Most horses can go throughout their entire lifetime without suffering from an allergy, while a small percentage present with allergies that impact their health and frustrate owners and managers. Allergic reactions occur when a horse's immune system becomes hypersensitized to ubiquitous substances (pollens, dust, etc.) that may only affect one out of many horses in a facility. Clinically, allergies are an abnormal reaction to foreign substances (called allergens) by the immune system. Allergies can develop over time, without warning and with prolonged exposure to an irritant.

All horses have the potential to develop allergies during their lives, and age, breed or gender do not normally influence allergies. When a horse comes in contact with an allergen to which it has been sensitized, this triggers a set of reactions involving histamines and prostaglandins, which lead to inflammation and a corresponding reaction by the horse’s immune system.1 With repeated exposure to the offending allergen, the horse will show stronger reactions.

Factors and Types of Allergies in the Horse

Allergic reactions in horses are triggered by varied factors and manifested in various ways. The most common types of allergic reactions in horses present in the skin or respiratory system. In the skin, vigorous itching of a specific area of its body may be observed, or it may be overcome with hives. With respiratory reactions, the horse may cough, sneeze or wheeze without nasal discharge.

Skin allergies such as hives are among the most common allergic reactions seen in horses. Hives usually appear between 12-14 hours after a horse is exposed to an allergen. Hives may initially be observed around the neck and shoulders, then over the rest of the body. Hives may give rise to itching, and can manifest as numerous small or sparse, larger-sized bumps.

As indicated above, allergic reactions can be brought on by a variety of triggers: pollen, dust, mold, bedding, insect bites, application of a topical product (shampoos and lotions) or vaccinations. One of the most allergenic type of insect bites comes from midges, the Culicoides species of flies.1 Some breeds of horses have a higher sensitivity to these bites and are more likely to show allergic reactions to them than others. 

Respiratory allergies can also affect horses. Horses are typically stabled in barns with restrictive air flow, where they can inhale dust and other particulates. Depending on the regimen in a given facility, time out in the pasture may be restricted and the horse’s movement limited. These factors can potentially affect the immune system, allowing allergic reactions to occur.

Respiratory allergies occur when a horse inhales an offending allergen. As with humans, when a horse breathes in dust, molds or pollen, its immune response reacts, making it difficult for the horse to breathe. 

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.

2Kuroda T., et. al. Pharmacokinetics and pharmacodynamics of d-chlorpheniramine following intravenous and oral administration in healthy Thoroughbred horses. Vet J. 2013 Aug;197(2):433-7.