A guide to causes and management of Equine urticaria.

Equine Urticaria can be a common skin complaint during the summer months when easy access to a wide range of potential allergens such as insects, plants and pollens is possible.  Often the cause of urticarial can be mystifying leading to frustration in the management of these potentially very itchy horses.

“A comprehensive horse management plan is crucial if you are to overcome Equine Urticaria”

What causes Urticaria?

The lumps noted over the horse as the result of ‘mast cell degeneration’ and the release of inflammatory mediators into the surrounding tissue.  These inflammatory mediators in turn dilate blood vessels causing plasma to leak into the surrounding skin layers creating the skin odema or hives and also contribute to the general itchiness of the skin. These hives can vary in size from a few millimetres to large ‘plaques’ covering the ventral abdomen.

The degranulation of mast cells is attributed to either an acute (Type I ) immediate, hypersensitivity reaction, mediated by immunoglobulin E (IgE) or by a delayed (Type III) hypersensitivity where the body has been pre-sensitised to an antigen.

Signs of Urticaria.

The characteristic signs of urticaria are raised skin lesion with steep sides and flat tops.  Hair loss is usually secondary to pruritus (itching).  The skin odema allows lesions to be ‘pitted’ when touched.  Lesions can vary in size from a few millimetres to large plaques.  Variations do occur with small ‘papular hives’ attributed to insect bites, to ‘giant urticaria’ presenting as plaques of oedema upto 40cms in diameter.  More bizarre forms and shapes of lesions occur when ‘coalescing hives’ form circular collections of lesions with a central indentation, appearing like a doughnut.

Most urticarial reactions occur rapidly and resolve likewise.  Chronic and persistent cases can last for several weeks or months.  Persistent itchiness will lead to skin damage and secondary bacterial infections.

“Identification of the underlying trigger in these cases is of paramount importance..”

History is everything in working out what caused the urticarial reaction.  What recent change in management that may have caused an acute (Type I) reaction or what re exposure may have caused a Type III delayed hypersensitivity?  These can include:

• Husbandry… a new bag of shavings smelling a little too much of pine?  A new skin shampoo or a new saddle cloth?
• Changes in the environment….  A new field, bale of hay or recent presence of flies.
• Diet… a new bag of feed or change in supplement… perhaps a new weed in the field?
• Health.. events such as vaccinations, worming or application of topical insecticide..

Although signs of urticaria may be obvious sometimes they also mimic other conditions such as vasculitis, bacterial folliculitis and cutaneous lymphoma.  A skin biopsy may be required to differentiate these cases.

“Skin and blood tests…. Neither serum IgE nor intra dermal skin testing will diagnose an allergic cause of urticarial in a horse…. Allergies can only be diagnosed by exclusion…”

However skin intradermal testing (and to a lesser extent, blood testing) will help rule out some of the causes of urticarial.  Food allergens are best excluded from diets in elimination diet trials where a single, novel food is fed for at least four weeks.

Conclusion: What can I do?

• Understand you are unlikely to find the answer to every case of urticaria.
• If a specific change in management can be pinpointed then obviously avoid future contact, inhalation, ingestion or application.
• Most simple urticarial reactions will resolve in 2-3 days.
• Medical expertise will be required in acute lesions causing intense pruritus and where secondary skin trauma has occurred.
• Therapy can involve Corticosteroids either given orally or topically.  Short acting Prednisolone is well tolerated in most animals though care should be exercised in ‘Laminitis Risk’ cases.
• Antihistamines offer an alternative therapy to steroids but published dose rates can be expensive and may cause drowsiness.

Practical solutions will involve:

• Fly prevention with Rugs, Hoods, fans in stables and modification of turn out periods.

• Constant diet with quality feeds from reliable sources.

• Pasture management to remove dung heaps and boggy areas which may attract flies.