Long-term management of horses with atopic dermatitis in southeastern England: a retrospective questionnaire study of owners' perceptions

Pruritic Horse: Approach to Allergic Skin Diseases in Horses

Allergy to insects is the most common skin allergy in horses. Pruritus in affected patients can be extreme. Face, ears, mane, and tail area are commonly affected areas. Diagnosis of insect bite hypersensitivity (IBH) is clinical and is based on history, clinical signs, and response to repellents. Allergy tests are not to be used for diagnostic purposes. Currently, there is no specific treatment for IBH other than insect avoidance, treatment of secondary infections, and symptomatic relief of pruritus. Many allergic horses become also sensitized to pollens. For these patients, allergen specific immunotherapy is beneficial.

Eosinophils Play a Surprising Leading Role in Recurrent Urticaria in Horses

Urticaria, independent of or associated with allergies, is commonly seen in horses and often shows a high reoccurrence rate. Managing these horses is discouraging, and efficient treatment options are lacking. Due to an incidental finding in a study on horses affected by insect bite hypersensitivity using the eosinophil-targeting eIL-5-CuMV-TT vaccine, we observed the prevention of reoccurring seasonal urticaria in four subsequent years with re-vaccination. In an exploratory case series of horses affected with non-seasonal urticaria, we aimed to investigate the role of eosinophils in urticaria. Skin punch biopsies for histology and qPCR of eosinophil associated genes were performed. Further, two severe, non-seasonal, recurrent urticaria-affected horses were vaccinated using eIL-5-CuMV-TT, and urticaria flare-up was followed up with re-vaccination for several years. Eotaxin-2, eotaxin-3, IL-5, CCR5, and CXCL10 showed high sensitivity and specificity for urticarial lesions, while eosinophils were present in 50% of histological tissue sections. The eIL-5-CuMV-TT vaccine reduced eosinophil counts in blood, cleared clinical signs of urticaria, and even prevented new episodes of urticaria in horses with non-seasonal recurrent urticaria. This indicates that eosinophils play a leading role in urticaria in horses, and targeting eosinophils offers an attractive new treatment option, replacing the use of corticosteroids.

Efficacy and Safety of Subcutaneous Allergen-Specific Immuno-Therapy in Horses with Allergic Cutaneous and Respiratory Diseases-A Systematic Review

Allergen-specific immunotherapy (AIT) is the only current intervention that has the ability to modify the immune response toward a tolerogenic state. This study aimed to assess the efficacy and safety of AIT in horses with allergic diseases in a systematic manner. Three databases were searched to identify articles reporting clinical outcomes and adverse events associated with AIT. The articles were evaluated for beneficial responses to AIT, defined as a ≥50% reduction in clinical signs, and clinical remission. Horses with respiratory diseases, urticaria, and pruritic dermatitis receiving insect monotherapy or multi-allergen AIT were included. All adverse events were graded, and analytical and confounding biases were assessed. The results showed that multi-allergen AIT had a beneficial response in 75% of horses with respiratory diseases, 88% with urticaria, and 56% with pruritic dermatitis. However, horses treated solely with insect AIT for pruritic dermatitis had a lower response rate (36%). Self-limiting local reactions were the most common adverse events, with systemic reactions grade II accounting for 11% of reported events. Analytical and confounding biases were identified as major limitations in the available studies. Further research is needed to address these biases and provide stronger evidence on the efficacy and safety of AIT in horses with allergic diseases.

Equine allergic skin diseases: Clinical consensus guidelines of the World Association for Veterinary Dermatology

BACKGROUND

Allergic skin diseases are common in horses worldwide. The most common causes are insect bites and environmental allergens.

OBJECTIVES

To review the current literature and provide consensus on pathogenesis, diagnosis, treatment and prevention.

MATERIALS AND METHODS

The authors reviewed the literature up to November 2022. Results were presented at North America Veterinary Dermatology Forum (2021) and European Veterinary Dermatology Congress (2021). The report was available to member organisations of the World Association for Veterinary Dermatology for feedback.

CONCLUSIONS AND CLINICAL RELEVANCE

Insect bite hypersensitivity (IBH) is the best characterised allergic skin disease. An immunoglobulin (Ig)E response against Culicoides salivary antigens is widely documented. Genetics and environmental factors play important roles. Tests with high sensitivity and specificity are lacking, and diagnosis of IBH is based on clinical signs, seasonality and response to insect control. Eosinophils, interleukin (IL)-5 and IL-31 are explored as therapeutic targets. Presently, the most effective treatment is insect avoidance. Existing evidence does not support allergen-specific immunotherapy (ASIT) using commercially available extracts of Culicoides. Hypersensitivity to environmental allergens (atopic dermatitis) is the next most common allergy. A role for IgE is supported by serological investigation, skin test studies and positive response to ASIT. Prospective, controlled, randomised studies are limited, and treatment relies largely on glucocorticoids, antihistamines and ASIT based on retrospective studies. Foods are known triggers for urticaria, yet their role in pruritic dermatitis is unknown. Recurrent urticaria is common in horses, yet our understanding is limited and focussed on IgE and T-helper 2 cell response. Prospective, controlled studies on treatments for urticaria are lacking. Glucocorticoids and antihistamines are primary reported treatments.

Approach to the pruritic horse

Pruritus in the horse may be due to several causes, the most common being a hypersensitivity response to salivary proteins in the Culicoides genera, which may coexist with atopic dermatitis, also known as an environmental allergy to pollens, molds, dust, storage mites, etc. Less common etiologies are food allergy and contact allergy, the latter often caused by owners applying various products to the skin. Other ectoparasites, such as Chorioptes mites, may also initiate pruritus. Secondary bacterial infections (usually Staphylococcus spp) may be pruritic in and of themselves. This article reviews the questions that need to be asked of owners to obtain a relevant history, always important for any organ system, but perhaps none more so than the skin. The various clinical findings such as alopecia and crusts and their location on the horse, diagnostic methods such as intradermal or serum testing for allergies, and subsequent hyposensitization are also discussed. Therapeutic options currently available for the potential underlying diseases, in particular for the hypersensitivity reactions to Culicoides spp or environmental allergens, are reviewed with the studies of hyposensitization over the last 40 years, as well as medications that may be effective. While the most common causes of pruritus in the horse are known, the current understanding of the pathophysiology still needs to be investigated, and consequently, the most effective treatments for those causes need to be improved. Newer research is discussed that may eventually add to the diagnostic and therapeutic options currently available for the pruritic horse.

Atopic Dermatitis in Domestic Animals: What Our Current Understanding Is and How This Applies to Clinical Practice

Atopic dermatitis is a clinical syndrome that affects both people and animals. Dogs closely mimic the complexity of the human skin disease, and much progress has been made in recent years in terms of our understanding of the role of skin impairment and the identification of new treatments. Cats and horses also develop atopic syndromes which include both cutaneous and respiratory signs, yet studies in these species are lagging. It is now recognized that atopic dermatitis is not a single disease but a multifaceted clinical syndrome with different pathways in various subgroups of patients. Appreciating this complexity is clinically relevant as we develop more targeted treatments which may work well in some patients but not in others. Different phenotypes of atopic dermatitis have been described in dogs, and it is possible that phenotypes related to breed and age may exist in other animals similar to how they are described in people. The awareness of different mechanisms of disease leads to the desire to correlate different phenotypes with specific biomarkers and responses to treatment. In this review, the current understanding and updated information on atopic syndrome in animals are described, highlighting opportunities for further studies in the future.

Discrepancies in the bilateral intradermal test and serum tests in atopic horses

Background

In equine atopic patients intradermal testing (IDT) and immunoglobulin (Ig)E serology are used frequently. There is little evidence regarding the reproducibility of the IDT and IgE serology in horses.

Objectives

To compare the results of a simultaneously performed IDT on the left and right side of the neck in atopic horses, and to compare these results with allergen‐specific IgE serology.

Animals

Ten equine patients from a university hospital population with chronic urticaria and/or pruritus.

Methods and materials

The IDT was performed using 16 allergens and the results were evaluated after 30 min, 1, 4 and 24 h. Thirteen allergens also were analysed in duplicate with two monoclonal allergen‐specific IgE enzyme‐linked immunosorbent assays (ELISA).

Results

Good agreement (Kappa > 0.6) between left and right IDT was found only for Dermatophagoides farinae, Lepidoglyphus destructor, birch pollen mixture and perennial rye at 30 min, birch pollen mixture at 1 h, and Acarus siro and nettle and common mugwort mixture at 4 h. The bilateral comparison of the other allergens and even the same allergens at other time points showed little or no concordance between left and right IDT. The interlaboratory comparison between both ELISAs, and the comparison between the ELISAs and IDT, showed a good agreement for two of 13 allergens: D. farinae and Dermatophagoides pteronyssinus.

Conclusions and clinical importance

Based on these preliminary data, IDT and IgE serological test results should be interpreted with great care and further studies are needed to indicate the clinical relevance of these findings.

Evaluation of multiple allergen simultaneous (sIgE) testing compared to intradermal testing in the etiological diagnosis of atopic dermatitis in horses

Although intradermal testing (IDT) is commonly used in the etiological diagnosis of allergies, in vitro testing for specific IgE (sIgE) is an attractive alternative. Currently, new laboratory techniques in veterinary allergological practice, including multiple allergen simultaneous tests (MASTs), gradually supersede in vivo tests. Both, serological (sIgE) and IDTs in fourteen atopic Malopolski horses were performed. Correlation and agreement between test results were evaluated. Receiver operating characteristic analysis showed that sIgE to Acarus siro had the best diagnostic performance (Area under the ROC curve [AUC] = 0.969), followed by Dermatophagoides pteronyssinus (AUC = 0.844), Dermatophagoides farinae (AUC = 0.813) and Tyrophagus putrescentiae (AUC = 0.803). A significant positive correlation between IDT and MAST was found for A. siro (r_S = 0.870; p = 0.00005), and D. farinae (r_S = 0.657; p = 0.011). There was significant moderate agreement for 2 of 5 allergens, A. siro (κ = 0.569) and D. farinae (κ = 0.485) in semiquantitative assessment and significant fair to substantial agreement for 3 of 5 allergens, D. pteronyssinus (κ = 0.689), A. siro (κ = 0.569), D. farinae (κ = 0.432) in dichotomic assessment. Sensitivity ranged from 44% to 89%, depending on the allergen, while specificity was significantly higher for all allergens in MAST (60%–100%); the mean accuracy was 73% (manufacturer cut-off) and 77.4% (optimal cut-off) based on the Youden index. Compared with IDT, serological MAST showed good detection performance for 60% allergen sIgE in dichotomic assessment with substantial diagnostic capability, but careful clinical interpretation is needed for some allergens.

Co-reactivity between related and unrelated environmental allergens in equine allergen-specific IgE serology testing in the UK

BACKGROUND

Identification of environmental allergens in horses with allergic disease facilitates allergen avoidance and targeted immunotherapy.

HYPOTHESIS/OBJECTIVES

To evaluate allergenic co-reactivity between 44 environmental allergens.

ANIMALS

Horses with suspected allergic disease (n = 344) whose sera were submitted for environmental allergen testing.

METHODS AND MATERIALS

Allergen-specific IgE serological assays were performed using 44 allergens divided into six taxonomically related groups: house dust/storage mites, moulds, insects, grass, tree and weed pollens. Using pairwise comparisons, odds ratios (ORs) were calculated for each environmental pair to determine if there was increased or decreased likelihood of a positive result for one allergen, given a positive result to another. The OR significance was set (using Holm-Bonferroni correction) at P < 0.00006 for all horses (n = 344) and P < 0.00005 for horses with at least one positive reaction (n = 239). Using one-way ANOVA with Tukey's post hoc tests (significance at P < 0.05), differences in mean log e ORs between three groups, taxonomically related allergens with a statistically significant association (related-associated), related allergens lacking a significant association (related-nonassociated) and unrelated allergens were tested.

RESULTS

Statistically significant associations were found between both related and unrelated allergen pairs, the former being more frequent. For all horses (n = 344) and horses with at least one positive reaction (n = 239), co-reactivity ranged from 100% (grasses) to 0% (moulds). The weeds group was exceptional in having more co-reactions with another group (grasses).

CONCLUSIONS AND CLINICAL IMPORTANCE

Co-reactivity was shown within and between certain related allergen groups. Further studies are required to determine whether this is the result of antigenic cross-reactivity.