Insect Bite Hypersensitivity (IBH) in Horses

Insect Bite Hypersensitivity (IBH), also known as summer eczema or sweet itch, is the most common allergic skin disease in horses. It is a chronic, seasonal hypersensitivity reaction caused by bites from insects, such as Culicoides spp. (biting midges) and, in some cases, such as Simulium (blackflies) and Stomoxys (stable flies). This IgE-mediated hypersensitivity leads to severe pruritus, self-trauma, and secondary skin infections, significantly affecting the welfare and performance of affected horses. The condition is particularly prevalent in temperate and subtropical climates, where these insects thrive.

Pathophysiology and Immunological Response

IBH is an immune-mediated Type I and Type IV hypersensitivity reaction triggered by salivary allergens from biting midges. Horses that develop IBH exhibit increased production of allergen-specific IgE, leading to mast cell degranulation and histamine release upon re-exposure to insect bites. This results in intense pruritus, erythema, edema, and eventual skin thickening due to chronic inflammation. The reaction also involves T-cell activation, which contributes to delayed-type hypersensitivity and sustained tissue damage.

Genetic predisposition plays a significant role in IBH, with certain breeds such as Icelandic horses, Friesians, and Welsh ponies showing increased susceptibility. Environmental factors, including pasture location, insect exposure, and climate, also influence disease expression and severity.

Clinical Signs

IBH typically presents as seasonal pruritus, occurring in spring and summer when Culicoides activity peaks. The hallmark clinical signs include:

Severe pruritus, leading to rubbing, biting, and self-inflicted trauma.
Lesions localized primarily to the mane, tail, and dorsal midline, but can also extend to the face, ventral abdomen, and limbs.
Alopecia, excoriations, hyperkeratosis, and lichenification in chronic cases.
Secondary bacterial or fungal infections due to persistent skin damage and barrier disruption.

Diagnosis of Insect Bite Hypersensitivity

The diagnosis is based on history, clinical signs, seasonality, and ruling out other pruritic conditions such as lice, mites, atopic dermatitis, or food allergies.

The offending allergens can then be identified and confirmed by serum testing or Intradermal testing.

The PAX Horse Allergy Xplorer (PAX) is an advanced serum-based allergy testing system designed to identify allergen-specific IgE antibodies in horses, providing a comprehensive profile of hypersensitivity reactions. Unlike traditional intradermal allergy testing (IDAT), which requires in vivo skin injections and sedation, PAX is a minimally invasive blood test that allows for detailed allergen mapping using cutting-edge molecular allergology.

The PAX test utilizes recombinant allergen technology and nano-bead immunoassays to detect IgE antibodies against a broad range of natural and molecular allergens, including Culicoides spp., stable flies (Stomoxys), black flies (Simulium), dust mites, pollens, molds, and food allergens. By identifying specific molecular allergen components, rather than whole allergen extracts, PAX helps distinguish between primary sensitization and cross-reactivity, improving diagnostic accuracy.

Management and Treatment Strategies

Managing IBH requires a multimodal approach, including insect control, immunotherapy, anti-inflammatory therapy, and skin barrier support.

1. Insect control and Enviromental Management

Preventing insect exposure is the cornerstone of IBH management. Key strategies include:

Stable confinement during peak midge activity (dawn and dusk) to reduce exposure.

Fine mesh fly sheets and hoods to provide physical protection.

Application of insect repellents containing permethrin, pyrethroids, or citronella, ensuring frequent reapplication.

Use of fans in stables to disrupt midge flight patterns.

Elimination of standing water and manure management to reduce breeding grounds.

2. Pharmacologycal and Immunomodulatory Therapy

Glucocorticoids such as dexamethasone or prednisolone are commonly used to suppress inflammation and pruritus in acute flare-ups. However, long-term use should be minimized due to the risk of laminitis and metabolic complications.

Allergen-Specific Immunotherapy (AIT), also known as desensitization therapy, is a long-term treatment designed to reduce hypersensitivity to specific allergens by gradually retraining the immune system. In horses with Insect Bite Hypersensitivity (IBH), AIT involves the administration of increasing doses of Culicoides-derived allergens over time to induce immune tolerance. The goal is to shift the immune response from a Type I hypersensitivity reaction (IgE-mediated) toward a more balanced immune profile, favoring regulatory T-cell activation and increased production of allergen-specific IgG antibodies (particularly IgG4 and IgG1). These blocking antibodies compete with IgE for allergen binding, reducing mast cell degranulation and histamine release upon subsequent insect exposure. Unlike symptomatic treatments such as corticosteroids, which only suppress inflammation, AIT modifies the underlying immune response, potentially leading to long-term clinical improvement and reduced dependency on medication.

The effectiveness of AIT varies, with some horses experiencing significant symptom reduction after several months to a year of therapy, while others may show only partial improvement. Success rates can be influenced by factors such as cross-reactivity among Culicoides species, individual immune variability, and the degree of pre-existing sensitization.

3. Skin Barrier Support and Nutritional Supplementation

Supplementing with essential fatty acids (omega-3 and omega-6), zinc, and vitamin E may help enhance the skin barrier, reduce inflammation, and promote coat health.