As in humans, horses can become allergic to all sorts of things. These can include plants, pollens and other substances in the air, as well as chemicals or food products. These allergens are substances that, when inhaled or absorbed through the skin, respiratory tract, or gastrointestinal tract, stimulate histamine production, leading to inflammation. Allergic reactions can also be triggered by medications or vaccination.1 In some horses, allergic reactions can become a chronic, frustrating and potentially debilitating part of life and a bane to horse owners and managers. 

Allergic reactions occur when a horse's immune system becomes hypersensitized to the above substances, which ordinarily do no harm, and which may only affect one out of many horses in a facility. When this occurs, the horse’s immune reaction runs out of control.2 An overabundance of antibodies are produced which stimulate the release of a flood of prostaglandins, histamines and other substances.1 Once a horse has had an allergic reaction to a substance, subsequent exposures increase the severity of the immune response, and the horse’s physical distress.

Types of Horse Allergies 

Airborne allergies are the most common allergies in the horse because horses usually live in environments with a high level of dust, mold, or other particulates. In such cases, the horse’s immune system overreacts to the presence of the offending allergen(s). Once an allergy is determined, the first line of treatment will involve avoiding the allergen. Allergy testing is available in some areas; this will facilitate your veterinarian to identify the specific allergens that are a problem for the horse and to formulate a treatment.2

Food allergies are also not uncommon in horses, although documented cases are rare. In some cases, food allergies have been associated with high-protein diets.1 A common approach to identifying a food allergy in a horse begins with a trial or elimination diet (avoiding the allergen). Once the offending allergen is isolated, control of the food allergy involves strict avoidance of the food. Concurrent diseases may complicate the identification of underlying food allergies.1 Occasionally, a horse will react to new food allergens as it gets older.2

Symptoms of Equine Allergies 

The symptoms of allergies in the horse are usually observed in the skin and respiratory system. An allergic reaction in the skin usually causes itching (pruritus) and/or hives (urticaria).1 Other possible signs can include hair loss, bumps and crusting. Coughing and nasal discharge may also occur, and are the most frequent displays of allergies in horses. Respiratory allergies in horses can present as an exaggerated response to inhaled pollens (similar to hay fever in humans).1,2 Horses with allergic rhinitis or rhinoconjunctivitis may have a runny nose as well as red, swollen, and watery eyes.

Chlorpheniramine for Allergies in the Horse

In healthy Thoroughbred horses, administration of 0.5 mg/kg IV plasma drug disposition was very rapid, with the mean terminal half-life and total body clearance calculated as 2.7 hours and 0.7 L/hr/kg, respectively. The observed maximal inhibition of wheal formation at 0.5 hours after administration of 0.1 and 0.5 mg/kg IV were 37.8% and 60.6%, respectively; administration of d-chlorpheniramine at 0.5 mg/kg PO resulted in bioavailability of 38%, with a peak plasma concentration at 1 hour and a maximal inhibition of wheal formation of 39% at 2 hours, suggesting that a larger dose rate and more frequent intervals may be needed to maintain therapeutic concentrations in horses.4

Where to buy Chlorpheniramine 

Chlorpheniramine is available in the U.S. through several pharmaceutical manufacturers and through veterinary custom compounding companies. 

Chlorpheniramine carries the potential for several drug interactions. Please consult your veterinarian prior to beginning any treatment regimen.

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

1Merck Veterinary Manual.

2equusmagazine.com.

4Kuroda T, Nagata S, Takizawa Y, et al. Pharmacokinetics and pharmacodynamics of d-chlorpheniramine following intravenous and oral administration in healthy Thoroughbred horses. Vet J. 2013;197(2):433-437.