HEREDITARY UNSOUNDNESS IN HORSES
The question of hereditary unsoundness in all its aspects is by no means an open book to the horse-breeder, but it is one well worth studying by him, if as thorough a knowledge as is possible of this subject will tend to lessen the percentage of unsound stock produced. Of the various causes which tend to make horse-raising disappointing and unprofitable, this is one of the important ones, if not the most important. Literally speaking, it is not so much the inheritance of a disease itself that we fear, but it is the tendency to the development of this disease. Comparatively few foals are dropped with unsoundnesses, that impair their value ultimately, but there are many that are foaled with a predisposition to the development of unsoundness. The study of this subject, then, practically resolves itself into a consideration of what constitutes the predisposing causes of unsoundness.
They may be divided into several heads, viz: Defective formation, defective quality, and insufficient quantity of tissue and temperament. Within certain limits, the practised eye of the observant horseman can determine in sire or dam the existence of these defects that are transmissible to the offspring, and predispose it to the development of unsoundness.
It is claimed by some authorities that some horses and mares possess a peculiar habit of body, an indefinable something about them which predisposes them and also their progeny to the development of some unsoundness. Such cases are rare, however, and their supposed existence is very frequently the result of the inability of an observer to appreciate the existence of detectible predisposing causes. If this indefinable something is the determining cause of predisposition in some cases, then the only positive evidence of its existence is the developed unsoundness. This theory presupposes that none of the detectible predisposing causes already mentioned exist in sufficient degree in such cases, so that when subjects of them are subjected to more than ordinarily exciting ones they would develop unsoundness unless the peculiar habit of body exists. It is also an acknowledgment of the helplessness of breeders in a considerable degree to prevent breeding colts with an inherent tendency to unsoundness. No matter how capable and careful a breeder is he will produce a certain percentage of unsound stock, due to heredity, but with care it can be reduced to a small one. Many breeders use a sire from convenience or from some quality he possesses, such as speed, action, style or disposition, knowing they are taking a chance of perpetuating some tendency to unsoundness that he possesses. Favorite mares are also bred, with a disregard to the well-established principle that “like begets like,” even when they are the victims of hereditary unsoundness or a strong predisposition to it. The successful breeder must divest himself of all sentiment and be capable of appreciating all defects which constitute predisposition to unsoundness. Of the predisposing causes, defective formation is the most fertile one. The predisposition to navicular disease is hereditary. Horses with narrow, deep heels are predisposed to it. Some observers may say: “But look at the large number of horses one sees with narrow, deep heels that have not got navicular disease.” Of course, such an argument is fallacious, as there may be many compensating conditions that will tend to neutralize the tendency to this disease in some subjects. A horse with the formation of foot described, even although he is subjected to the exciting causes of hard, fast and steady work, and irrational and infrequent shoeing, may be endowed by nature with a very light step. Nothing tends to prevent “wear and tear” of the legs and feet like light stepping. Here, then, is an example of an influence which tends to counterbalance the ill effects of a defective formation, but renders complicated the study of formation as a predisposing cause of unsoundness. One, then, has to weigh the influence of compensating conditions in determining the ill-consequences likely to result from defective formations of feet when transmitted to offspring. That defective formation of feet is handed down to progeny there is no manner of doubt. The predisposition to ringbone is undoubtedly hereditary, and when the pastern is of good formation one seldom finds it unless it is the result of some extraordinarily exciting cause. The two extremes of long, light oblique pasterns, and the short, straight coarse ones, are both predisposed, the former from the tension to which the ligaments are subjected, and consequent tendency to sprain, and the latter from the increased tendency to concussion. As we proceed up the front leg we find defective formation in the neighborhood of the knee, predisposing to unsoundness. Many horses that are more or less knee-sprung are practically sound. Others become progressively weak and are decidedly unsound; so that we are not much amiss in characterizing the condition called knee-sprung as an hereditary unsoundness. A commission was appointed some years ago in England to make out a list of hereditary unsoundnesses. The list included navicular disease, ringbone, spavin, sidebone, periodic ophthalmia or moonblindness, and roaring. It will be observed that knee-sprung was not included. In order not to complicate matters they made the list as short as possible, and only included the most serious forms of unsoundness or those that are very obstinate in yielding to treatment or are incurable. Unsoundnesses such as knee-sprung, curb or splint, though the tendency to them is undoubtedly hereditary, were not included. This was because these conditions seldom permanently interfere with a horse’s practical soundness. A horse is what is called “tied in” below the knee when the leg immediately below that joint is narrower from before backwards, than it is just above the fetlock. In this condition the tendons behind the knee are not well developed, and are placed too close to the shank-bone at this point. This imperfect tendonous development in a front leg is not usually confined to the tendons behind the leg, called the flexor tendons, as an imperfect development of these tendons is usually associated with an impaired development of the tendons running down the front of the leg called the extensor tendons. Such a condition frequently results in shaky knees, and if the subject experiences hard work, he is likely to become progressively worse. In some cases the flexor tendons appear to be much better developed than the extensors, and in an individual so formed, particularly if he is inclined to stand with his forefeet well back under him, there is an inordinate strain on the extensors, leading to relaxation and a corresponding tendency to contraction of the flexors. Here we have a lack of balance between the extensors and flexors, and the result is a knee-sprung condition. Although shaky-kneed or knee-sprung horses are not frequently incapacitated for work, the breeder should not lose sight of the fact that this condition is apt to considerably depreciate a horse’s value in the market, particularly for some purposes, and he can seldom afford to ignore this fact. It is not intended to advise breeders never to breed to a sire that stands a little over in the knees, especially if sufficient cause can be assigned for it, but we should recommend them to be very wary about using sires and dams that show a congenital tendency to this defect. Purchasers of horses usually look with considerable disfavor on a horse that is at all knee-sprung. Personally, the writer would rather buy a horse for his own use that is a little forward in the knees than one that stands back in them or is what is called calf-kneed. A horse with the former defect is almost sure to be much more elastic in his step than one with the latter and, consequently, will not suffer to the same extent from the ill-effects of concussion. A calf-kneed horse is also much more likely to suffer from strains. Knee-sprung, unlike the other unsoundnesses given in the list of hereditary troubles, is not very infrequently congenital. Swan-necked horses and those with thick, coarse throttles are considered to be of the formations most liable to develop the defect of the wind called “roaring.” In Great Britain and Ireland and on the continent of Europe, breeders are usually very particular about avoiding roarers for breeding purposes. The climatic conditions there seem favorable to its development; but in this country it is not nearly so much to be feared, and one should not hold aloof from an otherwise desirable sire on account of his being a roarer, though it must be admitted that the predisposition to the trouble may be transmitted. The predisposition to periodic ophthalmia or moonblindness is handed down from parent to offspring, but it is not nearly so common in this country as it once was, when the sanitary conditions were not so good; and it may be that some care has been exercised in breeding, so as to avoid its propagation. The only appreciable evidence of a predisposition to this unsoundness, outside of the existence of the disease, is the small or what is called “pig eye.” The writer knew a sire well that had “pig eyes” but sound ones, and they remained sound throughout his long life, but certainly ten per cent. of his progeny developed moonblindness.
The tendency to string-halt is undoubtedly transmissible from parent to offspring. Horses with snappy hock action are most likely to develop it. Nowadays it is not feared nearly so much as it was formerly, for if it should develop, a very large percentage of cases are curable by a not very difficult and by no means dangerous operation, which consists in the cutting of a tendon. A change has taken place in the generally accepted view as to the nature of string-halt. It was considered a purely nervous malady until it was found that the cutting of a tendon would in many instances cure it.
Curb, although not, as I remember it, included in the list of hereditary unsoundnesses, is decidedly hereditary, as well as the predisposition to it. It is not an infrequent occurrence to find foals “dropped” with curbs which frequently disappear in a large measure. But there is always some trace of them remaining. It is the rarest possible occurrence to find a broad hock, and one with the tendon standing well out behind, with any sign of curb. Narrow hocks and those with the point dipped forward towards the body of the joint; a leg with small circumference immediately below the hock, or what is commonly called tied in; those with the back tendon not standing out posterior to the bone at the back and outer part of hock—in addition to these indications of weakness and tendency to curb, if the joint is crooked, or what is called sickle-shaped, the predisposition is much increased. Curb is not so very much feared by some breeders, as it seldom causes permanent lameness. It is, however, a great eyesore; it depreciates a horse’s value very considerably, and it is liable to cause recurring lameness.
Crooked hocks, unduly straight hocks, narrow hocks, small hocks, those bent inwards or outwards, are all of weak formation, and are consequently predisposed, amongst other unsoundnesses, to bone spavin. Even though a horse should happen to have a bone spavin, provided he has a strongly formed hock, it would be safer to take a chance of breeding to him than one with a sound hock but of weak formation. So far we have run over briefly most of the defective formations that predispose to serious hereditary unsoundness. We have divided the causes into four heads, the second one of which was “insufficient quantity.” The old saying, “Size is strength, others things being equal,” applies to a horse’s extremities. We frequently hear it said that such and such a horse has “plenty of timber under him.” By this is meant that the individual in question has sufficient substance in the various structures that make up his legs and feet to give them strength and ability to stand “wear and tear.”
The practical horseman of experience learns that the horse with disproportionately small feet seldom stands much work without going sore from some unsoundness of these organs. So with the slender-pasterned horse. He is not only subject to strain at that point, but predisposed to ringbone. The horse light under the knee is apt to suffer from strains of the tendons and ligaments in that situation, as well as troublesome splints. Proportionately large joints give wearing ability to the legs. This is well exemplified in the case of hocks with plenty of tissue in them. The sire that transmits small hocks to his offspring has handed down to him one of the most prolific sources of unsoundness in these joints. Defects of formation of these joints are often a cause of trouble, as has already been pointed out, but not so much so in my experience as lack of size. Stating that a horse has plenty of timber under him does not cover all cases, as some horses have plenty of tissue in their front legs and are deficient in their hind ones. In addition to formation and quantity of tissue, “quality” is of vast importance in influencing the wearing ability of the legs and feet. Parents transmit with great faithfulness to their progeny defects in the quality of the horn of hoofs. Shelly, brittle hoofs are strongly predisposed to crack, developing sand and quarter cracks on slight provocation, and giving rise to that very troublesome inability of being unable to hold the shoes tightly. Brittle hoofs are not necessarily coarse in fibre. Hoofs of coarse fibre lack the density of structure which generally contributes to toughness. Undue size of foot, low heels and flat soles, with a tendency to be easily bruised, are apt to be associated with a lack of quality in the horn structure. A horse with bone of a spongy character or lacking in density is deficient in quality. Such an individual is predisposed to inflammatory diseases of bone, such as splints, sore shins, ringbone and spavin. The lack of quality in a horse is particularly well shown in the skin of his legs. The tendency to the development of cracked heels, stocked legs, mud fever and grease is very evident on slight provocation. Sires deficient in quality are apt to transmit to their progeny the tendency to what are called soft-legs, in which there is not only the inclination of the skin to swell up from little cause, but windgalls, puffy sheaths of tendons and boggy hocks are easily induced. If, then, we accept these statements with regard to quality, as it would appear that every practical horseman must, we must admit that coarseness or lack of quality is by no means an unimportant factor in contributing to hereditary predisposition to unsoundness. Temperament is the last of the four heads into which we divided the predisposing causes of hereditary unsoundness. Although it must be admitted that it has an influence, we look upon it as the least important of the four. The nervous horse that jumps and gets excited on slight provocation; the anxious horse that is always up in his collar and against the bit, are more taxing on the physical mechanism than easier-going horses. We cannot afford to quite ignore this question of temperament in selecting sires and dams, but if the legs and feet are well formed, have sufficient substance, and are made up of a good quality of tissue, they will generally stand any taxing that may result from a high-strung temperament.