Urticaria

Description

Urticaria is a condition of the skin characterised by the development of multiple flat-topped, oedematous swellings (wheals) in the dermis that dent or 'pit' with digital pressure. Episodes may be acute, chronic or recurrent. The condition occurs in all domestic animals but most commonly in the horse.

Pathogenesis

The aetiopathogenesis of urticaria is complex and many causes have been suggested. The basic pathogenesis is thought to be due to degranulation of mast cells and basophils leading to the release of inflammatory mediators and an increase in vascular permeability. This may be due to either an exogenous or endogenous type I hypersensitivity reaction. Exogenous urticaria may occur as a result of exposure to chemicals, medication and insect bites or stings, shampoos or insecticides. Endogenous urticaria occurs following inhalation of allergens such as pollens. For a large number of cases no underlying cause is identified and these cases are termed idiopathic urticaria.

Signalment

Horses between 1 and 10 years of age are most prone to developing the condition. There is no sex predilection but Thoroughbreds and Arabs appear to be more commonly affected than other breeds.

History and clinical signs

Multiple wheals are often observed and occur within a few minutes or hours of exposure to the causative agent. A history of recurrence or repeated episodes is often reported. Affected areas can vary from small, localised lesions up to 3 cm in diameter, to extensive generalised areas involving large areas of skin. Lesions can occur on any part of the body but usually develop on the back, legs, flanks, neck and eyelids. Dermatographism is a specific pressure-induced type of urticaria in which wheals form in reponse to exogenous pressure, commonly these are observed in the saddle area of the horse. In severe cases the mucous membranes of the nose, mouth, anus and vulva may be involved. Pruritus may or may not be present and the lesions usually disappear rapidly.

Diagnosis and treatment

Because of the numerous underlying causes of urticaria, definitive diagnosis may be difficult. Historical and clinical findings may lead to suspicion of the condition. The localisation of lesions (e.g. site of contact with saddle or insect repellent) or history of medication or exposure to chemicals may help to determine an underlying cause.

Acute urticaria often resolves spontaneouly without requiring treatment. If urticaria is susupected all current medications should be stopped. Dexamethasone given once at a dose of 0.1 mg/kg IM usually results in resolution of clinical signs within 24 hours. In rare cases, epinephrine may be required if the urticaria is associated with signs of anaphylaxis.

Recurrent cases may require further investigation in order to determine the underlying cause. This may include further questioning about the animal's history and response to withdrawal of drugs or topical products. If no underlying cause is found, an investigation for allergies is indicated and may include intradermal allergy testing, insect control trials and an elimination (novel protein) diet. Biopsy of lesions is generally unrewarding but helps to rule out vasculitis as an underlying cause of the clinical signs. Antihistamines such as Hydroxyzine may be useful in the management of chronic urticaria but are ineffective in treating the acute form of the condition. Acupuncture has been reported to be effective in one horse.

Prognosis

The prognosis with urticaria is dependent on the underlying cause. If this can be identified and removed the prognosis is excellent. Horses with recurrent (idiopathic) urticaria generally have a poor prognosis.

References

• Mair, T. S., Love, S., Schumacher, J., Watson, A. E. (1998) Equine Medicine, Surgery and Reproduction Elsevier Health Sciences
• Pascoe, R. R., Knottenbelt, D. C. (1999) Manual of Equine Dermatology Elsevier Health Sciences
• Rose, R. J., Hodgson, D. R. (2000) Manual of Equine Practice Elsevier Health Sciences
• Scott, D. W., Miller, W. H. (2003) Equine Dermatology Elsevier Health Sciences