here - Australian College of Veterinary Scientists

Introduction:
Equine Chronic Urticaria
Greg Burton
Urticaria (hives) are relatively common in horses. Chronic urticaria is an ill-defined
syndrome in the horse whereby transient urticarial eruptions occur in a recurrent
fashion over a period of greater than 6 to 8 weeks. Chronic urticaria in people is
more clearly defined as urticarial eruptions that occur at least twice weekly for greater
than 6 weeks.
Pathogenesis:
The pathogenesis is poorly understood. Both immunological and nonimmunological
mechanisms are considered likely. The major effector cell is likely to
be the mast cell however basophils, mononuclear cells, platelets and endothelial cells
may also play a role. Increased IgE bearing cells have been identified in skin
biopsies of horses with urticaria.
Potential triggers
- type 1 and type III hypersensitivity reactions (atopy, adverse food
reactions, contact reactions, arthropod bites, parasites)
- physical triggers (sunlight, pressure, heat, cold and exercise)
- psychological stress
- drugs (ivermectin, various antibiotics, NSAIDs)
- idiopathy
Clinical features:
There is no reported age, breed or sex predisposition in horses with urticaria.
Urticarial lesions appear suddenly, are often generalised and somewhat symmetrical,
are associated with variable pruritus and usually not associated with oozing of serum
or haemorrhage. They are variable sized from a few millimetres to several
centimetres with sharply demarcated edges and flat tops. The lesions are not painful,
have normal body temperature and pit on pressure. They are effervescent and
resolve within 24 hours. A clinical classification exists and may have some
aetiological significance.
I. Conventional urticaria: papules and wheals of 2-5mm diameter
II. Papular urticaria: uniform 3-6mm papules and often associated with
insect bites, especially mosquitoes
III. Giant urticaria: up to 40cm diameter
ACVSC Proceedings Dermatology Chapter Science Week 2005 47