They're hard to diagnose, but some horses suffer spring allergies. Typical allergy symptoms come on seasonally, and range from hives to coughing, sneezing and watery eyes.
Difficult to diagnose, the best way to determine if a horse is allergic to something in his environment is to observe his health carefully and keep notes. If he has respiratory allergies, allow the horse as much fresh air as possible, soak his hay in water to keep the dust down, and contact a veterinarian for a possible prescription for corticosteroids.
Allergies manifest in skin rashes are the most common equine allergies, and usually appear 18 to 24 hours after the horse has come in contact with the allergen. They usually resolve themselves; if it's a recurring problem, ask a veterinarian for help.