Pathogenesis, diagnostics and management in practice
Pruritus is a common reason for presentation in horses and a diagnostic challenge. The main allergic causes are insect bite hypersensitivity (IBH, sweet itch), equine atopic dermatitis (EAD) and urticaria. Accurate differentiation is the basis for veterinary management.
Pathogenesis
Allergic skin diseases result from dysregulation of the immune system against otherwise harmless antigens.
- IgE-mediated reaction (type I): immediate reaction with histamine release. It plays a role in urticaria and in IBH.
- Cell-mediated reaction (type IV): delayed reaction that may contribute in chronic cases. The evidence is inconsistent.
- Atopy: environmental allergens play a central role in EAD. Clinically, EAD can overlap with sweet itch and may be underdiagnosed.
In IBH, biting midges of the genus Culicoides are the classic trigger (Schaffartzik et al. 2012).
Clinical presentation
The leading signs are pruritus and urticaria. The distribution of lesions provides diagnostic clues:
- mane crest and tail base: classic in Culicoides-related IBH
- ventral midline: typical for blackflies (Simulium)
- head and distal limbs: suggests atopy or contact allergy
- generalised wheals: acute urticaria, for example due to drugs, feed or inhaled allergens
Chronic cases show alopecia, crusting, lichenification and excoriations. Secondary bacterial and fungal infections are possible and intensify the pruritus.
Diagnostics
- History and clinical examination: seasonality, husbandry, insect exposure, feed, medication
- Exclusion of differentials: ectoparasites (for example lice, mites), dermatophytosis, bacterial folliculitis
- Elimination diet: over several weeks if a food allergy is suspected (rare)
- Intradermal testing: an established method for identifying environmental allergens; interpretation requires experience. Serological tests are viewed controversially.
- Cytology and culture: if secondary infection is suspected
- Biopsy: helpful for ruling out autoimmune disease, for example pemphigus foliaceus
Veterinary management
Management of allergic skin disease is multimodal. A standardised approach is rarely successful. The decision lies with the treating veterinarian. Components include (Marsella 2024):
- Reducing allergen exposure: in IBH, for example, insect rugs, stable management around midge activity times, keeping away from water
- Antihistamines: used in urticaria; evidence for pruritus is limited
- Glucocorticoids: effective for short-term control of pruritus, but to be used with caution (risk of laminitis, particularly in horses with equine metabolic syndrome)
- Allergen-specific immunotherapy (ASIT): an option in EAD
Skin care and hygiene
In allergic skin disease the skin barrier is impaired. Crusts, exudate and dirt on damaged skin are part of the overall picture. Cleansing and care of the affected areas can accompany veterinary assessment and management.
Conclusion for practice
Allergic skin diseases in horses are chronic conditions that require ongoing management. The basis is careful diagnostics, allergen management and an overall plan agreed with the veterinarian. Skin care can complement that plan.
References
- Schaffartzik A, Hamza E, Janda J, Crameri R, Marti E, Rhyner C. Equine insect bite hypersensitivity: what do we know? Vet Immunol Immunopathol. 2012;147(3-4):113-126. PMID: 22575371.
- Marsella R. Pruritic Horse. Vet Clin North Am Equine Pract. 2024;40:219-235.