Equine Allergy

Equine asthma encompasses a spectrum of respiratory disease, ranging from mild to moderate asthma (previously known as inflammatory airway disease (IAD)) through to severe asthma (formally recurrent airway obstruction (RAO)). This new all-encompassing terminology is particularly helpful in aiding owner understanding of the condition, and rationalising subsequent management options.

Severe asthma is reported to affect 10–20% of adult horses in the northern hemisphere, with mild to moderate asthma affecting as much as 60–100% (dependent on population size and geographic region). ¹   

Mild to moderate asthma is more commonly observed in younger horses, but can occur at any age. These horses are known to be at an increased risk of developing severe asthma, which typically occurs at seven years or older.²

An allergic reaction to inhaled allergens (most commonly those found indoors) is likely to be central to the development of equine asthma; a seasonal pasture associated version of the disease also exists.

Excess tracheal mucus is expected, accompanied by coughing, poor performance and airway hyper-responsiveness. Laboured breathing at rest, pronounced bronchoalveolar lavage fluid (BALF) neutrophilia and marked exercise intolerance in addition, would indicate a more severe case. It is important to note that clinical signs may be subtle or absent in horses in remission from severe asthma.

Atopic dermatitis is an allergic condition which is now more frequently recognised in horses³, with environmental allergens being the primary trigger in genetically predisposed horses. The peak age of onset is observed at around 5-6 years of age.⁴

Horses with atopic dermatitis may present with a range of clinical signs including;

• Pruritis, in some cases leading to alopecia from traumatic hair loss, scaling and subcutaneous oedema
• Urticaria (linear and horizontal forms can be seen in addition to classical hives)
• Papules (if occurring with folliculitis this would be indicative of a possible infection)
• Pustules (suggestive of a secondary infection to an underlying primary issue)

It is important to note that atopic dermatitis commonly co-exists with IBH (insect bite hypersensitivity) in the horse.

Sweet itch is the most commonly recognised and discussed equine allergy. It is caused by a hypersensitivity response to the saliva of Culicoides spp (midges). It is important to note however that it is just one of the many flying insects which can generate an allergic response, with the UK estimated prevalence of IBH being between 3-12%.⁵

IBH is particularly common in Icelandic and other native ponies and cobs, with a peak age of onset of 2-4 years. ⁶

Horses with IBH may present with a range of clinical signs including;

• Pruritis, in some cases leading to alopecia from traumatic hair loss, scaling and subcutaneous oedema (classic predilection sites are often observed)
• Urticaria (linear and horizontal forms can be seen in addition to classical hives)