As a rule, however, the disease is acquired by a healthy animal being near an affected one and receiving the contagion direct. Affected animals may give off the contagion in the early stages of the disease before the symptoms are apparent to the observer; also, they may retain this infectious character, if they survive the attack, for six months and probably for a year after all symptoms of the disease have disappeared.
Incubation.—The time which elapses between exposure to the contagion of pleuropneumonia and the first appearance of the symptoms of this disease varies greatly with different individuals and with different outbreaks of the disease. Ordinarily the symptoms of disease make their appearance within three to six weeks after exposure; they may be observed, however, within two weeks or they may not become apparent until nearly or quite three months. It is this long period of incubation and the great length of time that an animal may disseminate the contagion after apparent recovery which give the plague that insidious character so often spoken of, and which greatly increase the difficulties of eradication.
Symptoms.—The symptoms are such as would be expected with inflammation of the lungs and pleuræ, but they vary considerably, according to the type which the disease manifests. If the attack is an acute one, as is frequently seen in hot weather, the symptoms appear suddenly; the breathing becomes rapid and difficult, the animal grunts or moans with each expiration, the shoulders stand out from the chest, the head is extended on the neck, the back is arched, the temperature is 104° to 107° F., the milk secretion is suspended, there is no appetite, rumination is stopped, the animal may bloat and later be affected with a severe diarrhea. Such cases are generally fatal in 7 to 20 days.
Very often the attack comes on slowly and the symptoms are much less clear. In the mildest cases there is a cough for a week or two, but no appreciable loss of appetite or elevation of temperature. The lungs are but slightly affected and recovery soon follows. Such animals may disseminate the contagion for a long time without being suspected, and for that reason are the most dangerous of all.
A more severe type of the plague is the most frequently seen. In these cases the cough is frequent, more or less painful, the back somewhat arched, and the milk secretion diminished. The prominence of these symptoms increases, the appetite is affected, the animal loses flesh, the breathing becomes more rapid, the cough more painful, pressure of the fingers between the ribs shows tenderness, the hair loses its gloss and stands erect, the skin becomes adherent, little, if any, milk is secreted, and the temperature rises, varying in different animals from 103° to 107° F. Animals thus affected may continue to grow worse and die in from three to eight weeks, or they may after a time begin to improve and make an apparent recovery. The inflammation of the lung does not, as a rule, subside and the organ return to its normal condition, as is the case in ordinary pneumonia, but with this disease the life of the affected portion of the lung is destroyed, the tissue dies, and a fibrous wall is formed around it to shut it away from the living parts. The tissue, thus encysted, gradually softens, becomes disintegrated, and breaks down into pus. The recovery, therefore, is not complete; it is only apparent and partial.
To those accustomed to examining the lungs of cattle, other and extremely important symptoms may be apparent during the course of the disease. By applying the ear over the walls of the chest an area of a certain extent may be found in which the natural breathing sound is diminished or entirely lost. This represents the diseased portion of the lungs. In other cases a loud blowing sound may be heard, quite different from any sound produced when the lung is in a healthy condition. In some cases crepitation is heard near the border line of the diseased area and friction sounds produced by the roughened pleura; these can be appreciated, however, only by those whose ears have been trained to distinguish between the different sounds which reach the ear when applied to the chest wall. By percussion—that is, by pressing the fingers of the left hand firmly against the wall of the chest and tapping upon the middle finger with the ends of the fingers of the right hand—an area of dullness may be discovered corresponding to the portion from which the respiratory murmur has disappeared. This loss of respiration detected by auscul tation, and the dullness brought out by percussion, are the most important evidences of an inflamed or consolidated lung.
Seriously affected animals remain standing if they have sufficient strength, but those which lie down always lie on the affected side.
The proportion of animals which become affected after being exposed varies according to the virulence of the outbreak, the susceptibility of the animals, and the length of time during which exposure is continued. Sometimes not more than 15, 20, or 30 per cent will contract the disease when a large herd is exposed; on the other hand, however, 80 or 90 per cent may be affected. The proportion of cases in which the disease proves fatal also varies greatly—it may not exceed 10 and it may reach 50 per cent. In general, it may be said that about 40 per cent of the exposed animals will contract the disease and about one-half of these cases will prove fatal.
Post-mortem appearances.—Owing to the complexity of the structure of the lung tissue, its ramifications of bronchial tubes and blood vessels, and its abundant supply of lymphatics, the pathological changes in pleuropneumonia are interpreted with great difficulty. Furthermore, there are certain kinds of pneumonia which present some resemblances to pleuropneumonia and which may therefore be confused with it in some of its phases.
If we kill an animal affected with acute pleuropneumonia and examine the cavity of the chest and lungs, the following appearances will be noted: