Causes. These are the ordinary causes of chest diseases. As special causes may be noted severe compression of the larynx as in roughly and repeatedly coughing an animal; the sudden contact of piercing cold air, of irritant gases, powders or liquids with the membrane, and the rapid, forcible and continuous current of condensed air through the glottis during severe exertion.
Among the general causes the most fruitful are the high feeding, hot, close stables, heats and chills, and other circumstances attendant on domestication. Laryngitis may be an extension from coryza, pharyngitis, bronchitis or pneumonia. It may further be but a local manifestation of influenza or strangles, contagious pneumonia, etc. The disease is acute, subacute, or chronic.
Symptoms of acute form. All acquainted with horses can recognize the general symptoms of sore throat. The nose is elevated and protruded to avoid compression of the larynx; it is carried stiffly for the same reason. There is some swelling around the throat or beneath the root of the ears. If the cartilages of the larynx are compressed between the finger and thumb, or if pressure is made in the median line below upon the connecting crico-thyroid membrane the patient instantly coughs and throws up the head to avoid a repetition of the suffering. This tenderness of the larynx to touch is peculiar to laryngitis and serves to distinguish it from pharyngitis. The cough is at first very hard and painful and only gives way to a soft mucous type when a free mucous exudation puts an end to the tense, thickened and dry state of the mucous membrane. The inspiratory act is accompanied by a whistling or deep bass sound, particularly after the slightest exertion. This may be heard at times during expiration as well, though not invariably so. Sometimes the animal drops the food from his mouth after mastication, because of the pain attendant on swallowing, but this is really a symptom of coexisting pharyngitis, and its absence implies the nonexistence of that complication. There is usually a slight pasty swelling between the branches of the lower jaw.
There are besides the general symptoms of fever more or less marked, such as increased temperature, accelerated pulse, red injected eyes and nose, slightly hastened breathing, the expiration being effected by a double lifting of the flank as in broken wind, etc.
In two or three days in favorable cases exudation takes place from the mucous membrane, the cough becomes softer and less frequent, the local tenderness decreases and the general symptoms subside.
If otherwise the symptoms may become more intense, and breathing may get loud and difficult in connection with thickening and rigidity of the mucous membrane, or a serous exudation into it and beneath it (œdema glottidis) which by closing the glottis renders breathing almost impossible. The same distressing symptoms may arise from spasm of the larynx excited by the inflammatory action. As arising from thickening or infiltration of the membrane these symptoms may come on comparatively slowly, but in the case of spasm they appear suddenly and have periods of intermission, reappearing on succeeding days and usually at the earlier part of the night. In such circumstances the loud, noisy breathing is heard at a considerable distance, the horse stands obstinately still, the fore feet apart, his elbows turned out to allow a firm action of the chest, the flanks working laboriously, the head low, the nose protruded, the nostrils widely dilated, the mouth open, the eyes standing out from their sockets red and wild looking, and the face constrained and pinched, the whole expression being that of intense agony from impending suffocation.
Lesions. In cases where death has supervened, perhaps in connection with another disease, the laryngeal mucosa, especially on and above the glottis, is soft, tumid, friable, with ramified or uniform redness and petechial spots. The epithelial layer may be softened, disintegrated and shed, leaving pointed or larger erosions, which are, however, usually superficial. In case of œdema glottidis the mucosa and submucosa are thickened by an abundant exudate which may extend to the connective tissue outside the larynx as well. In aggravated cases there may be dark red or brownish red discolorations of the mucosa.
Course, Duration. Fortunately these aggravated forms of the disease are rare and unless the patient perishes during such an attack or the inflammation extends down toward the chest, laryngitis rarely proves fatal. Its duration is from twelve to fifteen days. Its extension to the lungs may be suspected when the extreme tenderness of the throat subsides without any corresponding improvement in the health. Examination of the chest will then rarely fail to detect the presence of disease.
But although sore throat is rarely fatal its effects are not unimportant nor trivial. It occasionally merges into a chronic form, with a hacking cough, tenderness to pressure and an increased liability to other diseases of the air passages. More frequently it is followed by wasting and fatty degeneration of the laryngeal muscles and the horse becomes a confirmed roarer. In all cases it leaves a greater susceptibility to a second attack.
Subacute Laryngitis. This form has been chiefly seen in young animals and up to eight years old. At the outset its symptoms are moderate but as it is usually associated with serous effusions in and around the mucous membrane the symptoms above mentioned as indicating imminent danger of suffocation may suddenly appear and life can only be preserved by opening the trachea.