In dealing with the predisposing causes of disease, certain types of temperament are defined. The sanguine temperament refers to animals of a lively disposition with active circulation and quick movement, a condition of system which produces a disposition to inflammatory diseases. The lymphatic temperament, sometimes described as phlegmatic, is exactly the reverse of the sanguine; it is associated with a feeble circulation, a deficiency of red particles in the blood, pallor of the mucous membranes, and coldness of the skin, especially in the extremities, and it predisposes the individual to chronic diseases of a low type. The bilious temperament is probably always connected with a want of activity of the liver and other parts of the digestive organs, which tends to depress the vitality. A nervous temperament is indicated by excitement alternating with depression, both conditions resulting in a predisposition to what are called nervous affections.
Age has a marked influence in developing or fostering a tendency to special forms of disease.
It is well known that the foal suffers from diseases to which the state of the organism and the circumstances of its life render it peculiarly susceptible. The young animal is liable to the effects of cold, which induces irritation or inflammation of internal organs. Very trifling errors in dieting - in the sensitive state of the digestive canal - cause severe, sometimes fatal, attacks of diarrhoea. The brain is easily excited, and the process of teething increases the liability to various febrile and gastric disorders.
In advanced age the horse is predisposed to rheumatic affections, stiffness of joints from the increasing density of the ligamentous structure, while the muscular powers are impaired from loss of the true contractile tissue and the increase of fibrous structure, and the steady decrease of the vital powers adds to the predisposition to diseases of a chronic type. It may, however, be affirmed that the influence of the different age periods in the horse in the production of predisposition to disease is not to be compared to the changes which occur in the system of the human subject at different periods in his far longer average life.
Sex as a predisposing cause of disease relates chiefly to the generative system, and in the lower animals the female is predisposed to affections due to gestation and parturition from which the male animal is necessarily exempt; but in other respects no important difference has been observed. Mares are not more nor less liable than horses to those affections to which the equine race is prone.
Occupation is quoted among the predisposing causes of particular diseases in man, and it is surely the case that horses are likewise rendered susceptible to maladies of a kind which are incidental to their occupation, i.e. the kind of work which they are required to perform. It is only necessary to compare the occupation of the hunter with that of the agricultural horse on the farm, or the work of the carriage- or saddle-horse with that of the poor man's drudge, to realize that the position which the animal occupies, in other words the occupation as it would be termed in reference to man, exercises a very considerable influence on the susceptibility of the system of the horse to certain forms of disease: thus, racehorses are specially liable to sore shins; hacks, hunters, and harness-horses to splints and spavins; and cart-horses to side bones, etc. etc.