Hair loss in horses can result from inflammatory conditions caused by infection, allergies and burns. It can also be caused by a weakened immune system, hormonal imbalance or protein deficiency. Pregnant or lactating horses also often display hair loss.
A horse may be born with a condition that causes hair loss or else acquire that condition from an outside source. Hair loss can be caused by any disease that affects the hair follicles, sometimes preventing hair growth altogether or destroying the hair follicle or shaft. Many common diseases that cause hair loss are inflammatory and therefore also cause local irritation in the affected area.
Inflammatory skin conditions are most commonly caused by fungal, bacterial or parasitic infections; allergies; burns; and other skin traumas, such as excessive friction that results from a poorly fitted saddle. All these conditions are accompanied by constant itching, rubbing or biting at the irritated area. Among these causal types, the most common disease would be ringworm, a fungal infection.
Hair loss can also be caused by a protein deficiency, hormonal imbalance or weakened immune system; these conditions typically manifest without inflammation and may cause hair loss to appear random. A pregnant or lactating horse may also experience hair loss, according to Merck Manuals. The best way to determine the cause of hair loss is to see a veterinarian who knows what to look for and has the equipment necessary to find it.