Immune-mediated Skin Disease
In this class of conditions, the skin is damaged to varying degrees by the immune system. The damage can be mild or the horse’s skin can become ulcerated over the majority of its body. Most commonly, the skin is accidently attacked while the immune system is trying to fight off something it deems as foreign. Drug reactions fit into this category and are common in the horse.
The other group of immune-mediated conditions are the autoimmune skin diseases where the body tried to reject the normal skin. Pemphigus foliaceus is the most common autoimmune skin disease of the horse. All of these conditions are serious, lifelong and require vigorous treatment.
Staphylococcus bacteria is the most common cause of skin infection that we see. The bacteria can be transmitted easily from animal to animal and rarely from animal to human, and is found on all breeds of animals.
The most typical symptoms are a tender or itchy rash with numerous pustules.. The infection is diagnosed using skin tests and microbiology and treated with topical and/or oral antibiotics. Some strains are antibiotic resistant and require stronger medicines for treatment. Prognosis is good.
Dermatophilosis (Rain Rot)
This disease is an infection caused by D. congolensis, a gram positive bacteria thought to originated from the soil. Moisture and high temperatures contribute to the spread of the disease, which is fairly common among horses and usually is fairly easy to treat.
Horses are usually affected on the back, head, and neck and the legs. Initially, the horse will display a matted coat and bumps, which then progresses to scabs and lesions. The horse may also be itchy and display signs of discomfort.
In some diseases, the infection becomes chronic and mimics may other skin conditions of the horse. In these cases, skin biopsies are needed for the diagnosis and the treatment will involve a long course of oral and topical antibiotics.
Allergic Skin Disorders
Horses with allergies can get itchy skin, hives, or both. The most common allergen of the horse is the Culicoides gnat, a very small mosquito. There are many different species of gnats and each has its site of preference where it feeds on the horse. When the gnat pierces the skin to suck the horses blood it leaves some “saliva” in the skin that triggers an allergic reaction. In central New York, the most common feeding sites are the horses face, the mane and tail regions, or along the horse’s ventral abdomen. Control of the gnat and its associated skin disease can be very difficult.
Horses also can become allergic to environmental things like pollens, mold spores, and barn/food storage mites. The allergic reaction can cause hives which may or may not itch or itchy skin. These allergies can be treated with steroids or antihistamines but these drugs may be prohibited in competition horses. Allergy testing can be done to identify what the horse is allergic to.