• Horses are often exposed to allergens in the pasture.horse image by Penny Williams from Fotolia.com
  
  Swelling that occurs on a horse's face can have a variety of causes. Allergies, injuries, infections and even some diseases will cause the horses' lymph nodes to swell, which gives the appearance of swelling in the animal's jowls, including its throat and cheeks.
  
  Allergic reactions can take a variety of forms and no two animals are guaranteed to react to any allergen in the same way. Swollen jowls can be a symptom of a variety of equine allergies.
  
  

Plants, Pollen, Mold and Dust

• Exposure to plants, pollen, mold and dust can cause a horse to experience an allergic reaction, including a swollen jowl area.
  
  It is often difficult to figure out precisely what is trigger an animal's allergic reaction. Oftentimes, a plant in bloom or pollen in the air can cause a slight allergic reaction in a horse the same way it can in a human.
  
  Horses that have ingested the allergen may experience more severe allergy symptoms than those that only inhaled the allergen.
  
  

Insects

• Allergic reactions to insect bites and bee stings commonly cause swelling in the area where the horse was bitten or stung. If an allergic horse is bitten or stung on the face, the affected area will most likely swell noticeably.
  
  

Chemical and Product Allergies

• Much like humans, horses are also susceptible to chemical and product allergies. Pesticides can trigger allergic reactions in horses; so can veterinary drugs, supplements, shampoos or even products used to clean leather or tack--especially when used on horses with sensitive skin.