Erythema is the term applied to any redness of the skin, whether or not it is associated with an eruption. In its simple form erythema consists of patches of redness, which may be caused by friction, stimulating applications, and, in short, by any irritating substance which will bring about congestion or undue fulness of the vessels.
Erythema intertrigo is the name given to the redness produced by friction between two surfaces of the skin, as sometimes occurs between the arm and the brisket - that part which is called the axilla; it occurs also in the groin, especially of fat animals.
A very characteristic form of erythema in the horse is familiarly known as "mud fever", on account of the disease occurring in wet seasons, when the ground is soft and sloppy. It is a common ailment among hunters, especially when cold winds prevail, and the legs and body become splashed with mud and made repeatedly wet and dry in the course of a hard day's hunting. It is somewhat curious that in particular districts the irritation caused by the mud is most marked, and certain parts of the country are credited with having soil contaminated with some irritating matter, which, however, cannot be discovered by most careful examination.
The disease is manifested by irritation, soreness, and tumefaction of the skin, generally accompanied by a certain degree of fever, swelling of the legs sometimes incapacitating the animal for several days. It now and again occurs that the hair falls off in patches, but the blemish thus caused is not as a rule permanent.
A similar disease of the skin is also recognized among hacks and working horses under certain circumstances, irrespective of locality. Animals used for any purposes during muddy weather, and particularly when the nature of their work renders it necessary that they should be constantly in use, often suffer severely. The disease is commonly most marked in the winter-time when snow is on the ground, and more particularly when a sudden thaw takes place and cold easterly and northeasterly winds prevail. In this connection it has been noticed that in certain establishments, where the pressure of work or the shortness of hands occasioned neglect of grooming, the animals which on coming in from their work were turned into their stalls without having the mud washed from the legs and body escaped the mud fever altogether, while those which were carefully groomed and had the mud thoroughly washed from their skins were invariably attacked. As a matter of course, this fact came under the observation of veterinary surgeons, who were, from the nature of their avocation, in the best possible position to test the truth of the presumption that washing the mud from the legs and body of the horses was a cause of mud fever. It took a long time to reconcile horse-owners to this apparently unreasonable view, but it is now perfectly well known that in establishments where washing has been discontinued mud fever and cracked heels are of rare occurrence.
In the human subject erythema exhibits itself in a great number of forms, according to the situation of the disease and the character of the cause which produces it.
Erythema intertrigo is one of the forms which occur in the horse when two surfaces of skin rub against each other, and the term may conveniently be extended to include all those instances of redness following friction from any part of the harness, saddle, or collar. It will be noticed, however, that the irritation induced by friction is not likely to be detected in dark-coloured animals, and the cause therefore is frequently allowed to continue until abrasions occur, and the so-called "shoulder galls" appear. These conditions are common enough among working horses wearing a badly-fitting collar, and are difficult to deal with on account of the impossibility of the healing process taking place so long as the use of the collar is continued. The device which is resorted to, of hollowing out a part of the collar which causes the abrasion, is only partially successful. In order to dry up the abraded surface and harden the skin, an astringent lotion is usually employed; a solution of chloride of zinc, or bichloride of mercury, is generally effectual. A very convenient lotion to be kept in the stables is made by mixing Sir William Burnett's disinfecting fluid, in the proportion of one part to fifty of water, the bottle, of course, being marked with a poison label.
Purpura is a form of erythema which occurs in the horse under the name of purpura haemorrhagica. This disease consists in extravasation of blood into the tissues of the true skin, either from dilatation and overdistension of the vessels from deranged nerve-function, or from a morbid condition of the blood itself. It can hardly be looked upon as a distinct disease of the skin, but rather as an indication of some serious pathological state of the system, which would generally be classed under the heading of blood diseases. So far as the skin itself is concerned, the disease is exhibited in the form of patchy elevations, associated with blood-spots on the visible mucous membranes. The affection appears to be peculiar to the horse, and is met with at the termination of debilitating diseases, such as influenza and strangles. (See Purpura Haemorrhagica.)
Urticaria - commonly described as nettle-rash - frequently occurs in the horse during the spring and summer months. The causes are a sudden change of diet, especially from hard corn to fresh succulent herbage, such as grass, clover, rye-grass, vetches, etc, drinking cold water when the animal is heated, or the sudden movement of an animal from a hot stable to a cold, wet atmosphere. The form of urticaria which arises from dietetic errors is distinguished as urticaria ad ingestis.
The eruption in this affection occurs very suddenly in the form of flattened, more or less rounded elevations, differing much in size, the majority of them being something from an inch to two inches in diameter. They commonly occur on the neck, and frequently over a considerable part of the body, and sometimes the head also. The lumps appear very quickly, and may be so closely packed as to run together into a single patch the size of a dinner-plate. Sometimes when the eruption is scattered, and the lumps are not much larger than a shilling, the disease assumes a chronic form, and is attended with loss of hair from the raised patches. It is curious that the new hair is always lighter in colour than the rest of the coat. The treatment of the disease consists in the administration of a mild laxative, to be followed by small repeated doses of bicarbonate of soda and chloride of sodium in the food night and morning.
Liquor arsenicalis, in doses of an ounce to an ounce and a half, given in the food night and morning, may also be tried where the disease proves obstinate, and failing with this, some vegetable tonic, as gentian and columba root, or quinine, deserve a trial.
If there is much irritation, a little Goulard water may be applied to the seat of eruption, or a liniment of glycerine and oxide of zinc.