Splenisation is that condition of lung already described under the head of pulmonary congestion, and if affecting both lungs throughout, necessarily destroys life by arresting the æration of the blood.
Abscess. Diffuse suppuration is very common in the stage of gray hepatization. In this the affected lung becomes more or less extensively infiltrated with pus limited by no distinct membrane like the pus of an abscess, but exuding freely from the cut surface of the lung or escaping from its interstices when it is pressed. It is preceded and in its early stages associated with the formation of granular masses and corpuscles. Its existence cannot be certainly ascertained though it may be surmised when after hepatization of a portion of lung a mucous râle, a sort of gurgling, is heard in the adjacent bronchium and an abundant muco-purulent discharge takes place from the nose. It threatens extensive destruction of lung tissue.
Circumscribed suppuration or abscess is infrequent though occasionally met with in the horse and ox. In this case the excessive exudation at one point liquefies and the surrounding lymph becoming organized into a vascular membrane an abscess is formed. This may burst into the bronchial tubes and be discharged by the nose. In less favorable cases it makes its way toward the pleural surface and opens into the cavity of the chest. It is impossible to detect the existence of a pulmonary abscess though after it has burst into a bronchial tube the existence of the cavity may be ascertained by the amphoric sound heard on auscultation.
Animals may recover from such pulmonary suppurations or if they are too extensive the consequent depletion may induce hectic and death.
Gangrene of the lung is happily rare and has appeared to be connected with close, foul stables, previous ill-health, and work after the onset of pneumonia. It is characterized by high temperature (106° to 108° F.) by great dulness and prostration due to the poisoning of the nerve centres, by weakness and unsteadiness, by complete loss of appetite, and at length an intolerable fœtor of the breath as if from putrefying animal matter. In rare cases recovery may take place, the dead portion having become detached and expectorated.
Consolidation from hepatization is the condition in which the inflamed lung is always found, in the second stage of the disease. The lung has then the density and brownish red appearance naturally belonging to the liver (red hepatization), which changes on the occurrence of softening of the exuded products to a grayish hue (gray hepatization). But after the subsidence of the acute symptoms, the process of liquefaction and absorption is not always complete, a portion of the exuded product becomes vascular, is developed into fibrous tissue and remains permanently impervious to air. Such is the state of the lung in many cases of thick or short wind in horses when these have occurred as a sequel of pneumonia. A horse suffering in this way has the breathing habitually accelerated and is thrown into a state of great distress by any attempt to make him perform hard work such as galloping, dragging a load up hill and the like. A chronic cough may equally accompany this condition.
Pathological Lesions. These differ according to the stage of the disease. In the first stage, that of congestion the lung tissue is engorged with blood as described under the head of congested lungs. As early as 6 or 7 hours after artificial irritation, the alveoli of the affected part are already filled by exudation and cell proliferation. Until this has taken place the alveoli can still be distended by blowing into the bronchial tube.
In the second stage the condition of the lung is that of red hepatisation, so called from its resemblance in color and consistency to the liver. There are gradations between congestion and red hepatisation. In the earlier stages of the latter, the lung retains a measure of its softness, elasticity and permeability to air, though it is considerably firmer and less permeable than that which is in a state of congestion and differs further from it in exuding from its cut surface not a grumous, dark bloody pulp, but a clear straw colored fluid. In the advanced red hepatisation the lung is of a firm consistency and granular liver like appearance. In color it varies from a bright red to a dark liver hue, the darker shades being chiefly met with in old animals or when the inflammation and fever have been intense and prostrating. Varying shades are seen in different lobules of the same lung. Its air cells are no longer pervious, it no longer crepitates under the pressure of the finger, nor floats in water, and its friability is such that it breaks down readily when the finger is thrust into its substance. Its surface is distinctly granular from the fibrinous plugging of the alveoli. Such a lung does not collapse when the chest is opened but retains its bulk and shape and in some cases the diseased portion may, by reason of the abundance of the exudation, be really larger than the same portion of lung in a normal state of dilatation. Its surface may thus retain the imprint of the ribs. Owing to the stasis of the blood in the vessels a hepatised portion of lung cannot be injected. The exudation which infiltrates the lung tissue and obliterates the air cells contains in the vicinity of the bloodvessels numerous granular masses and corpuscles and in the darker colored portions blood globules, owing to the action of diapedesis the red cells and the rupture of minute vessels. The smaller bronchial tubes stand out white and empty showing that they have escaped the inflammatory action. Hepatization usually extends from the anterior lobe or lower border upward.
Gray hepatization is a sequel of the red and presents the same firmness, friability and usually the same granular aspect; the lack of crepitation on pressure, and the higher density than water. From the cut surface a fatty or purulent fluid exudes spontaneously, or in other cases only when pressure is applied. The granular masses and corpuscles have disappeared, and if suppuration is not so abundant as to prove extensively destructive to lung tissue, that is gradually cleared up and restored to health. This state is always a very perilous one.
Abscess of the lung sometimes met with in animals dying of pneumonia shows a circumscribed area of inflammation and induration with the liquid pus in the centre immediately surrounded by a vascular (limiting) membrane. Abscess may be single or multiple though in the latter case it is commonly a symptom of pyæmia.