Skin problems in horses are some of the most frustrating disorders to manage for both owner and veterinarian. Skin problems can disfigure a horse, and even can cause unsoundness. In addition, many skin diseases, such as allergies, have a complex cause so that one easy treatment is not available.
Signs of allergies in many horses appear with the arrival of summer and become progressively worse each year. Often horses with this history are allergic to insect bites (insect hypersensitivity). Several different clinical syndromes have been associated with insect hypersensitivity, such as Queensland or sweet itch, which is caused by Culicoides species (no-see-ums). However, any biting insect can be involved in insect hypersensitivity. In fact, many affected horses are allergic to the bites of more than one kind of insect.
The first signs can include redness and large, flat, circular swellings (wheals) or raised nodules with or without crusting. Intense itching (pruritus) often leads to skin damage, hair loss, secondary infections, and thickened, wrinkled skin.
The best therapy is to prevent insects from biting your horse, or at least reduce the number of bites. Effective management strategies include:
- Stabling during times of high insect activity;
- Directing fans to the surface of the horse when stalled;
- Using long-acting insect repellents (either on the horse or impregnated in mesh blankets or other equipment).
Intradermal skin testing (IDST) can help detect the insect or group of insects to which the horse is most allergic. If determined, a desensitizing vaccine can be custom-made for that horse. Approximately 50%-70% of horses respond favorably to desensitization; however, six to 12 months is needed before the horse receives the full benefit of this therapy.
Another strategy to control insect hypersensitivity is decreasing the horse’s immune reaction through corticosteroids. Depending on the situation, these compounds can be applied directly to the affected area(s) or given systemically.
Other skin diseases that can cause itching include:
- Hives (recurrent urticaria);
- Parasites (onchocerciasis);
- Ringworm (dermatophytosis);
Recurrent urticaria can be recognized by the sudden appearance of wheals in the skin, some of which might coalesce, or grow together. The skin lesions are a response to allergens that could be inhaled, ingested, or contacted in the environment. The wheals might disappear quickly only to recur, or they might remain over several days.
Identifying the allergen is done by IDST or by trial and error. This involves exposing the horse to different allergens individually, which is a tedious process but yields a more specific answer.
Onchocerciasis (awn-koe-sir-KIE-uh-sis) is hypersensitivity to the larval stage of a parasite that can live in the horse’s skin. Transmitted to the horse by Culicoides, these parasites are effectively controlled by the dewormers ivermectin and moxidectin. Therefore, hypersensitivity to them is much less common today than in the past.
Ringworm is characterized by circular, patchy, or coalescing lesions of hair loss–is another skin disease that can cause itching. Contrary to popular belief, the organism that causes ringworm is not a worm but a skin fungus, or dermatophyte. A diagnosis of ringworm can be confirmed by culturing hairs plucked from the edge of a lesion.
Many horses with allergic skin disease can develop contact hypersensitivity to medications or insect repellents used to treat the disease, making the original lesions worse. For that reason, your veterinarian might request that you stop treatment with all topical compounds and shampoos (except for one or two) while preliminary diagnostic work is performed.
Finally, horses kept in hot, humid areas can develop dermatophilosis. This disease, also called rain scald or rain rot, is caused by bacteria and often is mistaken for a fungal disease. The bacteria live in the outer layer of skin and cause from pinpoint to large, crusty scabs. When removed, the base of the hairs can be seen sticking through the bottom of the scab. In early or less-severe cases of this disease, simply removing the scabs with shampoos and currying will take care of the problem. More severe cases in which the infection has affected deeper layers of the skin might require antibiotics.
Your veterinarian should be an active partner in diagnosing and treating skin disease, particularly one that does not resolve in one to two weeks. By carefully examining your horse and following the progression of the skin lesions, you can help your veterinarian choose a place to perform a skin biopsy–the best diagnostic procedure for troublesome or persistent skin disease. Once a diagnosis is made, specific therapy can be recommended to resolve the condition.