Emphysema may be generalised. Such an accident is rare, but may occur in the ox as well as in the sheep and goat.
Provided the emphysema remains confined to the subcutaneous tissue, the animals are not necessarily in danger. Where, however, it also extends to the interstitial tissue, and particularly if the cause to which it is due continues, death may result in a very short time. This occurs, for example, when the emphysema extends into the mediastinum, and thus gains the pleura and lung.
The symptoms of emphysema are then complicated with respiratory and circulatory disturbance and with signs of asphyxia.
Causation. Subcutaneous emphysema may be produced in many different ways.
If, for example, in puncturing the rumen the canula be carelessly withdrawn so that the skin is slightly separated from the subjacent tissues, gas may pass from the rumen into the channel produced by the instrument. It then becomes distributed throughout the subcutaneous tissue, and if the cutaneous opening is displaced its escape is confined to the connective and interstitial tissues in the region of the flank. Diffuse suppuration may then be set up in these parts, and may extend far beyond them.
In the ox emphysema rarely becomes generalised, but in the sheep and goat extension is more common; the patients perish of intoxication, caused by reabsorption of septic gases.
Under other circumstances emphysema may be due to an injury in some region where the connective tissue is loose and pliable, as for instance the region of the elbow, the internal surface of the shoulder, or the fold of the flank. Every time the animals move the tissues are displaced, and air being drawn in, it is imprisoned by the valve-like action of the injured part and gradually finds its way into the subcutaneous tissue.
Accidental injuries to the trachea, particularly injuries produced by dogs biting sheep or goats, are always accompanied by local emphysema, unless the wounds in the skin and trachea correspond, which rarely happens. At every respiration a portion of the air expelled passes into the peritracheal tissue, from which it gradually invades neighbouring parts, and may attain the mediastinum, etc. The injured animal thus inflates its own tissues and dies from asphyxia.
The open lesions due to pulmonary echinococcosis, and the accidents associated with pneumo-thorax, tuberculous caverns and abscesses, or pulmonary emphysema may become points of departure for local, general, interstitial or subcutaneous emphysema.
The diagnosis of accidental emphysema presents no difficulty, for the local swellings can only be mistaken for those of black-quarter. In the latter disease, however, fever is a constant accompaniment, whilst in simple emphysema it is absent.