21 Mar 2016
Andy Durham reviews techniques for managing skin and respiratory conditions in equines caused by seasonal allergens.
Andy Durham
Job Title
Figure 1. Typical sweet itch – or is it? Why not another allergenic cause such as pollens, for example?
Allergies are commonly encountered in equine practice – especially during spring and summer.
The respiratory tract and skin are the main target organs, with wheezing, coughing, tachypnoea, dyspnoea, pruritus and urticaria being commonly encountered presenting signs.
Insect bite hypersensitivity (sweet itch) is the most commonly diagnosed allergy in horses, although this is almost invariably an assumed diagnosis without firm supportive evidence (Figure 1).
Differentiation of allergy to insect bites from other possible inhaled, feed or contact allergens is quite difficult and often not attempted. Consideration should also be given to the possibility of ectoparasitism.
A sweet itch diagnosis is generally based on typical signs in an adult animal during the fly season, often supported by previous history of seasonal pruritus and perhaps familial association.
It should be realised this is a diagnosis with weak security and, although often probably correct, many cases diagnosed as sweet itch may well be related to non-insect allergens (such as pollens, moulds and feeds) either as well as, or instead of, suspected fly bites.
Although serum allergy tests are easily performed, a lack of evidential support for their meaningfulness in horses does not support their use. Intradermal testing remains the preferred method of identifying causal allergens in horses, although it is offered by relatively few centres in the UK, which does not encourage its use. More than 20 years’ experience of the technique at Liphook Equine Hospital has generally been positive, allowing an evidence-based way forward in otherwise highly frustrating cases.
Intradermal testing cannot be meaningfully conducted with four weeks of glucocorticoid therapy and owners must also be advised response to subsequent immunotherapy (if applicable) may take several months, therefore a “quick fix” is unlikely.
Nevertheless, where an allergen has been identified, then attempted avoidance might be possible in limited circumstances (for example, dog hair, washing powders and dust mites in rugs). Fly avoidance, however, is rarely, if ever, absolute.
Thus, even a 99% success in fly bite avoidance might have negligible impact on clinical disease in some highly sensitised cases. Serious thought should be given to fly avoidance tactics and should take into account fly behaviour, as well as straightforward barriers and repellants (Figure 2).
Furthermore, even if absolute allergen avoidance was achieved then it might still be several weeks before good clinical improvement is noted, so patience is essential when awaiting results and judging the effectiveness of management changes.
Suspected dietary allergies can only really be investigated with properly controlled feed exclusion trials. However, these are remarkably inconvenient as they must involve the strict elimination of all enteral intake other than water and one basic nutrient type.
To achieve this, grass cubes, alfalfa, single species grass (for example, rye grass) hay or haylage are possible, but box confinement is essential to prevent grazing multiple plant types.
The length of a feed exclusion trial probably needs to be at least four to six weeks, or until the time at which signs resolve. Trial exposure of individual feedstuffs would hopefully establish the original cause.
Fundamentally, the preferred and most effective method for managing allergies of any kind is allergen avoidance, although failure to specifically identify causal allergen(s), in many cases, may confound this basic approach.
Ideally, attempts to identify causal allergens represents best practice, although often wholesale avoidance tactics can be employed by moving an affected horse to new premises and taking the opportunity to eliminate exposure to all previous items of food, bedding, tack, brushes, rugs and so on (that is, take the horse to the new yard and absolutely nothing else).
Gradual reintroduction of items following resolution of signs might identify the cause, although, in many cases, the cause may be invisible inhalant allergens.
Allergen-specific immunotherapy (ASIT) is an attractive technique when avoidance is not possible or practical, although, again, can only be attempted where the specific allergen is known and available as a sterile extract.
This involves controlled exposure to a potential allergen to gain tolerance, or at least an altered and less harmful immune response against it.
The aim of the technique is to cause immunoglobulin switching such that antigen exposure results in normal IgG synthesis rather than reaginic IgE antibodies that fix to dermal mast cells and mediate allergy. All horses produce IgG antibodies against insect salivary proteins, but IBH cases produce IgE. Sterile ASIT solutions are commercially available for a wide range of potential allergens and usually best selected on the basis of intradermal test results. Protocols usually comprise initial frequent injections of increasingly concentrated allergen solutions leading to less frequent (every one to two weeks) maintenance therapy that should continue for at least two years and, perhaps, for life.
Prerequisites for successful ASIT must logically include both identification of the specific allergen concerned and access to pure preparations of the specific allergen for re-presentation to the horse. Although these prerequisites are satisfied reasonably well in many atopic dermatitis cases, they are, thus far, not satisfied in the case of sweet itch.
For example, work from several studies has identified a number of specific Culicoides salivary proteins relevant to sweet itch that differ between geographic areas and between individual horses (Ferroglio et al, 2006; Hellberg et al, 2006; Wilson et al, 2008). One Swiss study of 42 sweet itch cases detected up to 10 specific Culicoides salivary proteins the horses reacted to, with typically three to four specific and different allergenic proteins per horse (Hellberg et al, 2006).
Further work has suggested allergic responses in IBH cases may not be exclusive to salivary proteins and that other proteins, such as those found in the eye, abdomen and thorax of the flies, may be relevant in a few cases also, perhaps resulting from sensitisation when flies are crushed into the skin when the horse rubs in a form of contact allergy (Wilson et al, 2001). Thus, the important clinical questions to be addressed with proposed ASIT for IBH are:
With the availability of intradermal testing solutions and ASIT products, we cannot be as specific as this with IBH cases. Rather, a more widespread “blunderbuss” approach is common practice, with crude whole-body fly extracts used for diagnosis and ASIT, which will, inevitably, contain a wide variety of irrelevant proteinaceous materials, diluting the presence and potential efficacy of any specifically relevant proteins contained in the solution.
Hopefully, in time, specific proteins will be able to be synthesised using molecular biologic techniques and, potentially, a whole panel of sweet itch salivary and non-salivary antigens will then be available for diagnostic and therapeutic purposes in IBH cases. Clearly, this issue is less of a concern where more specific allergen identification has been made, such as pollens, animal danders and so on, which are available as pure extracts for use in intradermal testing and subsequent ASIT.
It is always difficult to know with certainty the effectiveness of various ASIT protocols and to separate the placebo effect or natural resolution from true efficacy. For example, in one study of attempted ASIT for IBH, 50% of owners of horses in the blinded placebo group felt their horses had improved on “treatment” (Barbet et al, 1990).
In that study, no significant benefit of ASIT was found for IBH during the six-month study, although some might argue this was too short a period for proper evaluation of results. Other studies, with larger numbers of cases studying a variety of allergens over a longer time period, have indicated far better efficacy of ASIT in horses (Tallarico et al, 1998; Stepnik et al, 2011).
One other study of 32 horses with allergic urticaria and/or pruritus found owner-reported improvement of clinical signs in 84% of cases (Stepnik et al, 2011). Further scrutiny of these cases revealed ASIT had allowed total discontinuation of all other therapies in 59% of the 32 cases, with a further 9% able to discontinue glucocorticoids at least.
Interestingly, of 15 horses where ASIT had been discontinued due to resolution of clinical signs, 5 experienced recurrence of clinical signs; 3 of these were placed back on to ASIT and signs resolved again in all 3.
Summer pasture-associated recurrent airway obstruction has become an all-too-familiar summertime problem in horses in much of the UK. It remains unusual in horses aged less than six, but appears to increase in prevalence as horses age.
In a similar spectrum to stable-associated RAO, the outdoor version may vary – from poor performance with or without coughing and mild dyspnoea, to a severe, acute “air hunger”, with the affected horse showing maximal nostril flaring and heaving, and reluctance to move due to lack of sufficient oxygen exchange.
This disease remains incompletely understood, although it is likely to be effected by a combination of allergic and irritant airborne particles, along with airway hyperresponsiveness. Clinical exacerbation of signs may be associated with warmer, more humid spells and higher pollen and fungal spore counts (Costa et al, 2006).
Unlike stable-associated RAO, often, little can be done actively to improve air hygiene in such cases and, therefore, appropriate therapy may only be partially effective versus the ongoing airway challenge from the environment.
Stabling affected horses may not reduce inflammagen exposure and may add further airway challenges from stable dusts. Reasonably chronic therapy is to be envisaged in such cases and an expectation of seasonal recurrence should also be explained to the owner.
Acute relief of severe clinical signs may be achieved with a combination of IV bronchodilators, such as atropine (0.02mg/kg IV) or hyoscine (0.3mg/kg IV), along with furosemide (2mg/kg IV). IV glucocorticoids (for example, dexamethasone 0.05mg/kg
to 0.1mg/kg IV) will also be useful, although have a slower onset effect. Intranasal oxygen may be required in severe cases. Short-term maintenance therapy with systemic glucocorticoids might be transitioned towards inhaled glucocorticoids (for example, beclomethasone) as signs improve.
Ongoing systemic glucocorticoid therapy may be risky in laminitis-prone individuals
while grazing through the summer. The author’s impression of initiating therapy of severe cases with inhaled medicines has generally been disappointing and this mode of treatment is generally reserved for continuation or preventive therapy of a clinically reasonably stable case.
Treatment and management of skin and respiratory allergies in horses are a frustrating and problematic challenge in practice, generally due to the practical difficulties of identification and avoidance of causal allergen(s). Intradermal testing is the preferred means of identification of causal allergen(s), followed by avoidance measures and/or ASIT.
Respiratory allergies generally require chronic treatment, although, in the face of ongoing summertime challenge, this is rather a “swim against the tide”. Good support exists in the scientific literature for the success of ASIT in many cases of skin allergy, making it a potentially useful solution where avoidance is not possible to achieve.
Immunotherapy in the field of respiratory allergies has attracted some research interest in recent years, but perhaps remains a little experimental for routine clinical applications.
