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What is Allergic (Urticarial) Reaction?

Urticaria is often caused by an allergic reaction your horse develops to a specific allergen, such as mold, pollen, insects, or a chemical that he is repeatedly exposed to. Once exposed, the immune system produces antibodies to attack that particular foreign invader. When exposed to the same allergen, the immune system produces many of these antibodies, which release histamine, the chemical responsible for the inflammation seen in the skin. Horses can develop allergies at any age, and can be mildly or severely affected in their airways, digestive systems, or skin. Often, it is a lifelong condition that will need constant monitoring and treatment.

Hives, or urticaria, is an allergic reaction that causes circular welts, wheals, or nodules that can cover large areas of the skin. These bumps can appear in various sites, and are often filled with a clear fluid that is just below the skin’s surface. A firm finger press will leave an indentation, confirming that the bumps are indeed hives.

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Symptoms of Allergic (Urticarial) Reaction in Horses

The raised bumps that appear can be an acute episode, or a chronic condition that can lead to a diagnosis of an allergy. Hives occur in the upper layer of skin as clear fluid collects in response to the allergen. Bumps are small, from millimeters to inches in diameter, and can occur on various areas of the body, most commonly on the face, neck, shoulders, limbs, and flanks. If hives are not treated, your horse can experience a worsening of symptoms, such as labored breathing and death from an anaphylactic reaction. Signs can include:

  • Circular bumps, wheals, or papules on the skin
  • Bumps that ooze a clear liquid
  • Crusting over bumps
  • Itching 
  • Thickened skin
  • Skin inflammation and irritation
  • Hair loss 
  • Bruising 
  • Swollen eyelids, lips, and muzzle
  • Swollen limbs
  • Self-trauma 
  • Tail rubbing 
  • Rubbing on the fence or barn 
  • Secondary infections
  • Coughing
  • Difficulty breathing
  • Exercise intolerance
  • Decreased performance
  • Anaphylactic shock
  • Death

Causes of Allergic (Urticarial) Reaction in Horses

Hives are an allergic reaction to an allergen your horse has become sensitive to. The allergens that could be responsible are numerous, and can be narrowed down through a history of symptoms and exposure to common allergens, and diagnostic testing. It is also possible that your horse’s skin has been irritated by a chemical through contact or ingestion, or from a stressful condition. Possible allergens and irritants can include:

  • Pollens, molds, fungus, and dust
  • Insect bites, such as flies, biting midges, gnats, chiggers, and mosquitoes
  • Bacteria, virus, fungus, and parasites 
  • Food allergies
  • Animal dander or feathers
  • Dietary changes
  • Diets that are cereal based 
  • Food additives
  • Physical pressure on the skin, such as from an improperly fitted saddle or tack
  • Drugs, such as antibiotics, nonsteroidal anti-inflammatory drugs, dewormers, vaccines, various vitamins and supplements, topical ointments, or penicillin
  • Aerosol or other chemicals, such as fly spray, shampoos, or detergents
  • Bedding 
  • Temperature extremes
  • Intense exercise
  • Stress

Diagnosis of Allergic (Urticarial) Reaction in Horses

A diagnosis of hives is easy to make based on the characteristic symptoms. The cause of the hives can prove more challenging, as there are so many possibilities. A complete history of symptoms, exposure to new animals or environments, the season in which symptoms occur, recent medications, changes in the diet, bedding or products used on your horse, and other factors, can help to narrow down possible allergens that your horse may be sensitive to. Finding the cause is essential to preventing the hives from recurring. 

A physical examination of the hives can show points of penetration, a sign that insect bites may be the culprit. A skin biopsy can be taken and tested to rule out skin issues. An allergy can be determined through allergy testing. An intradermal allergy test (IDT) injects small amounts of various allergens directly into the skin to elicit a reaction and determine a specific allergy. Serum allergy testing (SAT) and the ELISA test both use a blood sample to reveal specific antibodies targeted to a particular allergen. While these tests can reveal an allergen, they often produce incorrect results and need to be taken into consideration with other methods. 

Taking a survey of all of the items, feeds, and chemicals in your horse’s environment is often the best way to eliminate or reveal possible allergens or irritants. Correlating the hives and other symptoms to exposure to these allergens or irritants can detect what is causing the reaction. If hives only appear in the summer months, it may point to a pollen allergy.

Treatment of Allergic (Urticarial) Reaction in Horses

Treatment involves resolving the symptoms as well as preventing a recurrence through managing your horse’s exposure to the allergen or irritant. If the allergen or irritant is discovered, removing it from your horse’s environment is the first and best way to stop the allergic reaction or contact dermatitis from occurring. If the reason is unknown, making small environmental or food changes can help to determine the true cause.

Some cases of hives may resolve on their own in a few days’ time, while others can persist. For acute cases, or if the swelling is causing an interference with normal functions, such as breathing, epinephrine can be administered, as well as a fast-acting corticosteroid given intravenously. Sometimes, this can resolve the lumps and itchiness without any further treatment needed. 

Other cases may need more treatment. These can include slower-acting corticosteroids, non-steroidal anti-inflammatory drugs, antihistamines, glucocorticoids, omega fatty acids, tricyclic antidepressants, phosphodiesterase inhibitors, or immunomodulatory drugs. Immunotherapy may be used to reduce the reaction your horse has to a particular allergen, and is administered over many months. If it is effective, it can provide a permanent solution.

Other supportive treatments can include the use of bronchodilators to assist breathing, and using topical antiseptic and antibiotic creams and solutions on the hives and irritated skin areas. Soothing emollient shampoos and conditioners can also be used, as well as topical corticosteroids.

Management methods to prevent contact with an allergen or irritant include:

  • Reduce time outside during high pollen counts
  • Place fans in your horse’s stall or barn to keep the air moving
  • Fly repellents and blankets can be used for insect sensitivities 
  • Keep horses inside during peak insect hours
  • Keep living quarters free of dust and particles
  • Check all feed and hay for accidental contamination of plant allergens
  • Replace bedding, feed, shampoos and detergents as needed

Recovery of Allergic (Urticarial) Reaction in Horses

While most cases of hives are short lived and resolve quickly, or can be treated, there are times when a serious reaction, such as anaphylactic shock, can occur. If this happens, seek medical help immediately, as this is a life-threatening condition.

Many cases of an allergy or irritant can be treated and managed through medications, skin treatments, and removal of the cause. Your horse may have an acute reaction that never reappears, or may need medication when a reaction occurs, possibly for the rest of his life. Recovery from hives is dependent on what is causing the reaction, the severity of the reaction, and the exposure to that allergen or irritant. Prevention of hives is only possible if the cause is known and removed from your horse’s environment.