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Review

Understanding Insect Bite Hypersensitivity in Horses: A Narrative Review for Clinical Practice

by
Alexandra Nicoleta Mureșan
1,*,
Ilinca Maria Țăpuc
2 and
Daniela Mihaela Neagu
1
1
Internal Medicine Department, Faculty of Veterinary Medicine, University of Agricultural Sciences and Veterinary Medicine, 400372 Cluj-Napoca, Romania
2
Equine Hospital, Faculty of Veterinary Medicine, University of Agricultural Sciences and Veterinary Medicine, 400372 Cluj-Napoca, Romania
*
Author to whom correspondence should be addressed.
Allergies 2025, 5(3), 31; https://doi.org/10.3390/allergies5030031
Submission received: 1 May 2025 / Revised: 3 July 2025 / Accepted: 8 September 2025 / Published: 22 September 2025
(This article belongs to the Section Veterinary Allergy)
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Abstract

Insect bite hypersensitivity (IBH) is a seasonally recurrent allergic dermatitis representing one of the most prevalent dermatological conditions in horses worldwide. This condition, driven by hypersensitivity to salivary allergens of Culicoides spp., causes substantial discomfort, welfare impairment, and potentially economic loss in equine populations. The pathogenesis of IBH is complex, involving genetic predisposition, epithelial barrier dysfunction, and a skewed T-helper 2 (Th2)-mediated immune response with elevated IgE production and eosinophilic inflammation. Advances in immunogenetics and molecular immunology have improved the understanding of the disease’s multifactorial nature. Research on immunotherapy and cytokine-targeted treatments is contributing to the development of more effective therapeutic options. This review synthesizes current knowledge on the immunopathogenesis and genetic determinants of IBH and discusses both conventional and emerging strategies for its clinical management.

1. Introduction

Insect bite hypersensitivity (IBH), also known as summer eczema or sweet itch, is one of the most common allergic skin diseases in horses and ponies, particularly in temperate climates. Characterized by severe pruritic dermatitis, IBH primarily arises from hypersensitivity reactions to the saliva of biting midges of the genus Culicoides, which are prevalent in many regions, including Europe [1,2]. The most common species are Culicoides obsoletus/scoticus and C. circumscriptus [3]. These manifestations compromise equine welfare, affect performance and lead to substantial veterinary costs [4,5]. This review was conceived as a narrative, practice-oriented synthesis rather than a formal systematic review. Nevertheless, a structured, multi-database search was performed to ensure comprehensive coverage of the contemporary evidence base on equine insect-bite hypersensitivity (IBH). PubMed, Web of Science Core Collection, Scopus and CAB Abstracts were queried from 1 January 2000 to 31 March 2025. Titles and abstracts were screened for relevance to pathogenesis, genetics, diagnostics or management of IBH. Reviews, original research, case series and translational immunology papers in English, German or French were eligible; single-patient case reports and non-equine studies were excluded unless they provided mechanistic insight directly applicable to IBH. Priority was given to publications from the last ten years, seminal earlier studies still cited in current guidelines, and the most recent trials of biologic or vaccine interventions. Because the aim was a pragmatic synthesis, we did not apply a formal risk-of-bias tool.

2. Etiology

Horses can experience IBH primarily owing to Culicoides species, but other hematophagous insects such as Simulium (black flies), Tabanus (horseflies) and Stomoxys (stable flies) may elicit similar hypersensitivity reactions. Specifically, Simulium spp. can provoke IgE-mediated responses in horses, indicating potential cross-reactivity between allergens from different blood-sucking insects [6,7]. Such findings suggest that certain salivary proteins of these insects may share allergenic properties with Culicoides allergens, leading to co-sensitization in horses [7]. The immune response in horses can be influenced by multiple insect allergens, as sensitization to more than one source can exacerbate allergic symptoms. Documented evidence indicates IgE antibodies specific to allergens from both Culicoides and Simulium in horses, highlighting the need to consider a broader range of hematophagous insects when evaluating and managing IBH [6].
Environmental factors also influence insect prevalence and allergy risk, particularly in climates conducive to their proliferation [8]. Culicoides preferentially feed on horses because of their large body surface area and higher concentrations of attractants such as carbon dioxide and body heat [9]. The black flies (Simulium spp.) and horseflies (Tabanus spp.) exhibit similar feeding behaviors, preferring hosts that offer optimal feeding opportunities [10]. Adult female Simulium generally feed near aquatic environments where their larvae develop, whereas Tabanus spp. abound in rural and pasture areas where horses graze, increasing exposure [11]. Culicoides and horseflies are most active at warmer temperatures and typically feed at dawn and dusk [12,13]. Wind and humidity can further influence their feeding success by aiding navigation toward the host [11].
IBH is therefore a multifactorial disorder driven by immunological responses to insect saliva, genetic predisposition and environmental exposure.

2.1. Genetic Predisposition and Breed Susceptibility

IBH displays a notable genetic component. Heritability estimates vary among breeds, ranging from 0 to 0.36 [2]. This variation suggests a complex genetic basis for the condition, with influencing factors differing between breeds and populations. Icelandic horses, which show pronounced susceptibility to IBH when imported into Culicoides-populated environments, exhibit heightened allergic responses to novel allergens after migration [6,14]. Variants within the major histocompatibility complex (MHC) have been linked to IBH susceptibility, highlighting potential loci that modulate immune responses to insect allergens [14,15].
High-density SNP arrays have identified genomic regions associated with IgE responses to Culicoides antigens in several breeds [16]. Beyond single-nucleotide polymorphisms (SNP), copy-number variations (CNV) may also contribute to the phenotype [15,17]. Breed differences are evident: Icelandic, Friesian, Shetland and Belgian Warmblood horses display variable IBH prevalence linked to their genetic background and historical allergen exposure [2,17]. Genome-wide association studies (GWAS) and quantitative trait locus (QTL) mapping have identified regions of the equine genome associated with IBH susceptibility. These include loci linked to immune response genes, suggesting polygenic inheritance. Selection signatures in native Italian breeds likewise correlate with resistance to IBH, emphasizing the importance of genetic diversity in breeding programs [18].
Breed-specific immune responses differ further in cytokine and IgE profiles. For example, affected Shetland ponies show distinct cytokine polarization [19,20], and variations in IL-31 expression have been reported between breeds [21,22]. It has been reported that over 30% of Icelandic horses develop IBH after relocation to areas with diverse insect populations, reinforcing the idea that their immune systems may not have adapted to these new allergens [6].

2.2. Immunopathogenesis of IBH

Pathogenesis of IBH is a multifaceted process mainly rooted in an exaggerated immune response to allergens. These allergens are recognized by antigen-presenting cells (APCs), which process and present them to naïve T-helper (Th) cells near the bite. It is fundamentally a type I hypersensitivity reaction combined with type IVb, mediated by allergen-specific immunoglobulin E (IgE) and characterized by mast-cell degranulation and eosinophilic infiltration [23]. Affected horses exported from Iceland often generate IgE responses to several recombinant Culicoides allergens at the same time, implying that the reactivity stems from parallel co-sensitization events rather than cross-reactivity among the allergens [6]. Recent research using protein array technology has identified multiple Culicoides allergens contributing to the IgE response in affected horses [24]. In susceptible individuals, this interaction skews the immune response toward a T-helper type 2 (Th2) profile, characterized by the secretion of cytokines such as IL-4 and IL-13. These cytokines promote the differentiation of B cells into plasma cells that produce IgE antibodies specific to Culicoides allergens [1,25] perpetuating cutaneous inflammation and pruritus. Cytokine, chemokine, and immune cell receptor mRNA expression levels in the lesional skin of IBH horses are often increased []. The clinical symptoms are largely a consequence of this inflammatory cascade, resulting in the characteristic itchy and inflamed skin lesions observed in affected horses [25].
Recent studies have revealed that epithelial barrier dysfunction may contribute significantly to the initiation and amplification of the allergic response. Cvitaš et al. [26,27] found altered expression of keratinocyte-derived cytokines and tight-junction proteins in affected horses, suggesting compromise that facilitates allergen penetration and immune sensitization. In human beings, studies regarding asthma looked into epithelial-derived alarmins which may promote dendritic-cell activation and skew T-cell differentiation toward a Th2 profile [28] and may be of future interest in IBH as well. Immune-system dysregulation, evidenced by the presence of regulatory T cells (Tregs) and the production of pro-inflammatory cytokines, increases susceptibility to allergens from Culicoides saliva [29]. The implication of both innate and adaptive immune pathways supports the view of IBH as a multifactorial, chronic allergic condition rather than a transient hypersensitivity.
Overall, hypersensitivity disorders in horses are broadly divided into antibody-mediated (types I, II, and III) and T-cell-mediated (type IV) reactions, with clinical presentations ranging from pruritic dermatitis and urticaria to more severe systemic reactions [1,23,30]. Other types of hypersensitivity reactions include atopic dermatitis, which involves IgE-mediated responses to environmental allergens such as pollens, molds, and dust mites, and is supported by serological and skin test studies [1]. Urticaria (hives) is also common in horses and can be triggered by foods, drugs, insect stings, or environmental allergens but also atopic dermatitis; it is thought to involve both IgE and T-helper 2 cell responses, though the exact mechanisms are not fully understood [1,31]. Equine asthma is a particular case, where aeroallergens are involved [32]. Both type I (immediate, IgE-mediated) and type IV (delayed, T-cell-mediated) hypersensitivity reactions are activated, with the balance between these types varying by disease severity. Severe equine asthma is primarily associated with type I hypersensitivity, where environmental allergens trigger rapid immune responses, while mild to moderate forms often involve type IV hypersensitivity, characterized by delayed, cell-mediated immune reactions [33]. Studies using intradermal testing and allergen provocation have shown that severe cases frequently display immediate reactions typical of type I hypersensitivity, whereas milder cases may show more delayed responses, implicating type IV mechanisms [33,34]. Additionally, molecular and cellular analyses reveal that different inflammatory pathways and immune cell profiles are active in various asthma phenotypes, supporting the involvement of both classical (type I) and nonclassical (type IV) hypersensitivity processes [35,36]. Because different manifestations of hypersensitivity can occur together, it has been shown that IBH is associated with airway hyperreactivity to inhaled histamine [37]. While IBH and equine asthma are distinct conditions, they are connected by underlying immune dysregulation and a tendency for affected horses to develop more than one type of hypersensitivity reaction [38]. Less commonly, horses may develop hypersensitivity reactions to drugs or vaccines, which can manifest as either immediate (anaphylactic) or delayed (cell-mediated) responses [30].

3. Clinical Presentation and Diagnosis

The most prominent clinical manifestation of insect bite hypersensitivity (IBH) is intense pruritus, which can cause substantial discomfort in affected horses. This itching typically localizes to regions with frequent insect activity, particularly the mane (Figure 1 and Figure 2), tail, and face [1]. In response to the itch, horses often engage in vigorous scratching or rubbing, leading to self-inflicted trauma, open wounds, and an increased risk of secondary infections [39]. Early cutaneous reactions to insect bites often present as erythematous papules, which may become inflamed and swollen. Persistent scratching and biting results in localized alopecia (hair loss), primarily due to mechanical damage (Figure 2 and Figure 3). Over time, abrasions and ulcerative lesions from self-trauma can exacerbate the dermatologic condition, perpetuating a vicious cycle of itching and skin damage which can also become superinfected [40]. Chronic scratching-induced skin trauma predisposes the horse to both bacterial and fungal secondary infections and discomfort can even lead to severe weight loss (Figure 1). These infections further complicate the clinical presentation and often necessitate adjunctive antimicrobial or antifungal therapy, in addition to addressing the underlying allergic hypersensitivity.
IBH typically follows a seasonal pattern, correlating with the lifecycle of Culicoides spp. Clinical signs usually emerge during warmer months, coinciding with the peak activity of these biting midges. These manifestations are especially pronounced in horses that relocate from Culicoides-free regions to endemic areas, where their naïve immune systems are suddenly challenged by multiple allergens [6,41]. There are also increased altered behavior patterns during biting seasons and moments of the day. The chronic discomfort associated with pruritus can cause notable behavioral changes, including increased irritability, restlessness, and difficulty concentrating, particularly in performance horses. These changes can impact training, handling, and overall athletic output [42].
Histological analyses of skin biopsies from IBH lesions consistently demonstrate eosinophilic dermatitis, with prominent infiltration of eosinophils. This finding aligns with a type I hypersensitivity reaction, reinforcing the immunological basis of IBH [43].
The diagnosis of IBH in horses is multifactorial and involves a combination of clinical evaluation, serological testing for specific antibodies, and consideration of environmental factors. Given the significant impact of IBH on equine welfare, accurate diagnosis is crucial for effective management strategies.
The initial diagnosis of IBH primarily involves a comprehensive clinical evaluation. The clinical history and presenting symptoms, including intense pruritus, localized dermatitis, and alopecia, particularly around the mane, tail, and face [44] are important. The condition is typically seasonal, aligning with the activity of Culicoides midges, and findings may worsen during the summer months when these insects are prevalent [6]. The disease can occur at a young age (around 2 years old) depending on climate [2]. Clinical scoring systems have been developed to quantitatively assess the extent of the skin lesions and pruritic response in affected horses, aiding in standardizing observations for better comparisons across populations and geographic regions [41].
Allergen-specific IgE serological testing and intradermal testing may support diagnosis but require cautious interpretation due to cross-reactivity and variable specificity [45]. Different assays are employed, with the equine Allercept® test being noted as a viable alternative to intradermal skin tests, demonstrating a satisfactory correlation with the latter [46]. This serological approach allows for the identification of sensitization to multiple Culicoides allergens, but affected horses often show IgE responses to several protein components in the saliva of these insects [47]. Complex sensitization patterns appear, wherein IBH-affected horses display elevated levels of specific IgE antibodies compared to healthy control horses after exposure to Culicoides [5]. However, a significant proportion of affected horses may present with varying patterns of sensitization, complicating the predictive value of serum IgE testing alone [7]. While serological tests can indicate sensitization, they cannot definitively confirm the clinical diagnosis without correlating clinical symptoms. Skin biopsies are not recommended for diagnosis as no finding is characteristic for IBH [1].
In addition to clinical signs and serological testing, a comprehensive history that accounts for environmental factors is important for diagnosis. This includes understanding the horse’s geographic history and exposure to potential insect allergens, especially for horses that have been moved from regions without Culicoides populations [6,44].

4. Management Approaches

Providing effective shelter is important in managing IBH. Horses should be stabled during peak insect activity hours, typically during the evening and early morning when Culicoides midges are most active [48] and kept away from bodies of water which are breeding grounds for these insects [49]. Limiting access to grazing during the high-risk seasons for Culicoides activity can further reduce exposure. It may be beneficial to rotate grazing areas or house horses in insect-free zones [50]. Utilizing physical barriers such as fly sheets, masks, and other protective gear like net protection [13] can help prevent insect bites. These products should be lightweight and allow for movement while providing adequate coverage to sensitive areas as otherwise, sweating can occur, and secondary bacterial infections might develop [1]. They also require consistent use and proper fitting to avoid friction-induced lesions and discomfort. High-speed fans within stables may also reduce midge entry and biting frequency, as these insects are weak fliers [51]. By minimizing exposure to these insects through suitable repellent use, the likelihood of allergic reactions can be reduced. Commercial fly sprays are available with different combinations of synthetic and natural components. 3.6% Permethrin [51], 1% Deltamethrin [52], and 0.3% Cypermethrin-based sprays have been unsuccessful at protection against biting midges, whereas N, N-Diethyl-meta-toluamide (DEET) 15%-based products have been consistently efficient in horses [53] and other species as well [54]. Essential oils derived from plants such as citronella (Cymbopogon nardus) and lemon eucalyptus (Corymbia citriodora) oil are often recommended; however, certain combinations can attract insects [55]. Interest towards novel alternatives has led to different plant extracts such as neem (Azadirachta indica), Moroccan lavender (Lavandula dentata) or tea tree (Melaleuca alternifolia) oil [56,57,58] being used in different combinations for other insects, but their efficacy remains to be established for Culicoides biting midges. Furthermore, the potential for dermal irritation or allergic reactions as described in humans [59], in horses with pre-existing skin lesions, necessitates cautious use and patch testing.
While essential oils may reduce exposure to midges, they do not alter the underlying immunopathogenesis of IBH. As such, they are best employed as supportive tools in integrated pest management, particularly in horses with mild to moderate clinical signs or in combination with physical barriers such as fly sheets.
Stable hygiene and landscape management are also essential. Regular removal of manure at least twice daily and reduction in moist organic matter can diminish midge breeding sites [57]. Any sources of standing water such as leaking irrigation systems, overflowing troughs, or persistently wet areas on pasture should be eliminated [3]. The use of insecticides or larvicides around barns has been explored but concerns regarding environmental toxicity and non-target species have limited their widespread application [60].

5. Advances in Immunotherapy and Other Therapeutic Interventions

Recent research has shifted toward modifying the underlying immunological response. Key treatments include allergen-specific immunotherapy (ASIT) and various symptomatic therapies aimed at alleviating the symptoms associated with this condition. ASIT using Culicoides allergens through whole Culicoides extracts or recombinant allergens aims to desensitize affected horses and has shown promise in moderating the immune response and mitigating the adverse effects associated with IBH, offering a potential long-term solution for affected horses. Multi-allergen allergen immunotherapy was effective in 56% of horses with pruritic dermatitis, but horses receiving insect-specific AIT alone for pruritic dermatitis showed a lower response rate of 36% [61]. Novotny et al. [5] reported on the efficacy of component-resolved allergen microarray analysis, which enables the identification of specific allergens contributing to IBH in affected horses. Also, studies by Meulenbroeks et al. [62], Jónsdóttir et al. [63] Stefansdóttir et al. [64] demonstrated that intralymphatic or intramuscular injection of recombinant Culicoides allergens can induce T-cell tolerance and reduce clinical signs. Transgenic barley-produced Culicoides nubeculosus (Cul n) allergens Cul n 3 and Cul n 4 seem to be promising for their use in immunoassays [65].
One of the more promising advancements in the management of IBH is the use of vaccination against IL-5, a cytokine crucial for the growth and survival of eosinophils, which play a significant role in eliciting the allergic response. Research by Fettelschoss-Gabriel et al. [66] demonstrated that immunization targeting IL-5 leads to a significant reduction in eosinophils and improves disease symptoms in IBH-affected horses. A virus-like-particle vaccine (VLP) induced IL-5-specific autoantibodies, with a significant reduction in eosinophil levels in blood and clinical signs in a clinical trial on horses [67]. Vaccinating against IL-31, a cytokine associated with itch in allergic diseases showed successful mitigation of IL-31-mediated pruritus, leading to a decrease in self-inflicted trauma and associated skin lesions [21]. However, the sheer size of horses prohibits the use of these VLP vaccines from a financial point of view [68].
In addition to immunotherapy, corticosteroids and antihistamines are routinely prescribed as adjunct therapies both systemically and topically to manage the acute inflammatory responses and itching prevalent in IBH cases [1]. However, antihistamines such as Cetirizine in horses have no real effect, as IBH is not only a histamine mediated pathology [69]. Systemic prednisolone or dexamethasone have not been investigated in the context of IBH but are commonly prescribed in pruritic disease of horses [70]. Side effects of these drugs are often invoked but a retrospective study evaluating glucocorticoid therapy as a potential risk factor for laminitis found that significant associations were identified with breed, body weight, and the presence of an underlying endocrinopathy, rather than with steroid administration itself [71]. Clinical practices vary widely, with treatments often tailored to the individual horse based on allergen-specific serological testing and clinical presentation.
Olacitinib (Apoquel®), a Janus Kinase inhibitor used in allergies in other species [72], while administered empirically to equines with IBH, has only been recently investigated for its pharmacokinetic properties [73].
Owners and veterinarians alike have found great interest in topical use of different formulations. Various essential oil combinations have had good results in reducing clinical signs [74], and skin repair takes place faster with omega-3-rich ointments [75]. Chamomile (Matricaria recutita L.) oil can aid in pruritic conditions through anti-inflammatory effects in human beings [76] and has been preferred by owners as a healing aid. Tea tree oil has been used as a treatment in other pruritic skin conditions with success [77].

6. Nutritional Support

The role of nutrition in modulating immune responses is a developing area in the context of IBH. Supplementation with omega-3 fatty acids, particularly EPA and DHA from marine sources, has demonstrated anti-inflammatory effects and they are better incorporated into tissue compared to flaxseed supplements [78]. While specific data in IBH are limited, anecdotal reports and small-scale studies suggest that omega-3 supplementation may reduce pruritus severity and improve coat quality in affected horses [79].
Other nutraceuticals, including vitamin E, zinc, and selenium in sunflower oil, may support skin-barrier function and antioxidant defenses, although evidence for efficacy in IBH is currently inconclusive [80]. In human beings, Rooibos tea (Aspalathus linearis) is considered safe and rich in quercetin, a known antiallergic, and is being used empirically by owners in their horse’s feed [81]. An overview of all therapeutic strategies can be found in Table 1.

7. Comparisons to Other Species and Equids

Insect-bite hypersensitivity (IBH) is not unique to horses. Comparable, IgE-mediated conditions occur in several domestic animals and in humans, providing useful clinical and mechanistic parallels.
Dogs—mosquito-bite hypersensitivity (MBH). Pruritic papules and wheals appear on sparsely haired areas (pinnae, groin, facial folds). Lesions peak within hours of Aedes exposure and histology shows eosinophilic perivascular dermatitis, mirroring equine IBH [82]. The first contact prompts B cells to switch toward immunoglobulin E (IgE). The newly formed IgE coats resident mast cells, priming them for activation; subsequent allergen contact triggers degranulation and the release of prostaglandins and leukotrienes, leading to the hallmark redness and swelling at the bite site [83]. The condition remains extremely rare.
Sheep—Culicoides-associated dermatitis. Although much less common than in equids, signs are similar, depending on Culicoides species, and due to moderate superficial interstitial-to-perivascular infiltrate characteristic of chronic, widespread eosinophilic dermatitis this profile most strongly points to a hypersensitivity reaction [84].
Human beings—In human beings, bites from hematophagous insects frequently sensitize individuals to salivary antigens, and in more than 90% of people trigger local hypersensitivity—appearing either as an IgE-mediated wheal-and-flare within minutes or as a T-cell-driven papular reaction that develops later [85]. In mosquito bites, histamine appears central to the reaction, released either directly by components of mosquito saliva, by IgE-dependent mast-cell activation, or via IgE-independent pathways. Clinically, the bites elicit a sequence of reactions—an immediate phase, a delayed phase, and, in certain individuals, an extensive local response. Within minutes, a circular wheal 2–10 mm in diameter, surrounded by erythema, appears and reaches its maximum at 20–30 min. The delayed phase follows: pruritic papules of similar size emerge, peak after 24–36 h, and then subside gradually over the next few days. In individuals with mosquito allergies, larger wheals can appear in the first minutes [86] and in some individuals, severe symptoms such as vomiting and fever with IgE-positive testing for mosquitoes can manifest through Skeeter syndrome [87]. Long-term exposure to mosquito bites might induce natural desensitization, in contrast to horses [88].
Current research on insect bite hypersensitivity IBH in equids focuses exclusively on horses, with little published data on differences in IBH between horses and other equids such as mules, donkeys, or zebras. One recent paper has presented an association between allergic dermatitis and Culicoides ocumarensis Ortiz in donkeys in Brazil [89]. It is hypothesized that in zebras, their stripes act as fly (Tabanus spp.) protectors [90], whether this applies to Culicoides or other information on allergic dermatitis is unknown.
Table 1. Overview of the therapeutic approach in IBH with dose regimes.
Table 1. Overview of the therapeutic approach in IBH with dose regimes.
Treatment/
Class		Formulation and
Concentration		Suggested Dose/FrequencyEvidence Level and Key CommentsKey Refs.
Prednisolone (systemic GC)Oral tablets/
powder		1.5–2 mg kg q24 h × 7–10 d → taper to 0.5 mg kg q48 hWidely used; no prospective trials; long-term monotherapy discouraged, low risk for laminitis[40,91,92]
Dexamethasone (systemic GC)Oral solution/inj.0.02–0.10 mg kg q24 h (load) → 0.01–0.02 mg kg q48–72 hUseful for prednisolone-non-responders or severe cases[40,91,92]
Permethrin (pour-on)3.6%20 mL topline; repeat q14 dSmall field trial: non-sig. midge reduction; clinical value possible[51]
Permethrin (spray)2%Spray to coat q14 dRCT: ↓ lesion scores vs. control[1]
Cypermethrin (spray)0.15%Spray; protection ≈ 2 hNet study: brief repellency only[1]
Cypermethrin (spray)1%Label q24 hNo controlled data[1]
Deltamethrin (spray)1%Label directionsIneffective at repelling Culicoides[52]
DEET (spray/lotion)15%Apply; repels ≈ 6 hProven midge repellency[53]
Citronella + lemon-eucalyptus5–10% blendApply q2–4 hField study: ineffective or attractive[55]
Essential-oil herbal sprayCamphor/lemongrass/
may-chang/peppermint/
patchouli		Daily × 28 dDBPC cross-over: 95% owner-reported relief[1]
Omega-3/6 topical creamFatty acids + humectantsApply q24 hSplit-body RCT: ↓ lesions on treated side[66]
Phytogenic ointmentPlant extractsApply bid × 21 dDBPC: ↑ owner comfort vs. placebo[93]
CetirizineOral tablets0.4 mg kg−1 PO bid × 3 wksPlacebo-controlled: no benefit[69]
ChlorphenamineOral0.1–0.5 mg kg−1 PO bidHistology benefit; clinical effect untested[1]
DoxepinOral0.75–1 mg kg−1 PO bidAnecdotal only; competition restrictions[1]
DiphenhydramineOral1–2 mg kg−1 PO q8–12 hVariable response; competition restrictions[1]
Linseed (flax) oiln-3 source1 lb/1000 lb BW q24 h × 6 wkRCT: no sig. change; some owner improvement[79]
Flaxseed (ground)DietSame as above × 42 d↓ intradermal reactions in AD horses[79]
Evening-primrose + fish oil (80:20)Oral20 g day−1 × 13 wk10/14 IBH horses ≥good response[1]
Sunflower oil + vit/AA/peptidesOral30 d courseDBPC n = 50: placebo limbs worsened; owner scores NS[80]
PentoxifyllineOral10–15 mg kg−1 PO bid (may ↑ to 30 mg kg−1 day−1)No IBH trials; empiric use[1,94]
Oclacitinib (off-label)Oral0.1–0.25 mg kg−1 PO q24 hSmall RCT: pruritus ↓ at 0.25 mg kg−1[95]
Hydrocortisone aceponate 0.058% sprayTopical2–4 sprays to focal lesions q24 hMinimal systemic absorption; useful for mane/tail[96]
Fly sheet/mask—Continuous wear; remove dailyEffective barrier; heat/sweat risk in humid climates[1]
Stable fans—Airflow > 2.5 m s−1 dusk/night↓ Culicoides landings[48]
Allergen-specific immunotherapy (SC)Aqueous or
alum-precipitated
allergens		Escalating course → maintenance ~20,000 PNU mL q21 d~70% improve; customize mix; evaluate ≥12 mo[1,97]
Abbreviations: GC = glucocorticoid; RCT = randomized controlled trial; DBPC = double-blinded placebo-controlled; NS = not significant; BW = body weight; PNU = protein nitrogen units.

8. Future Perspectives and Integrated Approaches

Given the multifactorial etiology of IBH, integrated management approaches that address genetic, environmental, and immunological factors are essential. Breeding strategies that incorporate genetic resistance markers may contribute to long-term control. Early-life exposure to endemic allergens may play a protective role, suggesting a potential window for immune tolerance induction and future research in this direction. Large-scale studies are needed to define which allergens are important in different geographical locations.
Leveraging advancements in biomarker research, such as the identification of specific IgE and cytotoxic factors associated with IBH, would refine diagnostic accuracy. Biomarker identification could lead to the development of rapid diagnostic tests that offer timely results and allow for quick intervention strategies. A recent study identified different significant increase in associated cytokine, chemokine, and immune cell receptor mRNA expression in the lesional skin of horses with IBH [98].
Ongoing research into virus-like particle (VLP) vaccines targeting these cytokines aims to provide a more effective and long-term solution for managing allergic reactions in IBH-affected horses. Vaccines designed to elicit specific immune responses and decrease eosinophil activation hold potential for significantly improving horse welfare.
Unfortunately, medical treatment in these animals does not truly address the underlying pathology and has not shown clear long-term benefit in patients. Treatment is limited to either prevention or alleviation of symptoms but is not curative.
Complementary therapies, including herbal repellents, essential oils, and nutritional modulation of immune function, are increasingly explored, although robust clinical evidence remains limited. Future research should aim to evaluate these modalities through standardized, controlled trials and integrate them with immunological interventions.

9. Conclusions

IBH represents a significant welfare concern and economic burden in the equine industry. It results from a complex interplay of genetic predisposition, epithelial barrier integrity, and immune dysregulation. While conventional treatments remain primarily palliative, advances in immunotherapy, cytokine targeting, and allergen-specific vaccination hold promise for disease modification. Complementary and environmental strategies, especially insect avoidance, natural repellents, and immune-supportive nutrition, can play a role in reducing allergen exposure and supporting overall skin health. A comprehensive, multimodal approach that combines these modalities with targeted immunological therapies represents the most promising pathway toward effective and sustainable IBH management.

Author Contributions

A.N.M. conceived and designed the review, A.N.M., I.M.Ț. and D.M.N. contributed to the writing of the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

No new data was created.

Acknowledgments

The authors would like to thank Alexandra Preda for the images of her affected horse.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Figure 1. Weight loss in a horse affected by severe IBH. Notice the lacking mane due to rubbing and breakage of the hair.
Figure 1. Weight loss in a horse affected by severe IBH. Notice the lacking mane due to rubbing and breakage of the hair.
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Figure 2. Excoriations and crusts as well as hyperkeratosis in the mane area in the same animal as above.
Figure 2. Excoriations and crusts as well as hyperkeratosis in the mane area in the same animal as above.
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Figure 3. Vigorous scratching of the tail brought on by local increased insect activity.
Figure 3. Vigorous scratching of the tail brought on by local increased insect activity.
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Mureșan, A.N.; Țăpuc, I.M.; Neagu, D.M. Understanding Insect Bite Hypersensitivity in Horses: A Narrative Review for Clinical Practice. Allergies 2025, 5, 31. https://doi.org/10.3390/allergies5030031

AMA Style

Mureșan AN, Țăpuc IM, Neagu DM. Understanding Insect Bite Hypersensitivity in Horses: A Narrative Review for Clinical Practice. Allergies. 2025; 5(3):31. https://doi.org/10.3390/allergies5030031

Chicago/Turabian Style

Mureșan, Alexandra Nicoleta, Ilinca Maria Țăpuc, and Daniela Mihaela Neagu. 2025. "Understanding Insect Bite Hypersensitivity in Horses: A Narrative Review for Clinical Practice" Allergies 5, no. 3: 31. https://doi.org/10.3390/allergies5030031

APA Style

Mureșan, A. N., Țăpuc, I. M., & Neagu, D. M. (2025). Understanding Insect Bite Hypersensitivity in Horses: A Narrative Review for Clinical Practice. Allergies, 5(3), 31. https://doi.org/10.3390/allergies5030031

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