Allergic disease in horses is a lifelong condition which can change over time. A multimodal therapeutic approach is the best way to achieve long-term management after first identifying all underlying primary triggers.
The concept of an individual animal having an allergic threshold above which clinical signs are seen, has been long established. However, there is now a greater understanding that allergies often occur in combination and it can be this accumulative effect that pushes them above the threshold. This is why when dealing with the allergic horse, even if Culicoides hypersensitivity it suspected as the primary trigger, it’s important not think of this in isolation but also consider other insect bite hypersensitivities, atopic dermatitis, asthma and food hypersensitivity. Doing so enables these other components contributing to the clinical signs to be addressed where necessary and so help bring the horse below the allergic threshold.
It has been suggested that 10-20% of equids will suffer from allergic disease at some point during their life although, there is limited data available. The estimated incidence of insect bite hypersensitivity (IBH) is 3-5% of the equine population with respiratory allergies being significantly higher. Atopic dermatitis in horses is also becoming more commonly recognised. Allergies can affect any breed, age or sex of horse, however IBH is particularly common in Icelandic and other native ponies and cobs.
AGE OF ONSET
The peak age of onset for dermatological and gastrointestinal signs is from 2 to 5 years, whereas respiratory signs tend to occur at a later stage usually 7 years or older.
Changes in environment inducing a heavier allergen load should be considered when taking the animal’s history such as; moving to pastures with a higher incidence of midges, stabling with high dust levels or seasonally higher pollen counts.
CLINCAL SIGNS AND OTHER POSSIBLE AETIOLOGIES
Pruritus – can lead to alopecia from traumatic hair loss, scaling and subcutaneous oedema.
Most cases of pruritus in the horse are due to:
- Ectoparasites: Culicoides, various other biting flies, mites (Chorioptes, Trombiculidae)
- Allergies: atopic dermatitis, fly-salivary protein allergies (especially Culicoides spp.), contact allergy or food allergy (less common)
- Bacterial, yeast or fungal infections: pyoderma (staphylococcal), dermatophytes , Malassezia
The distribution of the pruritus, or presence of other lesions, can help ascertain likely causes and aid in site selection when sampling for skin scrapings)
- Ventral Midline – Culicoides hypersensitivity (rarely Chorioptic mange).
- Mane and Tail – Culicoides hypersensitivity
- Pastern – Chorioptic mange (occasionally vasculitis, bacterial or dermatophyte infection, rarely contact allergy or atopic dermatitis)
- Face – Atopic dermatitis, black flies (especially if between rami of mandible), or stable flies (front legs and cranial thorax may also be affected).
- Tail only – Culicoides hypersensitivity, pinworms, atopic dermatitis, Malassezia infection (the yeast are found between the mammary glands in mares or preputial fossa in males but manifests as tail pruritus) and food allergy (rare).
Urticaria – Can occur with or without pruritus. The most common causes are; biting flies, atopic dermatitis and adverse drug reactions (but could also be due to vasculitis, fungal infections, or pemphigus foliaceus)
Papules – Most likely to be caused by allergy, infection (bacterial, yeast or fungal), or ectoparasites. Folliculitis (hair shaft involvement) would be more suggestive of dermatophytes or bacterial infection.
Pustules – Suggestive of an infectious cause but this maybe secondary to an underlying primary issue
- Frequent coughing
- Increased respiratory effort at rest
- Exercise intolerance
Equine asthma is associated with airway inflammation and narrowing of the airways along with increased mucus production. It was previously referred to as recurrent airway obstruction or chronic obstructive pulmonary disease with summer pasture associated versions . It can be caused by airborne pollution from mould/fungal spores, dust mites and pollens, both outdoors and in the stable.
The term equine asthma is helpful as aids owner understanding. It enables the focus to be on the elements we can help control:
- the clinical reaction – by using medication
- the environment – by using allergen avoidance/reduction techniques
- Chronic colic
- Long-term weight loss
Food allergy in horses is much less understood than in either humans or dogs. The prevalence is not currently known in the equine population. As previously noted, dermal signs can also be associated with food allergy. Please see our section on dietary management for more information and guidance on conducting a food trial in horses.
DIAGNOSTIC WORK-UP OF THE PRURITIC HORSE
Both serological and intradermal testing should be used after systematically ruling out other causes of pruritus. Their purpose is to identify allergens which may be responsible once the diagnosis of allergy has already been established. The results of either test must always be interpreted alongside the clinical history.
STEP 1 – Rule out bacterial and yeast infection and ectoparasites and fungal infections
- Skin scrapes for ectoparasites
- Cytology and hair shaft examination
- Bacterial and fungal culture
STEP 2 – Consider a dietary trial to rule out adverse food reaction
- SENSITEST® FOOD TEST can be used to aid in the selection of an appropriate exclusion diet by using negatively scoring foods.
DIAGNOSIS OF ALLERGY
STEP 3 – Identify allergens involved (alongside clinical history) for avoidance, general environmental management and when appropriate allergen-specific immunotherapy
- SENSITEST® ENVIRONMENTAL TEST and/or intradermal testing (please see our review of recent literature supporting the use of serological environmental testing in horses for allergen detection)
Allergic disease in horses is a lifelong condition which changes over time. The objective with treatment is to obtain good control of the clinical signs with minimal flare-ups, while accepting flare-ups will occur and educating owners to expect them and know how to respond when they do. A multimodal therapeutic approach is the best way to achieve long-term management of these cases and reduce the dependence on glucocorticoid therapy other than as targeted treatment of flare-ups when appropriate to do so.
Allergen avoidance / reduction
Although unlikely to offer a complete solution, reduction in allergen exposure is the best treatment option for all types of equine allergies and so should always be incorporated in any long-term treatment plan. This could involve extreme actions such as changing the horse’s overall environment by moving yards to reduce exposure to known pollens or insects or changing the horse’s entire routine at the current yard. Alternatively, smaller adaptations could be implemented such as switching to rubber mats and pelleted food to minimise dust. Some further detailed examples are listed below, are included in our equine specific results packs.
Pasture management options
- Move the horse away from standing water, manure piles, compost piles, and sheep or cattle
- Restrict access to the edge of woodland, sheltered area (where wind will be reduced) and any marshy, mossy or boggy areas
- Stable during the relevant insect’s peak activity time period/s, for Culicoides this is dusk and dawn but for many other biting flies it is the middle of the day
Horse management options
- Use fly rugs and/or fly masks impregnated with a repellent such as permethrin covering the horse’s head, neck and tail base, or use a repellent in addition to using a rug
- Make sure rugs and masks fit well and are free from tears, mud and faecal contamination as this will attract flies or allow them access
- Ensure fly repellents are true repellents such as permethrin or similar (which stop the insects from approaching and biting) not just insecticides (which may kill after biting occurs)
- Washes and sprays containing permethrin will need to be reapplied frequently if the horse gets wet or sweaty
- Tags and collars which help repel insects are also available can be tied into the mane and tail
- Apply vitamin E to where permethrin is applied (typically under the mane and tail head) to prevent permethrin-induced paresthesia (burning or prickling sensation)
- Some biting insects will not bite through layers of oil, so certain oil-based products may help by providing a barrier on the skin
Stable management options
- Install large fans to circulate air within barns / stables as midges are poor fliers
- Open windows and doors where possible covering with a fine meshed screen and use fly tape
- Insecticide sprays released by a timer can be used within enclosed areas
- Add fish to any ponds / permanent stagnant water to ingest insect ova and larvae
- Use midge trapping machines in the immediate vicinity of stables or shelters
Dust and storage mite management
- Wash rugs and other textiles at hot temperatures and dry in the sun when possible as mites struggle in hot dry conditions
- Store rugs over winter (once thoroughly clean and dry) in air tight packaging
- Change rugs, and other textiles such as saddle cloths, as much as possible to keep fabrics mite free
- Dry rugs and other textiles as soon as possible to avoid damp conditions
- Reduce humidity and increase ventilation within the stable environment
- Regularly dust the stable to decrease the food sources available to the mites
- Try to source good quality dust free hay for your horse
- Buy smaller bales of forage to prevent build-up of dust
- Empty dry feed out of its packaging and store in resealable plastic containers, discarding the dust at the bottom of the bags
- Clean these plastic containers regularly, again discarding any dust at the bottom and always do this before adding new food
- Consider purchasing smaller bags of food to ensure batches are kept fresh
- Wipe the horse’s muzzle with a damp cloth after eating to remove any dust and food residue
- Rinse the horse’s feed buckets daily
- Keep the food in dry, cool conditions
- Never use any foods after the use by date
- Keep stables well ventilated and treat damp walls with a mould inhibitor
- Don’t allow mould to form in damp areas of stabling
- Check any food sources – ensure they are kept in dry conditions and there is no sign of mould or rot especially on fruit and vegetables.
- Treat any damp walls with a mould inhibitor.
- Avoid heavy vegetation and climbing plants such as ivy, around or over barns and stabling
- Keep the stable yard free of fallen leaves and other plant debris
- Try to position the muck heap away from / up wind from stabling
- Keep tack dry and clean as some moulds are common on leather
- Source good quality forage free from mould and dust
- Keep all rugs and other textiles in dry conditions
Grasses and weeds
- Exercise horses on well-cut tracks and paths, or include road work where feasible, to reduce exposure to long flowering grasses and weeds
- Keep paddocks and areas around arenas and stabling weed free
- Regularly groom and bathe/rinse the horse, especially after exposure to long grass and /or on high pollen count days, this will help to physically remove some of the pollens on their hair
- Encourage the owner to make a note of where the horse has been and what it could have been exposed to when the horse’s condition flares up. Keeping an allergy diary will help to identify patterns over time.
- Keep the horse stabled if possible when grass is being cut, crops are being harvested in the area and when the pollen count is high
- Encourage the owner to keep up to date with the local pollen reports by checking weather reports
- Consider using a face mask or nose net when the horse is turned out
- Be aware that species related to the named allergens may also prove problematic to the horse
- Avoid exercise routes which pass alongside or through wooded areas during peak pollen season
- Turn out away from trees where possible, or fence off areas to avoid direct contact
- Keep hedge lines and trees well cut to limit flowering
- Avoid areas with a high concentration of the named allergen e.g. beech woods
- Be aware that the pollen calendar is seasonal but often earlier than grasses or weeds although is dependent on species
- Be aware that species related to the names allergens may also prove problematic for your horse. e.g. Privet is related to the ash family
If food allergens are contributing to the clinical signs, and can be successfully identified, avoidance by removal from the diet can have a significant impact.
The protocol below outlines the current guidance for conducting a dietary trial in horses to diagnose an adverse food reaction:
- Provide the horse with a novel feed source not routinely fed for 4 to 6 weeks duration
- Selecting negative scoring foods from the SENSITEST® food test can be combined with dietary history to aid this process
- Ideally avoid alfalfa and molasses as they are commonly incorporated in various feeds, treats and supplements
- Ensure all unnecessary supplements, vitamins, and other drugs are avoided for the full duration of the trial as they may result in non-IgE mediated reactions
If an improvement in clinical signs is seen, a dietary challenge must then be performed to confirm that food is a contributing factor. This is done by reintroducing one previously given food source every 7 days and examining the patient for any evidence of exacerbation of clinical signs.
Corticosteroids – Contraindicated in some horses and have known potentially very serious side-effects particularly with prolonged use. Are on the FEI prohibited substances list. Can be useful for short-term effective relief especially when managing flare-ups.
Allergen Specific Immunotherapy (ASIT) – If the individual allergens have been identified, allergen specific immunotherapy treatment (ASIT) can be beneficial. The allergens thought to be contributing to the symptoms seen can then be made into a bespoke, individual treatment which is used to gradually desensitise the horse to the suspected problem allergens.
Antihistamines – limited evidence but anecdotally sometimes quite effective. Antihistamines rarely cause either drowsiness or excitability in horses.
Essential fatty acids – adjunctive to other measures and limited evidence for use, 2-8 weeks duration needed before assessing if they have helped
In addition to repellents listed in the insect control section above these topical therapies should be considered:
The act of bathing the horse in cool water is itself beneficial as it:
- rehydrates the skin (improving the integrity of the epidermal skin barrier)
- causes vasoconstriction minimising the number of inflammatory mediators reaching the skin
- mechanically washes off both allergens (reducing percutaneous absorption) and any bacteria or yeast (helping control secondary infections which can be further aided if required by using antibacterial/antifungal shampoos)
Numerous different veterinary shampoos are available, selection should be based on what the individual horses requirements are (i.e. to help control secondary infection, restore skin barrier, calm the skin etc.).
These have proven very useful in small animals for controlling small areas of inflammation although currently none are licensed for use in horses.
SAMPLE STORAGE SERVICE
Here at Avacta Animal Health it is essential we always test samples at the optimum time i.e when the animal is symptomatic and ideally prior to any intervention. Therefore, if a client is reluctant to go ahead with allergy testing at first, or you would like to try certain treatment/s initially then please make use of our storage facility. Send samples to us with the submission form clearly marked “for storage” and we will store them free of charge for up to 3 months. Then if you would like to proceed with testing at any point simply call us to upgrade your sample.
WHY SEROLOGICAL TESTING?
Serological tests can aid diagnosis and treatment selection in cases of atopic dermatitis by providing rapid and easy identification of potential offending allergens.
Although intradermal testing (IDT) was considered the “gold standard” for the diagnosis of allergic disease, recent data has suggested that the sensitivity and specificity of IDT vs serology is comparable1.
Furthermore, there are limitations to IDT such as; extensive patient preparation, sedation often being required and interpretation requiring both experience and expertise.
The ELISA technology has the following advantages:
- Quick and easy – serum sample sent to laboratory
- Requires minimal patient preparation
- No adverse reactions
- No need for referral
- Standardised procedure
- Excellent reproducibility
- Not influenced by existing skin pathologies or most medications (see our allergy withdrawal guide)
1. Favrot, C., Steffan, J., Seewald, W. & Picco, F. (2010). A prospective study on the clinical features of chronic canine atopic dermatitis and its diagnosis. Vet Dermatology, 21(1):23-31.
Since 1999 we’ve been assisting companion animal veterinary surgeons with their diagnosis of allergies in dogs, cats and horses.
- Our team of leading research and development scientists are highly regarded within the dermatology field, having submitted clinical papers and presented at successive World Congresses of Veterinary Dermatology since 2008. We work alongside leading figures and companies in veterinary dermatology.
- SENSITEST® complete allergy tests use a monoclonal antibody to detect allergen-specific IgE antibodies, making SENSITEST® allergy tests very specific and reliable.
- Tests are manufactured in our Yorkshire laboratory allowing for end to end control throughout the process with a specific focus on UK native species of plant allergens
- Our knowledgeable, friendly Customer Service team are on hand to provide first-class tailored support and guidance throughout your allergy cases. Technical support is provided by a certified dermatologist.
- Dedicated client support literature and website to inform your clients and aid compliance.
- Personalised results packs sent out with every test.
- Storage facility to hold samples free of charge for 3 months when you suspect allergy in your case workup, but are not ready for serological analysis.
- Free pre-paid postal packs to send in your sample submissions.
- Results provided within 2-8 days from receipt of sample.
FOOD, ENVIRONMENTAL & INSECT
- Comprehensive food panel
- Comprehensive indoor, outdoor and insect panel
ENVIRONMENTAL & INSECT
- Comprehensive indoor, outdoor and insect panel
- Dermatophyte PCR