Slight colds or sore throats may cause roaring so long as they persist. Tight reining with the nose drawn in toward the chest induces a stridor in certain animals by distorting the larynx and trachea. Some horses with thick necks, badly set on heads, and in a state of obesity, roar, yet the symptom subsides when the superfluous fat is got rid of and they are brought into hard working condition. Stallions are very liable to make a noise from this cause. In a case of roaring which disappeared when the horse had been exercised for some time Leblanc diagnosed an œdema of the glottis which was absorbed under the increased movement of the parts. He did not test his opinion by dissection.
Roaring sometimes hereditary. That roaring runs in families there can be no doubt, but the direct cause appears to be mostly the transmission of a faulty conformation. A head with faulty shape and badly set on; a thick, short neck, deficient in mobility, or a small, narrow chest, predisposed to acute diseases, descends from parents to offspring, entailing a predisposition to roaring. The large Normandy horse is notoriously subject to roaring, but then he is equally characterized by a big, coarse head, narrow forehead and nostrils, big jowl, and narrow intermaxillary space. In all breeds this form is very subject to roaring, because of the stiffness of the neck and tendency to compression of the larynx. With the head badly set on, as it is almost of necessity in these animals, everything is done to produce roaring. Not only is the head cruelly reined in at work, but the horse is kept a great part of his time in the stable in the same or even in a worse condition, the larynx meanwhile unnaturally compressed between his narrow jaws and the nerve compressed or the larynx distorted.
It must be added, however, that like some other acquired distortions or alterations roaring may repeat itself in the progeny. Goodwin mentions an instance of it on the female side through three successive generations of thoroughbreds. Of transmission on the side of the male the following instance is noteworthy: M. Liphaert, an extensive proprietor in Livonia, bought a first-class English thoroughbred stallion. His progeny were healthy until he became a roarer at ten years old. All his foals, got after this date, followed the sire in becoming roarers, and, it is important to observe, almost all at the age of ten years.
Symptoms. These, of course, are manifest enough while the animal is sufficiently excited to give rise to the noise. Certain indications may be obtained even while the animal stands in the stable. If cough is excited by pinching the upper rings of the windpipe it is prolonged into a groan. If suddenly threatened with a cane the abrupt inspiration which results is attended by a grunt. The absence of these symptoms is not, however, sufficient to establish the nonexistence of roaring. The horse must be galloped or put to heavy draught to fully test the breathing organs. Galloping up a steep hill is perhaps the best test. A gallop over a recently ploughed field is about equally good. Soft pasture land or an unpaved road is preferable to Macadam or pavement. Galloping in a riding school on the soft tan is an excellent measure as the sound is confined and the animal is always within earshot of the examiner. The person examining should either ride the horse himself or have a disinterested party, in no way connected with either buyer or seller, to mount him. If the rider is in the interest of the seller he may contrive to slacken the pace before he reaches the examiner, or by irritating the horse may make it difficult to approach him immediately on his being pulled up. If in the interest of the buyer he may succeed, by the use of a powerful bit, in drawing the horse’s nose in to the chest, or by compressing the larynx with a tight throat latch he may produce noise in breathing when the animal is suddenly brought to a stand. Unless the course is up a steep hill or over a ploughed field the horse should be galloped for from five to ten minutes; he should be then made to pass close to the examiner at full speed, and finally brought up suddenly by his side and without any previous slacking of his pace. The ear should be at once placed close to the nostrils, when the slightest abnormal sound accompanying the inspiratory act will be at once recognized.
Draught horses are sufficiently tested by driving them in a heavy vehicle or one with the wheels dragged. By walking alongside or keeping the ear near to the nostrils any harsh sound additional to the normal blowing noise of hurried breathing is easily noted.
The finger placed on the larynx detects the strong vibratory tremor, and Friedberger notes that the left arytenoid is much more easily displaced than the right, increasing the stridor.
If the horse is, at the time of examination, the subject of a cold, sore throat, or other acute disease of the air passages no importance is to be attached to any noise made in breathing, but he cannot be pronounced a sound horse until, this malady having passed off, it is found on careful examination that no such sequel has been left.
Among the most puzzling cases are those in which the roaring occurs with periods of intermission. If the horse has been fed for a short time on vetches this may account for its temporary access, and unless the same feeding is again allowed a recurrence is not to be looked for. If due to the occasional displacement of a pedunculated tumor of the nose or pharynx and its interference with the action of the larynx its existence may be recognized by careful examination, diminished current of air through one nostril, etc. But there remain some rare cases in which there are no such appreciable causes, and yet the horse would be pronounced sound or unsound as examined at certain intervals. On this subject more information is desirable.
The following varieties of roaring will be distinguished from that of paralysis by the occurrence of the sound in both acts of breathing (expiration and inspiration):—distortions, tumors or foreign bodies in the nose:—tumors about the throat, in the windpipe or bronchi:—distortion of the windpipe, from tight reining, fracture or congenital deformity:—and the presence of a false membrane stretching across the windpipe.
Examination by manipulation, auscultation and percussion along the whole length of the air passages alike during rest and after exercise, may enable one in unusual cases to recognize the structural changes that give rise to roaring.