Equine Allergy

EQUINE ASTHMA

 

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. 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.

 

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

 

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.

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EQUINE ATOPIC DERMATITIS

 

Atopic dermatitis is an allergic condition which is now more frequently recognised in horses, with environmental allergens being the primary trigger. The peak age of onset is observed at around 5 – 6.5 years with a range of 2 – 12 yr.3,4

 

Horses with atopic dermatitis may present with:

 

  • Pruritis – which will often lead to a range of secondary lesions including erythema, erosions, scaling and alopecia as a result of self-trauma and potentially also secondary infections

 

  • Urticaria – linear and horizontal forms can be seen in addition to classical hives

 

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

EQUINE INSECT BITE HYPERSENSITIVITY (IBH)

 

IBH (colloquially known as sweet itch) is the most commonly recognised and discussed equine allergy. It presents a significant potential welfare issue for many affected horses and ponies. IBH is caused by a hypersensitivity response to proteins within the saliva of Culicoides spp. (midges) although it is important to note that many flying biting insects can generate an allergic response. The estimated UK prevalence of IBH is between 3-12%.5 As mentioned above, IBH commonly co-exists with atopic dermatitis.

 

IBH is particularly common in Icelandic horses 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 which can be indistinguishable from atopic dermatitis from symptoms alone (although some classic predilection sites are often observed such as the tail head):

 

  • Pruritis – which will often lead to a range of secondary lesions including erythema, erosions, scaling and alopecia as a result of self-trauma and potentially also secondary infections.

 

  • Urticaria – linear and horizontal forms can be seen in addition to classical hives
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EQUINE INSECT BITE HYPER – SENSITIVITY (IBH)

 

IBH (colloquially known as sweet itch) is the most commonly recognised and discussed equine allergy. It presents a significant potential welfare issue for many affected horses and ponies. IBH is caused by a hypersensitivity response to proteins within the saliva of Culicoides spp. (midges) although it is important to note that many flying biting insects can generate an allergic response. The estimated UK prevalence of IBH is between 3-12%.5 As mentioned above, IBH commonly co-exists with atopic dermatitis.

 

IBH is particularly common in Icelandic horses 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 which can be indistinguishable from atopic dermatitis from symptoms alone (although some classic predilection sites are often observed such as the tail head):

 

  • Pruritis – which will often lead to a range of secondary lesions including erythema, erosions, scaling and alopecia as a result of self-trauma and potentially also secondary infections.

 

  • Urticaria – linear and horizontal forms can be seen in addition to classical hives
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References

    1. Davis KU, Sheats MK. Bronchoalveolar Lavage Cytology Characteristics and Seasonal Changes in a Herd of Pastured Teaching Horses. Front. Vet. Sci 2019; 6:74.
    2. Couëtil, L., Cardwell, J., Gerber, V., Lavoie, J., Léguillette, R. and Richard, E. (2016). Inflammatory Airway Disease of Horses—Revised Consensus Statement. J Vet Intern Med, 30: 503-515.
    3. Cannon A. Clinical signs of allergy, in Proceedings. 21st North American Veterinary Dermatology Forum 2006;59–61.
    4. White SD. Advances in equine atopic dermatitis, serologic and intradermal allergy testing. Clin Tech Equine Pract 2005;4:311–313.
    5. McCaig, J. (1973). A survey to establish the incidence of sweet itch in ponies in the United Kingdom. Vet Rec., 93: 444-6.
    6. White S.D. The pruritic (itchy) horse: how to approach and treat it. In: Proceedings of the 29th annual congress of the European Society and College of Veterinary Dermatology, Lausanne, 2017.