Animal Breeding Heredity of Diseases
Animal Breeding Heredity of Diseases That certain forms of diseases are transmissible does not for one moment admit of question. That all forms of disease are transmissible is not true. It is equally certain that in many instances disease may be present, or the predisposing influences that lead thereto, in a form so subtle as to escape notice, and that when thus present it may be transmitted directly, or the tendency to it only may be transmitted, requiring only certain conditions to develop into the active form.
This question, then, is one of the great practical moments to the breeder of livestock and should receive at his hands the most careful consideration.
Heredity of Disease Defined. —By the heredity of diseases is meant the transmission to the progeny of certain abnormal conditions of the system which characterized the parents. It has already been shown in Chapter VI. that all abnormal conditions do not constitute disease, and that while all disease is ab- normal, all that is abnormal is not disease. It should also be remembered that only certain kinds of disease are transmissible.
While it may not be easy in all instances to distinguish between diseases that are transmissible and those that are not, it would be correct to say that all diseases of that class known as constitutional are transmissible. Those of a tuberculous character are by far the most numerous.
Many contagious and infectious diseases are not only not transmissible, but their Laving been once borne by the individual would, in some instances, appear to render the progeny less susceptible to the disease. Such it has been claimed is true of hog cholera, although authorities are not agreed as to this question.
Heredity of Diseases Structural and Functional. —As with the heredity of abnormal characters which do not constitute diseases, such inheritance may relate to a modification of structure or to a derangement of function. When such heredity relates to the modification of structure it is seldom or never questioned, since the evidence of its presence are so apparent to the eye, but when it relates to derangement of function, it is more liable to be overlooked.
For instance, when ringbone has been present in one or both parents and again appears in the offspring, the inheritance of ringbone by the latter from the former is not questioned, since the evidence of it is so apparent to the eye.
But suppose that the udder of a dam is tuberculous, the presence of the same, for a time at least, may not be patent to the eye. Should the cow so affected beget progeny it will inherit the tendency to tuberculosis, and should she suckle the same, the progeny will be almost certain to contract tuberculosis from the dam.
Now suppose such inheritance is exactly similar in kind, function in the udder of the progeny will be deranged, and yet it may not be possible to be absolutely assured of such derangement, at least for a time, save through the process of a postmortem on the cow, and even then, the functional de- arrangement would be accompanied by structural derangement.
It would seem to be true, therefore, that all inherited disease is accompanied by structural modification or derangement and that this may be so even when only the indications pointing to disease are present. It would also seem to be true that any peculiarity of the functional activity of an organ if long continued, is likely to result in a habit of the system which will be inherited by the offspring*.
Hereditary Disease Congenital or of Latent Transmission.— Hereditary disease is either con- genital or there is a predisposition to it. It is con- genital when it is apparent at birth. A brood mare may have certain joints greatly enlarged. If, in her offspring, the same is apparent at birth, it illustrates congenital transmission.
The same is true of goiter in lambs when it is present at birth. Of course, it makes no difference how the disease came to be trans- mitted, whether through the immediate ancestors or those more remote, it is congenital when the indications of it are present at birth. The predisposition to disease is inherited when the tendency only is transmitted but does not actually constitute disease until some exciting cause develops it at a later period.
The tendency to tuberculosis may be transmitted. This tendency for a time may be latent. The health-giving influences surrounding the animal may be such that for a time no evidence of tuberculosis is apparent. Later, however, the surroundings become unfavorable, and the disease appears.
Again, it may not appear until the next generation or even for a longer period, but finally develops. In such instances the conclusion is fair, that the predisposition to the disease was transmitted all the while. It is evident, therefore, that there is always more or less of a hazard in breeding from animals that are tainted with disease, though to the eye they may seem to be in perfect health.
The tuberculin test may single out animals that are affected with tuberculosis. It may be that to the eye they are in perfect vigor. Possibly they are pure bred and rich in blood lines of famous ancestral descent. While the owner consents to quarantine them he continues to rear progeny from them.
In such instances the fact should not be lost sight of, that the predisposition to tuberculosis has probably been transmitted. There is the possibility, however, that through wise management the predisposition thus transmitted may eventually be eliminated. On the other hand, the predisposition to certain forms of disease may become hereditary. The cattle beast with a narrow chest falls easy prey to the influences which produce tuberculosis. Breeding such animals in direct descent for generations ensures the heredity of a predisposition to tuberculous diseases.
Diseases that are Hereditary. —While various forms of disease are transmissible, those of a tuberculous “character are peculiarly so. To so great an extent is this true, that a close examination of the question would show that the instances of transmission of diseases, tuberculous in character, would outnumber those of the transmission of all other forms of disease combined. And this is true of domestic animals to a greater extent than it is true of the human family.
Tuberculous diseases are characterized by the formation of tubercles in various organs of the body and by a perversion of the nutritive functions. These tubercles are by no means uniform in their location, even with the same form of disease. For instance, in tuberculosis, the tubercles may be found in one instance in the lungs, in another in the bowels, and in a third instance, in the udder of females.
In aggravated cases all these organs and indeed various other organs are affected. The most common forms of tuberculosis include consumption, diarrhea, dysentery, mesenteric disease, hydrocephalus, and glandular swellings.
Tuberculous Diseases Frequent Among Domestic Animals. — Tuberculous diseases are frequent but not equally so among horses, cattle, sheep, and swine. Cattle are much more likely to be affected with such ailments than horses, sheep, and swine. Horses are more frequently affected than sheep, and the trouble in horses is much prone to assume the form of swollen and otherwise diseased joints and limbs. Tuberculosis may not have developed, and yet there may be a predisposition to it, the indications of which are manifest.
These indications are various. They include the following, viz. :
- A thin carcass and lacking in depth, a narrow chest and loin, flat ribs, large barrel depression and hollow flanks.
- Extreme thinness and fineness of the head, neck, and withers, want of fullness in the eyes, hollowness behind the ears, un- due fullness under the jaws, and a small and narrow muzzle.
- Much prominence of the bones in certain parts as at the joints, and a coarse and ungainly appearance. And
4, a hard, unyielding skin, thin and dry hair, and irregularity in changing the coat.
A thin carcass, of course, means one lacking in width throughout its entire length. Narrowness of chest, flatness of rib and smallness of muzzle are all associated with circumscribed respiration and low vitality. Want of width and depth in body are associated with a lack of digestive capacity. The low vitality and the lack of digestive capacity account for the lack of fullness in the eye, behind the ear and in the Hanks. They are the outcome of weak nutrition. which in turn is the outcome of the causes named.
The undue length of the limbs in such instances is probably a result of the law of correlation discussed in Chapter VIII. The undue prominence of the joints arises from a perverted nutrition. The harsh, unyielding skin, and the characteristics of coat mentioned are the outcome of a feeble circulation which in turn grows out of a feeble digestion. Animals thus formed fall an easy prey to tuberculous diseases, hence, to breed from them would be very unwise.
Variations in the Inheritance of Diseases. Diseases. —The inheritance of the so-called constitutional diseases varies in many instances in the organs affected. In these there are sometimes alternations in the trans- mission, which is to say, the parents may have diseased lungs, and the transmission in the next generation will manifest itself in the form of tumors or glandular swellings, while in the following generation it may again assume the form of lung disease in one or the other of its forms.
But it is also true that the location of the disease may depend in many instances on the method of infection, that is to say, whether through the digestive organs or through the lungs. The conditions to which the animals are subjected may, in part, account for such alternations in trans- mission, but they do not furnish the explanation of all the instances of such variable transmission. The injudicious treatment of animals predisposed to such diseases may also aid in determining the particular organ that will be affected. For instance, the injudicious use of a violent cathartic may locate the inherited tendency to chronic diarrhea.
Cause of Tuberculosis. Tuberculosis. —Tuberculosis is one of the most common forms in which disease appears in domestic animals. Because of the extent to which it prevails, especially among cattle, it has been thought that greater loss arises from this source than from all the other forms of disease combined. It should, therefore, not be out of place to give some special consideration to this question. The direct cause of tuberculosis is a rod-like microscopic parasite which may, in various ways, be transferred from one animal to another. In the congenital form the disease changes may be in process of development, or they may be developed.
The germs may be conveyed in the mother’s milk or in the nasal or bowel discharges. They may also be inhaled in the atmosphere of surroundings where tuberculous cattle have recently been kept. There is no way, however, in which the contagion will more certainly be conveyed than in the milk of the dams whose udders are tuberculous.
It does not happen with much frequency that the disease is trans- mitted in the congenital form, but the predisposition to it is probably invariably transmitted by tuberculous parents.
Predisposing Causes of Tuberculosis. —There are several predisposing causes of tuberculosis aside from inheritance. These include disorder of the digestive organs, food deficient in quantity and quality, impure water, confinement in dark, damp, filthy, unventilated apartments, and undue exposure to cold or to any other influence that lowers the action of the vital powers. The extent to which cattle have been confined in damp, dark, and ill-ventilated stables is responsible for the great extent to which tuberculosis prevails more than any other single external cause.
Cattle reared on the ranges are but little subject to tuberculosis, notwithstanding that in many instances they are frequently subjected to privation because of short supplies of food. This fact should be carefully considered by those who re- quire to keep cattle housed much of the time in winter. It emphasizes the necessity of supplying them with ample fresh air in the stables and with sufficient exercise.
Continued in-and-in breeding, or even protracted close breeding and breeding from immature or enfeebled parents are also responsible for much of the tuberculosis that prevails. Of this fact there is evidence of the greater extent to which tuberculosis has prevailed in the families of Shorthorns and Jerseys that have been thus bred, than in other families of these respective breeds.
It is also evidenced in the less extent to which the disease prevails in semi mountain breeds, such as the West Highland, which have been subjected to less artificial conditions than the breeds just named. But the conclusion must not be reached that artificial conditions of necessity conduce to the increase of tuberculosis.
It is only when these conditions are made such as to lower the vitality of the animals that they foster tuberculosis. Of course, when the predisposition to tuberculosis is inherited, the conditions named become intensified in their action. Similarly, when the predisposition to any form of disease is inherited, the action of the exciting causes becomes intensified.
Inherited Predisposition to Disease from One or Both Parents —The inherited predisposition to dis- ease may be derived from either parent or from both. When such predisposition is derived from both parents it becomes intensified.
It would follow, therefore, that the hazard of transmitting the predisposition to tuberculosis in the progeny will be greatly increased when both parents are thus affected. To lessen this danger herds should certainly be, at least occasionally, subjected to the tuberculin test as otherwise there can be no certain assurance that such a mistake will not be made.
There are some instances in which the limitation of disease is confined to one sex and transmitted by the other, as, for instance, when the inheritance of skin diseases is manifest in the male descendants only, although the females transmit the same with more or less of regularity to their male progeny.
Many instances of such transmission have been noted in the human family. Such transmission also occurs with frequency among domestic animals, although with them it has been les< noticed. This would seem to be akin to similar. transmission of abnormal qualities already noticed.
Suspension in the Transmission of Disease. Disease. —In the transmission of hereditary disease there may be suspension for a time as well as alternation, which is to say, the disease may not be transmitted for a time and may again reappear. The suspension may continue for several generations. This may be due to the absence of exciting causes, or it may be owing to favorable sanitary conditions, followed by those less sanitary or indeed of an entirely opposite character.
Such transmission is atavic in its nature and is subject” to the same laws that control atavic transmission. It is also true that alternations of development are frequent in the transmission of hereditary diseases. So frequent are those alternations such transmission would, in many instances, seem to be the rule rather than the exception.
These alternations in transmission, especially in the human. family, may relate, first, to alternation in generations, and second, to alternation in the individuals affected in the same family. For instance, certain diseases will sometimes be entirely absent in the first generation but will reappear in the second.
They will be absent in the third generation and reappear in the fourth. Again, in individual families, the first born uray be free from the taint, and the second may inherit it, the third is free and the fourth affected, and thus the transmission descends through the entire family. And yet again, the alternation may relate to sex the inherited dis- ease will manifest itself only in one sex.
In still other instances, the disease will be confined to one sex and transmitted only by or through the other, as previously pointed out. These alternations in transmission would appear to be under the control of influences which, though not understood, are regular in their action. They would seem to be the outcome of two antagonistic characters which alternate in dominating power, but why they should act thus is yet a veiled secret.
Disease may be Transmitted Potentially. Potentially. —Disease still future in the parent or the tendencies to it, though undeveloped, may be transmitted potentially to the offspring. Such instances are of frequent in the transmission of cancers. The tendency to these in the human family is probably trans- mitted in all instances before the disease has appeared in the parent. Nor does it follow that the tendency to disease in the parent thus transmitted potentially to the offspring Avill ever become operative, that is, it may never develop into actual disease in the parent.
In breeding horses instances of such transmission are not infrequent. Parents notorious for the development of swollen limbs, or ringbones in some instances beget progeny in which these ailments do not appear until subsequently to the birth of the latter, and in some instances, they do not appear at all. Such trans- mission is akin to transmission characterized by suspension, nevertheless they are not the same.
The strength of the predisposing conditions acting in conjunction with other conditions, particularly those of an external character, have an all-powerful influence in determining the exact character of the heredity.
Predisposition to Disease through Faulty Conformation. — Animals free from constitutional taint transmit indirectly to their offspring a predisposition to certain forms of disease through the faulty conformation of certain physical features of development.
1, A disproportion in the width and strength of the leg above the hock to the width and strength of the same below this part in the parent horse, pre- disposes to spavin in the offspring. And this predisposition may lead to the development of the disease even when spavin or the tendency to it has not been transmitted directly from the parents.
2, In draft horses round limbs containing an unusual proportion of cellular tissue predispose the offspring to such dis- eases as weed and grease.
3, Chests that are narrow, pasterns that are’ upright and toes that are turned out beyond a certain limit, beget a tendency in the off- spring to bone diseases of the foot, as, for instance, ringbone. Such instances could easily be multiplied.
The same principle has been referred to when treating the above indications of tuberculosis. Because of such danger, every care should be taken when selecting breeding animals to avoid selecting those that are thus constituted. In the choice of these, as much importance should be placed upon freedom from the taint of inherited disease as upon the absence of the various features that indicate inferior physical development.
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