probable therefore that the organism belonged to the necrophorus group, but that is was a variation from that species which causes the well-known conditions such as disseminated necrosis of the liver in cattle and foot-rot in sheep.