These are among the most frequent and grave of all affections of the domestic animals. They are especially important however in the case of animals that depend on the soundness of their wind. In horses and dogs accordingly any permanent injury to the organs of respiration will seriously impair the value, not only because of the diminished usefulness of the affected animal, but also because of the probable deterioration of their progeny. The rapid paces demanded of these animals and the strain to which the respiratory organs are subject are potent causes of respiratory disorder. In all animals, however, the extent of the respiratory surface and its extreme delicacy and tenuity especially predispose it to disease. Hales estimates that the mucous membrane covering all the air sacs and air cells is, in the calf, no less than 250 square feet. As the chest of the horse is at least double that of the calf, and as it contains much less connective tissue, and is made up of minute air cells from ¹⁄₇₀–¹⁄₂₀₀ inch in diameter and separated from each other by walls so attenuated that the contained capillary bloodvessels are equally exposed to the air on both sides, in two adjacent air cells, the estimate for the average horse must be considerably above 500 square feet. This membrane, incomparably the most delicate and susceptible in the animal economy, is constantly in contact with the air in all its variable conditions, and is necessarily affected by these variations.
The severe changes of temperature are not without their influence on this sensitive membrane. If these changes are sudden, as for example in our northern states where the temperature will vary from 50° to 70° Fah., in a single day, the danger of injury becomes imminent, and the lungs require to be strong indeed to resist their effects. Sudden transition from the hot close atmosphere of the barn or stable to the chilling winds of winter is equally hurtful. But it is not alone the transition from warmth to cold that is injurious. The general relaxation attendant on the sudden change from a cold bracing atmosphere to one unduly hot is even more injurious. How frequently do human beings suffer from colds as the result of a close sultry period at once supervening on a clear cold one? How extensively do chest diseases prevail among horses brought from the clear pure atmosphere of the field, and shut up in close, hot stables? Here, no doubt, there is superadded the impurity of the too often infected air, the change of diet, of exercise and of general care yet we find that affections of the air passages are to a great extent in ratio with the heat of the building. Hence their constant presence in dealer’s stables where it is thought desirable to keep the horses warm to hasten the improvement in the coat.
The suddenness of the transition is usually a principal cause of injury. Where the climate changes slowly the animal economy becomes habituated to it and resists successfully the injurious influences. Thus when spring merges gradually into summer and autumn into winter, diseases of this kind are far less frequent. But on the other hand a sudden and extreme variation of temperature, whether in the ordinary course of the season or from a wide change of latitude, is notoriously attended with diseases of the air passages. Ayrshire, shorthorn and Jersey cattle, when first imported into the Northern States of America, contract colds, consumption and other chest diseases to a far greater extent than the native races, though their progeny or even they themselves after acclimatization, exhibit powers of resistance nearly equal to the native stock. Sheep that have been shorn in midwinter or early spring often repay the inhumanity of their owners by dying of inflamed lungs. Southdown and Leicester sheep, sent from England to the north of Scotland, demand at first the greatest care to protect them against the increased rigor of the climate. The army veterinary statistics of France show that horses transported from the southern parts of the country to the more northern stations, suffer largely from pulmonary affections. But if the change is effected slowly the requisite powers of resistance are acquired and the novel conditions of life cease to be injurious. That this varied power of resistance is not confined to the higher animals would appear from the experiments of W. Edwards on cold blooded animals. He subjected them in winter and in summer respectively to a very low temperature and found that whereas in summer their temperature declined 3° to 6° Cent., in winter they had a greater resistance and barely declined ⁴⁄₁₀ths of a degree.
The action of cold on the surface of the body often leads to morbid states of the air passages as the result of nervous sympathy. A beast is subjected to a keen cold wind, is attacked with shivering, and inflammation of the chest supervenes. The result is rendered more certain if the wind is associated with rain and if the animal has been previously in a state of perspiration. A heavy coat of hair, a profuse perspiration, and a cold draught often combines effectively to produce respiratory disease.
It must be added that the chilling debilitates the nuclei of the animal tissues, and lessens their power of resistance to noxious influences. The excess of cold in the freezing of a part, is followed by congestion and even violent inflammation with perhaps sloughing after it has been thawed. The persistence of such tissue debility is familiar to us all in the example of chillblains. A less extreme application of cold affects the tissues and nuclei less powerfully, but none the less surely. The increased liability to disease of the chilled system is strikingly illustrated in the experiment of Pasteur with anthrax. The chicken which had proved refractory to an ordinary dose of anthrax virus, was dipped in water at ordinary temperature until the heat of its body was reduced, and then it fell an easy victim to the anthrax bacillus. In the same way the person who recklessly exposes himself to wet and chill falls a ready victim to intermittent or yellow fever from which he would otherwise have escaped. Debility from another cause, such as bruise or laceration, favors deep-seated invasion by pus cocci, and a resulting abscess, from which the patient would have remained free, but for such traumatism.
But the effect of cold is not confined to the sympathy between the skin and respiratory mucous membrane, nor the revulsion of the blood toward internal organs, nor to the debilitating of the tissues. The application of cold constringes the vessels and lessens the freedom of the circulation and suppresses the normal cutaneous exhalation. A somewhat similar condition may be induced by prolonged exposure to the rays of a burning sun, the skin becomes hot, dry and rigid, and incompatible with the maintenance of the respiratory function. In either case there is a retention of effete and deleterious matters in the circulation which it was the function of the skin to have eliminated. The danger of such retention may be best exemplified by noting the result of the complete repression of perspiration in the remarkable experiments of Fourcault and Bouley. The former covered dogs and other small animals with an impermeable varnish which induced death after some days or in some cases in a few hours. Bouley shaved three horses and covered the skin with tar. There resulted dullness, torpor, deep, slow breathing, weak and diminishing pulse, muscular tremors, manifest cooling of the body and expired air, and deep violet color of the mucous membranes. They died respectively on the seventh, ninth and tenth days. A fourth horse covered with a layer of strong glue and then with tar perished nine hours after the application. The bodies were like those of animals that had died of suffocation. The mucous membrane of the stomach and bowels was gorged with black blood, the lungs violently congested—dark red and heavy—the air tubes filled with frothy material, and the lining membrane of the heart had dark spots of blood extravasation. It is no longer then matter for surprise that temporary suppression of the insensible perspiration should be followed by diseases of the chest or abdomen, that extensive burns of the surface of the body should be speedily followed by inflammations of internal organs or that extensive and severe cutaneous inflammations should be associated with internal lesions.
Since the days of Hippocrates it has been universally acknowledged that moist seasons and localities are less salubrious than dry ones. As already observed moisture in a cold atmosphere intensifies its effect. In a hot, close atmosphere it strongly conduces to putrefaction in dead organic matter, and the air becomes loaded as a consequence with noxious gases, and in its lower strata with bacteria in a state of active growth. This condition is most intense in close, unventilated stables, and manifestly operates in both predisposing to and exciting those diseases of the chest and other parts, so frequent in such places. Winds raise and carry such germs, but also sooner rob them of virulence. (See Zymotic Diseases). Susceptible, young animals, newly housed, usually suffer the most severely from these injurious conditions. Often in their case frequent, extreme and sudden changes, and great atmospheric impurity, are combined with a diet to which they have been hitherto altogether unaccustomed. In young horses there are superadded the exertions—too often extreme—connected with training or work. There are the heats and chills, the soaking perspiration and the frigid winds and rain, the general exhaustion, but particularly the overwork of the respiratory organs, each of itself calculated to superinduce disease. Percivall justly remarks that among young horses, newly stabled and put to work, the prevailing diseases are “catarrh, sore throat, strangles, bronchitis, pneumonia and pleurisy.” His tables of the diseases attacking the horses of his own regiment (1st Life Guards), are so instructive that I here reproduce them:
| A TABLE (COMPILED FROM EXTRACTS FROM A “REGISTER OF SICK HORSES” LIMITED TO A GIVEN PERIOD) SHOWING THE COMPARATIVE AGES AT WHICH HORSES APPEAR MOST DISPOSED TO CERTAIN ORGANIC DISEASES. | ||||||
|---|---|---|---|---|---|---|
| No. of Patients Under 5 Years. | No. in Their 5th Year. | No. above 5 Years and Under 10. | No. 10 Years and Upwards but under 20. | No. 20 Years and Upwards. | Total. | |
| Disease of the lungs | 170 | 50 | 20 | 50 | 10 | 300 |
| Disease of the bowels | 10 | 20 | 40 | 70 | 20 | 160 |
| Disease of the brain | 4 | 2 | 5 | 14 | 2 | 27 |
| Disease of the eyes | 30 | 10 | 70 | 35 | 5 | 150 |
It will be seen that nearly one-half of the sicknesses, occurring among the horses of the regiment, were chest diseases, and that nearly three-fourths of these were in animals under five years old, or in those newly purchased from the country.
The subjoined table shows the relative prevalence of disease in different months of the year, deduced from the Register above referred to: