Roaring, popularly known as a disease, is only a symptom of disease in horses. It consists in a more or less loud unnatural sound emitted during the act of inspiration. As a rule it is first manifested by an animal making a slight noise, but this slowly increases in loudness and intensity, and in many cases the animal becomes useless whilst still comparatively young. Whistling is a modification of roaring, and is due to similar causes. The disease is found to be due, in the great majority of cases, to a wasting, atrophy, and fatty degeneration of the muscles of the larynx, but more particularly of those of the left side. This is partly at least explained by the fact that the nerve supplying the motor power to the left side is given off deep within the chest, winding round the posterior aorta, whereas that on the right is given off opposite the first rib, just at the entrance into the chest, and that the left nerve is more apt to be implicated in diseases of the organs within the chest. Still this theory is not quite satisfactory, as the same anatomical arrangement is found in other animals, yet roaring from muscular atrophy is not known among them, and many 'roarers' whose history has been known from birth have never suffered from chest affections, whilst others severely affected with chest disease have not become roarers. Again, mares and ponies are not nearly so prone to become roarers as males and larger horses.
The development of roaring is often due to catarrh, strangles, or some other disease affecting the respiratory organs; but it is generally concluded that these diseases are not sufficient of themselves to cause it, provided there be no hereditary taint, this hereditary taint alone being sufficient in many instances to induce roaring without the advent of another disease. There is no cure for it, all attempts made in this direction having hitherto proved abortive. In 1887 an operation for the cure of roaring was reintroduced by Dr Fleming, then principal veterinary surgeon to Her Majesty's forces. Similar operations had been performed by Günther, in Hanover, so far back as 1834. It consists in making a long incision into the larynx, the animal being under chloroform, and removing the arytenoid cartilage and vocal chord of the paralysed side. Some horses were slightly benefited, but many became worse than before the operation, and had to be destroyed. This proved a great disappointment to the veterinary profession, as hopes had been held out that at last a cure for roaring had been discovered.
Roaring is now included by the Royal Agricultural Society of England among the hereditary unsoundnesses, and their veterinary officers are instructed to disqualify all horses exhibited at the great national show that give any signs of this grave hereditary disease. See works by George Fleming (1889) and T. J. Cadiot (trans. 1893).