Navicular Bone.| Showing Small Ulcers invading its Structures.
Calks, and such means as are adopted to remove the frog from the ground, operate unfavourably on the feet, and especially where mutilation is part of the operation of shoeing.
Contraction of the foot not infrequently results where the hoof is allowed to grow unduly long and the frog is removed from the ground, or where, as the result of injury to some part of the limb, the foot is rested for a long period. Whether the result of the one cause or the other, it is a condition which, if not carefully rectified by shoeing, may excite navicular disease.
It is comparatively seldom that navicular disease is found to exist in the hind feet, and the preference which it shows for the fore ones may possibly be found in the difference which exists in the conformation of the limbs. The straight fore-leg allows the weight to fall directly on to the feet, while in the hind one it is first broken and diffused in passing through the angle forming the hock, so that by the time it reaches the foot the sharp edge of concussion is removed and the foot escapes the injury which is inflicted through the straighter column in front.
Prevalence of the Disease.—As to the horses which suffer from this affection, it may be somewhat difficult to say in which particular variety it is most prevalent. Between the light and heavy horses there is a great difference in favour of the latter, although since they have been called upon to do so much trotting work the malady has increased in the same proportion. There is a much greater number of cases among harness horses than any other description, but it must not be forgotten that they are the more numerous. And on this account we should look for more cases than are to be found in the other varieties.
Hunters are frequently found to be affected by navicular disease, although their work is for the most part over soft, yielding ground. These animals suffer most when made to jump from high banks into roads, especially when the muscles are tired and have lost much of their power. In these circumstances the full weight of the body falls upon the feet, with the result that the navicular bone may suffer by impact with the ground and become the seat of disease. But apart from these special accidents, hunters become subjects of the malady as the result of constant wear.
The racehorse, as such, is comparatively seldom the victim of this affection. So long as he is in training he is constantly on the turf, and at an early period is relegated to the stud, and ceases to be exposed to the causes by which the disease is excited. If, however, he does not himself contract the malady, his peculiar habits of life have a tendency to weaken the feet and to predispose his stock to contract the disease. In this way he becomes a factor in its propagation. His constant absence from the hard road does little to encourage the secretion of a thick, strong horn, and to impart to the feet that flinty hardness so much to be admired in a sire. “No foot, no horse,” is an axiom as true to-day as it was when first formulated by Lafosse. This is not said to prejudice the thoroughbred, for which the writer has a very high regard as a sire both of hunters and harness horses, but rather to hold out the caution to those who use him.
Symptoms.—There are few diseases in which the symptoms are so obscure and ill-defined in the earlier stages as they are in navicular disease.
The situation of the injured part forbids that inspection and manipulation which we may readily apply to other diseases, and there is too frequently no visible effects of the injury to guide the expert, but only an insidious and slowly progressive lameness.
It is more than probable that for some time the owner will be in doubt as to whether there is any defect at all. The only change observable to him is an uneasy sensation experienced when the horse is ridden in his fast paces. Soon, however, the defect becomes obvious in one leg or the other, and later in both.