A wide variety of conditions can produce unsightliness and illnessSome horses develop skin allergies that can be very irritating and troublesome. There are many forms of allergic dermatitis (inflammation of the skin) and a lot of these occur most commonly in spring, summer, and early fall, due to allergic reactions to insect bites, skin contact with irritating substances, infections, ingestion of certain proteins or a high protein content in the diet, or irritation from non-biting insects.
Allergy is an individuals overreaction to a substance or agent which normally causes little or no response by the body. Usually the body handles invading substances (antigens) with self-protective mechanisms that allow the horse to fend off or eliminate the problem without adverse effect. The body creates circulating chemicals called antibodies to counteract or fight the invading antigen. But if the horses defense system goes overboard in its efforts, producing excessive numbers of antibodies and creating an inflammatory response in the body, this hypersensitivity is called an allergy. Certain individuals and families of horses are more allergy-prone than others, being extra sensitive to various types of invaders. And the allergic response may become worse with each encounter, since the horses body is already sensitized.
In general, allergies tend to have seasonal or environmental patterns, appearing and disappearing as the seasons change or as the horse is put into different surroundings. These patterns, and the fact that the allergies become progressively worse with time, differentiate them from simple inflammatory responses to skin irritations such as from detergents or caustic chemicals.

Specific conditions
Summer itch, sometimes called sweet itch, is a condition that develops from a sensitivity of the outer layer of skin to insect saliva. It can also be caused by a parasitic worm larva. Specific species of insects may cause allergic skin reactions in some horses while being harmless to others. Allergy to the insect saliva produces an inflammatory, itchy thickening and scabbing of the skin, especially over the rump and/or withers, which may become bare and weepy as the horse scratches these areas.
Contact dermatitis is an allergic reaction due to direct contact with irritating substances. The irritant may be a chemical such as a dye, preservative used on bridles or saddles, soaps, insect repellents, constant exposure to feces and urine (as in soiled bedding), contact with certain pasture plants, or even a sensitivity to wound secretions or a topical medication. Ventral midline dermatitis is swelling and weepy, bare skin along the underline of the belly, caused by a sensitivity to horn flies that concentrate there, or to other insect bites that contribute to the problem.
Sometimes an allergic reaction will manifest itself as swellings (raised areas) in the skin. This condition is called urticaria and the bumps are called hives or wheals. These raised lumps may be one-third to one inch or more in diameter and often appear in clusters. The bumps are due to dilation of the small blood vessels (capillaries) near the skins surface, which is part of the bodys response to the offending substance. Seepage from the damaged capillaries results in the skin swellings. There is usually no itching with hives, unless the problem is caused by allergy to insect bites or stings.
The bumps come on suddenly after the horse comes into contact with the offending allergen. If you happen to be with the horse, you can actually see the fairly firm bumps growing as you watch. They usually pop up rather symmetrically on both sides of the body and sometimes on the head and neck.
Hives can be caused by a reaction to insect bites or stings, or contact with stinging plants that the horse is sensitive to, or can be caused by eating weeds or plants that produce the allergic reaction. The latter type of reaction is usually due to ingestion of some type of protein, which the body treats as a foreign substance. Plant pollens or poisons may also trigger a case of hives.

The livers role
The degree or severity of the reaction may be affected by liver impairment, since the liver is the bodys main filter and detoxification unit. Horses with impaired liver function tend to have more problems with allergic reactions or photosensitization. Some offending materials eaten or absorbed by the body are not efficiently filtered out and eliminated if the horse has liver damage, and end up in the skin (carried there by the bloodstream). If any of these substances are photosensitive, they produce chemical changes when exposed to sunlight, creating cell death and skin damage. The effects of photosensitization occur most readily on unpigmented areas (white markings) or thinly haired portions of the head such as the nostrils, muzzle, and eyelids. Itching, swelling, and redness are the first symptoms, then a dying and sloughing off of the affected skin if the horse is not treated (and removed from the offending pasture or feed and taken out of the sunlight).
Certain drugs or vaccines can also produce allergic reactions in some horses. Penicillin will cause hives or other manifestations of allergy in some individuals. Some horses are allergic to the vaccine combination or carrier in the four-way Eastern-Western encephalomyelitis, influenza, and tetanus vaccine, breaking out in hives soon after being vaccinated. It may take several days for the hives to subside, and they generally become worse with each subsequent vaccination. Youll probably have to try a different type of vaccine for future injections if you have a horse that is allergic to a specific kind.
In mild cases of hives, the lumps and bumps will generally subside without treatment within a few hours or by the next day. But in more severe cases they may persist for several days (generally due to the appearance of new swellings as the old ones subside), and the horse may need an injection of antihistamine or treatment with dexamethazone. A severe case may need every-other-day treatment with steroids for a number of days. In some cases, other allergic manifestations such as diarrhea, slight fever, or breathing difficulties may accompany the skin lesions.
Hives may recur if the cause is not correctedif the diet is not changed or the exposure to the causative insects or plants is not prevented. Many times the cause of hives is readily obvious, as when a horse reacts to a certain injection, drug, or feed change. Other times it may be more difficult to discover the culprit, and if the hives recur or do not disappear within a few days you may have to have a veterinarian run sensitivity tests (blood tests and localized skin testing on the horse) to try to discover the cause. In most cases, however, acute skin allergies are unsightly but not harmful, and they usually clear up on their own without treatment.

Chronic allergies
By contrast, chronic skin allergies can be long-lasting and hard to treat. Their characteristic large local skin lesions generally appear at least four hours after the horse has been exposed to the causative allergen, and they persist for days or weeks. Signs of this type of skin allergy include itching and swelling and surface reactions ranging from skin flaking to oozing. There is usually some hair loss due to the horses itching and scratching. The affected area is usually confined to the portion of the body where the antigen made contact or where the offending insects attacked. Poison ivy is the most well-known cause of delayed and persistent skin allergy in people, while horses most commonly suffer from sweet itch (also called summer itch, Queensland itch, summer eczema and culicoides dermatitis).
Caused by a reaction to the saliva of certain biting insects such as gnats, no-see-ums, culicoides, and others, the symptoms of sweet itch generally do not appear until several hours after the insect feeds on the horse. Then the lumps and intense itching develop along the mane, tail, head, belly, or topline. Eventually, the horse may lose patches of hair, especially in mane and tail, and the skin becomes thickened as he constantly rubs these areas. In horses that suffer from allergy to insect saliva, one or two bites can reactivate the allergic response on every part of the horses body that has encountered the antigen in the past. One of the most frustrating things about sweet itch and other delayed skin allergies is that once the horse is sensitized to a particular insects bite, he becomes even more sensitive, turning into an itchy, bumpy mess each season the insect attacks him, with reactions getting worse with each episode.
The treatment of skin allergy depends heavily on anti-inflammatory drugs (primarily steroids), which are more effective than Bute or Banamine. The steroid treatments are usually given with relatively high doses at first, then progressively reduced until the minimum satisfactory dose is being given, on an every-other-day schedule.
Delayed skin allergies are usually difficult to deal with because of the nature of the immune system-allergen battle that creates them; the allergen attaches itself to the protein in the skin, and thus the bodys defenses attack the skin proteins while trying to evict the invaders. This type of reaction can continue long after the external exposure to the offending substance has ended.
Careful management is the best weapon against skin allergies, attempting to minimize the horses reactions by protecting him from exposure to the things that set off his allergies. For a horse that is affected by flies and biting insects, the use of fly repellents, keeping him indoors during the worst times for insect bites, or using a light sheet to protect his body can be helpful. But if a horse continually develops hives or other symptoms in spite of all your precautions, you may have to treat him with steroids every second or third day to help clear up the allergy and alleviate his discomfort. Daily dosing with steroids is not recommended, since this may have harmful side effects over time, such as damaging his adrenal glands or compromising his immune system. If you have a stubborn cause of skin allergy in a horse, consult with your veterinarian.