We have seen, in speaking of roaring and whistling, that the muscles which regulate the size of the opening into the larynx are liable to become thrown out of use as the result of paralysis. In the disease now to be noticed the same narrowing of the orifice takes place, but in an entirely different manner. In the one case it is the result of a paralysed state of a certain set of muscles (abductors) which open the larynx, while in the other it arises out of abnormal action or a spasmodic contraction of another set (adductors), whose natural purpose it is to close it.
Spasm of the larynx may result from any sudden and violent irritation, and from milder stimulation when the larynx is in an irritable condition. The accidental entrance of food or other foreign agent will sometimes bring it about, and especially when the sensibility of the organ is heightened in the course of an attack of inflammatory irritation. Some of the most severe and dangerous cases, however, arise in the entire absence of any such condition, and from the manner in which they come and go it is clear that they have a nervous origin; but whether the actual seat of the disorder be in the brain or in the nerves of the larynx themselves cannot well be determined.
It is seldom that any premonitory indications mark the oncoming of the disease. Horses who suffer from it usually enjoy good general health, and are mostly in fair condition. The attack almost invariably comes on suddenly, and, with few exceptions, while trotting or galloping. While moving along, the horse is noticed to come suddenly to a stand-still. The head is raised and moved from side to side, the face is pinched and wears an anxious expression, the nostrils are dilated, and after a brief arrest in the breathing a loud shrill noise is heard with each inspiration. In severe cases the horse breaks out in patchy sweat, the gait becomes unsteady, the breathing gasping and troubled, and the body is generally convulsed.
The paroxysm may quickly pass away only to return when the animal is caused to move, or it may be renewed from time to time at varying intervals for several hours, when it disappears, leaving the patient but little the worse for his trouble.
At the outset of the seizure the horse should be brought to a stand-still at once and allowed to remain in a state of quiet until the paroxysm abates, when he may be removed to the nearest stable. The body should now be well rubbed down and plenty of fresh air provided for. A draught, consisting of 2 ounces of tincture of opium in water, or, what is better, a subcutaneous injection of morphia, is to be administered and repeated in two hours if the symptoms do not abate. Hot bandages to the throat after an application of glycerine and belladonna will be found useful, with inhalations of warm vapour, and a dose of physic should be administered when the spasm has disappeared.
An animal having suffered one attack is frequently the victim of a second, and sometimes a succession of seizures may follow the first at varying intervals of days, weeks, or months. Where this is the case the greatest care should be observed to guard against overfeeding, severe exertion, and, as far as possible, against the inhalation of dust and irritating gases, and to avoid exposure to such sudden and extreme changes of temperature as are met with in passing from hot, foul stables into the cold air.