Review of Recent Equine Allergy Publications by Professor Richard E W Halliwell

Professor Richard E W Halliwell outlines here three interesting and recent equine allergy publications. The first paper evaluates the immunological responses to allergen-specific immunotherapy in horses with IgE associated dermatological disease. The second discusses the long term management of horses with atopic dermatitis in south-eastern England. Then finally the third paper, by our very own Johanna Forsyth, Richard Halliwell and Robert Harrand looks at the cross reactivity between allergens in equine allergen specific IgE testing.


Longitudinal evaluation of immunological responses to allergen-specific immunotherapy in horses with IgE associated dermatological disease, a pilot study

Noel E Radwanski, Daniel O Morris, Raymond C Boston, Rosario Cerundolo and Kenneth W Lee

Vet Dermatol 2019; 30: 255-e78

Nineteen horses were studied with a “conditional” diagnosis of atopic dermatitis (AD). Three showed non-pruritic urticaria, eight had urticaria with pruritus, eight suffered from non-specific pruritic skin changes and one was pruritic with eosinophilic granulomas. Fourteen were seasonally affected and five showed perennial signs with seasonal exacerbation. All were given allergen-specific immunotherapy (ASIT) based upon results of either an ELISA for allergen-specific IgE or intradermal testing (IDT). The response was monitored by an owner assessed 10-point visual analogue scale (VAS) and the serum levels of allergen-specific IgE and IgG were assayed at four-monthly intervals.

The main conclusions were that ”overall, owner-assessed clinical improvement was evident in 13 of the 17 horses (76.5%) that completed the first 12 months of ASIT, whereas no substantial clinical benefit was noted in the remaining four animals. After 24 months of therapy, nine of 11 (81.8%) were judged to have benefitted from ASIT”. Those that did not respond fully required less anti-pruritic therapy to effect control. The response was the same whether allergens were selected based upon results of IDT or ELISA, and there was good concordance between the results of both. Successful ASIT resulted in a marked fall in allergen-specific IgE, and some horses showed a rise in the levels of IgG. The injections were generally well-tolerated although four horses showed post-injection swellings at the site. These were avoided by adjusting the dosage and therapy was continued. Such cases highlight the importance of regular client communications during ASIT induction protocols as these horses might not have continued to receive therapy without dosage adjustments. One horse developed a clotting abnormality that was clinically asymptomatic but observed on blood sampling. This was not further characterized and resolved on cessation of therapy.

Equine AD has not been well characterized, and there are no accepted diagnostic criteria. Thus the diagnosis is made by exclusion of other known causes of urticaria and/or pruritus. It can be difficult to distinguish from insect-bite hypersensitivity except in the case of sweet-itch (Culicoides hypersensitivity) which has a characteristic distribution. The spectrum of the clinical signs in the patients selected for therapy in this series was very similar to those in a study in California (Stepnick, CT, et al Vet Dermatol 2012; 23: 29-7e). In the latter publication, of 54 horses diagnosed with AD, 28 were suffering from non-pruritic recurrent urticaria, eight from pruritus and 18 from a combination. Of 41 cases that were treated with ASIT, 19 were controlled by this alone. In 15 cases the ASIT was discontinued after six months to eight years (mean 2.2 years) as the clinical signs had resolved. Of these 15, 10 suffered no recurrence of the clinical signs, but in five cases the condition recurred after a mean of two years. Thus it seems that ASIT may offer an actual cure in some cases of AD as is the case in dogs. Post-injection swelling was noted in five cases (16%) in this latter study. Neither of these studies were blinded or placebo controlled. In the only published study that was so controlled, no significant efficacy was shown when compared to the placebo. However, when the initial grade was used as the control, statistically significant improvement was attributable to ASIT during the blinded phase of the study (Rosenkrantz WS et al. Advances in Veterinary Dermatology Vol 3, Butterworth Heinemann 1998; 192-200).

Successful ASIT in dogs is generally accompanied by a change in the Th1/Th2 polarization, an increase in Treg cells and a fall in IgE with a concomitant increase in IgG (Keppel KE et al, Vet Immunol Immunopathol 2008; 123: 332-344). This is the first study to address this question in horses, and the results were far from clear cut apart from showing a fall in IgE levels in most instances.

Horses with AD generally present with pruritus and/or urticaria, and ASIT offers a safe and effective therapy for the majority of cases.


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

Anette Loeffler, Desiree Herrick, Serena Allen and Janet D Littlewood

Vet Dermatol 2018; 29: 526-e176

This was a questionnaire-based retrospective analysis of the clinical signs and response to therapy of 82 horses with a diagnosis of atopic dermatitis (AD) that were referred for intradermal testing (IDT). Thirty-four (41.5%) had non-pruritic urticaria, 12 (14.6%) had urticaria and pruritus and 36 (43.9%) suffered from pruritus alone. In most cases (81.7%) the disease was generalized. Where it was localized, it involved the trunk (n=8), the head (n=6) and the legs (n=1). All animals were affected perennially but with seasonal exacerbations in 19 horses (23.2%).

All but two showed positive IDT results to an average of 13 of some 50 allergens at 30 mins and/or 2-4 hr after injection, indicative of immediate or late-phase IgE reactions. Delayed reactions were noted infrequently and were presumed indicative of delayed hypersensitivity.

Management changes to limit exposure to allergens, particularly insects, dust and storage mites were implemented as follows:

For the stable:
· Replace loose bedding with rubber matting
· Quarterly vacuuming and pressure hosing of stable walls, ceiling and floors
· Reduce exposure to birds and poultry
For foodstuff:
· Storage in clean, sealed containers
· Replace hay with wilted, vacuum-packed grass products
· Replace loose with cubed concentrate
For the rugs:
· Avoid excessive rugging
· Launder rugs frequently at high temperatures or freeze for 2-3 days after laundry
· Add human anti-allergy duvets (in polycotton cover) between skin and rugs to provide a barrier to dust mite allergens
· Maintain at pasture for extended periods
· Use a broad-leaf weed-killer
· Insect prophylaxis with frequent applications or permethrin-containing products as per data sheet or manufacturer’s recommendations

Complete follow-up data was received in respect of 47 horses. Four of the original 82 had been euthanized on humane grounds due to the severity of their disease and a further seven for unrelated reasons. Medications employed to control the condition included:

· Systemic glucocorticoids (n=35) with a good response in 94.3% but with adverse effects in 20%
· Topical glucocorticoids (n=10)
· Antihistamines (n=28) with a good response in 60.7%
· ASIT (n=14) with a 75-100% improvement noted in 64.3%. Two horses were restarted on ASIT after discontinuation resulted in a relapse, and again responded well. Swelling at the site of     injection was seen at some point in six of the 14 horses, but none persisted or required veterinary attention.
· Management changes (n=22) with nine showing improvement, and in four horses no further treatment was necessary. In three of these, complete remission was achieved by removing treats from the diet.

The clinical signs were similar to the foregoing reports, and comprised urticaria, with or without pruritus in 85.4% of cases and pruritus alone in the remaining 14.6%. Thus the horse appears to differ from man and other animals in which urticaria is usually pruritic. All except two of the 82 horses in the study showed positive IDT results, but no normal horses were skin tested and so the significance of a positive IDT as a predictor of a final diagnosis of AD is not known. However, as in man and other animals, the diagnosis of AD in the horse is made on clinical grounds with skin testing and/or serology employed to aid in the selection of allergens for potential avoidance or use in ASIT. The existence or otherwise of atopic-like dermatitis in the horse, in which the clinical signs are identical but no evidence of allergen-specific IgE is found, has yet to be determined.

Allergens most commonly implicated in the horse are dust and storage mites, and so management changes to limit mite exposure is a logical approach. Of particular interest were the three horses who responded completely to the withholding of treats, suggesting that food can be an allergen in horses presented with clinical signs of AD.

The most prominent presenting clinical sign was again urticaria accompanied in approximately half the cases by pruritus, with some cases showing pruritus and non-specific skin changes. Most had a generalized distribution. The condition was so severe in four cases that euthanasia was undertaken on humane grounds. ASIT was again a safe and relatively effective treatment, but management changes were also helpful and should form part of the therapeutic approach.


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

Johanna Forsyth, Richard E Halliwell and Robert Harrand

Vet Dermatol In press

Cross-reactivity between allergens is a hot topic in current allergy research. Put simply, it implies that when animal (or human) is sensitized by exposure to allergen A it may also recognize structurally similar allergens, B, C and D. This is of importance when formulating allergenic mixtures for use in allergen-specific immunotherapy (ASIT), in that ideally one should only be hyposensitizing with allergens that incited the immune response, and inclusion of all allergens to which an immune response is shown may be counterproductive. The major reason for this is because such cross-reactivity is often mediated by antibodies against cross-reactive carbohydrate determinants that are non-pathogenic and do not contribute to the disease process. The existence of such antibodies is well known in man and they have recently been identified in the dog (Levy and DeBoer, Vet Dermatol 2018; 29: 243-e90; Bexley et al Vet Dermatol 2018; 29: 357). It would be surprising if they did not occur in horses.

The search for evidence of possible cross-reacting antibodies ordinarily starts by examination of results of serology for allergen-specific IgE or intradermal tests (IDTs). A search is made for evidence of co-reactivity – i.e. when multiple pairs of results are assessed, do positive reactions occur in individual pairs with significantly greater frequency than would be expected by chance. Such co-reactivity is often the result of cross-reactivity, but the definitive proof can only be obtained from cross-inhibition studies. If the same, or very similar IgE-binding proteins are shared between antigens A and B, then the addition of antigen B to the animal’s serum will result in disappearance of signal when assayed for reactivity against antigen A. If this is also true when done in reverse, then that is proof of cross-reactive sensitization.

In this study, which was done at Avacta Animal Health, results of serology for allergen-specific IgE in 344 horses with suspected allergic disease were evaluated retrospectively using pair-wise comparisons. The allergens were divided into six taxonomically related groups, namely house dust/storage mites, tree pollens, grass pollens, weed pollens, insects and moulds, and evidence of co-reactivity was sought between related allergens, or between allergens in unrelated groups. Two separate analyses were made with the first using all results, and the second restricted to sera with at least one positive reaction. Evidence of co-reactivity was found in both sera sets within most of the taxonomic groups, and to a lesser extent between unrelated groups. Grasses were 100% co-reactive, mites less so at 73% and 67% for the two analyses, followed by 49% and 42% for weeds, 33% and 20% for insects, 29% and 18% for trees and 0% for both groups of sera in respect of moulds. Unexpectedly, the weed group showed a greater level of co-reactivity with grasses than it did within its own group.

Greater precision in allergy diagnosis will enable more precisely targeted allergen immunotherapy and/or allergen avoidance where this is practical. This co-reactivity study is the first step in the search for evidence of cross-reactivity, and the positive results justify the pursuit of cross-inhibition studies that will provide definitive proof. The ultimate goal is to develop serological assays that reveal only the response to the inciting allergen and in which all cross-reactivity is eliminated. This would be extremely difficult to do for IDTs without a complete library of recombinant allergens, which implies that serology will increasingly become the diagnostic tool of choice once these enhancements have been implemented.

This study has shown that there is a high degree of co-reactivity within most of the taxonomically related allergen groups, which is likely to be the result of cross-reactivity.
It is suggested that the clinician should be suspicious of results that show a large number of positive results within taxonomically related allergen groups, and that when designing allergen mixtures for ASIT it could be prudent to pay particular attention to allergens that are known to be abundant in the animal’s environment and to which positive results are shown.