Horses experience a variety of situations related to plants, insects and chemicals that can be the source of discomfort or a variety of clinical symptoms.

These reactions can be broken down into several areas:

• A feed or a food allergy is an abnormal immune system reaction, generally to a specific protein in a specific ingredient in the feed or the food.
• This may result in what we call uticaria (welts), plaque, bumps or hives that we see on the skin. Sometimes called protein bumps, these are not a reaction to too much protein, but are a reaction to a very specific protein. (Equine Clinical Nutrition, Dr. Lon D. Lewis, Williams & Wilkins, 1995 p 20.)
• This protein may come from a feed ingredient, but also may come from ingestion or inhalation of other material or something like insect bites.
• One horse in a herd may develop an allergic reaction while other horses are not affected.  Identifying the specific allergen may require exclusion feeding or highly specific allergy testing.
• In humans, the extreme example is something like a peanut allergy where a very small quantity can create a life threatening situation.
• A feed or food intolerance is not the same as an allergy, although there may be some similar symptoms.
• An individual is not able to properly digest a specific ingredient, which can result in digestive disturbances.
• Human examples may include such things as lactose intolerance or gluten intolerance.  This has not been well identified in horses, but may be possible.
• An irritation may be caused when the horse comes in contact with specific substances that cause tissue irritation.
• A horse that is sensitive to the chemicals in a particular grooming product or fly spray may break out or experience skin irritation and loss of hair.
• Insect bites may cause both an allergic response and a surface irritation.
• Chemical irritants from plant, insect or synthetic sources may cause skin surface reactions.

Unusual skin symptoms or digestive disturbances may require careful observation (and sometimes a bit of luck) to determine the underlying cause.  It may be very useful to work with your veterinarian to determine the exact condition and search for potential causes.