Respiratory symptoms and skin rashes are the common symptoms of allergies among horses, although a general state of ill-health without obvious cause could be a sign of allergies as well, particularly in the spring.
Natural Horse Remedies: Apple cider vinegar added to your horse's feed or water can reduce allergy symptoms. Sprayed on its coat, it can also relieve hives or other allergic skin conditions. Red clover, burdock, and spirulina are dietary supplements that may also boost the immune system to relieve allergy symptoms.