By this term is understood a loosening of the union between the bones forming the shoulder-joint, in consequence of which the head of the humerus or arm-bone is outwardly displaced from the shallow cavity into which it fits on the lower end of the scapula or blade-bone. This disease is invariably associated with paralysis of the muscles of the shoulder, which sooner or later waste, and by ceasing to give support to the joint allow the bones to become displaced, and the limb as an organ of support to be weakened and deformed. It is more especially seen in young horses engaged in agricultural work, although adults are by no means exempt from it.
The precise origin of the disease is but imperfectly understood. That the muscles of the shoulder are paralysed is clear enough, but as to how that paralysis is brought about there is considerable diversity of opinion. By some it is regarded as the result of an injury to the suprascapular nerve by blows, and over-stretching in the performance of certain backward movements of the shoulder and limb, involving great strain. By others it is said to arise out of inflammation of the muscles of the shoulder, induced by the stretching they undergo when slipping in and out of the furrow in the act of ploughing, and in habitually walking on an irregular surface. Of the two alleged causes our view is in favour of the former. We cannot understand shoulder-slip being of such rare occurrence in face of the very large number of young horses which are every day at plough, following the occupation which is said to induce it.
These vary somewhat in different cases as far as refers to the onset of the disease. In some there are no clearly-defined local symptoms until the muscles have become noticeably atrophied, and as they waste the shoulder is seen to shrink, leaving the spine in the centre of the blade-bone standing out prominently as a sharp projecting ridge with a hollow on either side. At the same time the point of the shoulder bulges outward, and the foot of the affected limb is drawn inward towards the other one, so that the leg, instead of following a straight line from the shoulder to the ground, takes a slanting direction inwards. In respect to locomotion, the main features of these cases are a disposition to trail the toe, inability to bring the limb well forward, and the outward movement which it describes when an attempt is made to do so. When the weight of the body is placed upon it, the shoulder is forced outward, and there is a perceptible descent of the quarter on the affected side.
These symptoms may be preceded by more or less heat and swelling in the region of the joint, which subsides as the disease progresses and the muscles waste.
Rest from work in a straw yard or paddock is the first requirement to be complied with, but in no case should the animal be confined to the stable. Movement is most desirable as a means of exciting nutrition and restoring wasted muscles, so long as it is not carried beyond mere exercise. The frequent application of stimulating embrocations over the affected region, with vigorous rubbing, is also recommended to the same end.
With regard to general treatment, nux vomica and sulphate of iron in two-drachm doses will benefit the local ailment by improving the general health and toning up the wasting muscles. To this must be added good food in liberal rations.