here - Australian College of Veterinary Scientists

Introduction
Diagnostic Approach to Urticaria
Reg Pascoe
Horses show the greatest incidence of urticaria of all the species of domestic animals.
Urticaria is a specific skin lesion rather than a specific disease entity. It has many
different aetiologies and pathogenesis (Logas & Barbet 1999). Generally, it is discussed
as a single entity even though its clinical manifestations vary from a minor transitory
nature to major systemic life endangering problems.
Aetiology / Pathophysiology
The pathogenesis of equine urticaria is not well understood. Urticaria has been
associated with immunological and nonimmunological mechanisms leading to the
release of various mediators by mast cells. In a study by Rufnacht et al (2004) skin
biopsies of 32 horses with a history of urticaria were stained with toluidine blue, a
double-labelling method for chymase and tryptase, plus immunohistochemistry for
immunoglobulin IgE. These horses were compared with horses with pemphigus
foliaceus, insect bite hypersensitivity and control horses with healthy skin. Neither
formalin fixation time nor biopsy site influenced the staining methods.No chymasepositive
cells were found. In all groups of horses, cells staining with toluidine blue and
for tryptase and IgE were found in the epidermis and hair follicle papilla and
significantly more positively staining cells were observed in the subepidermal dermis
compared with the deep dermis. Horses with urticaria had significantly more IgEbearing
cells in the subepidermal dermis than control horses. However, horses with
urticaria had significantly fewer toluidine-blue-stained mast cells in both subepidermal
and deep dermis compared with the insect bite hypersensitivity and pemphigus
foliaceus groups.
This study supports IgE-mediated reactions play a role in the pathogenesis of urticaria.
The use of intradermal testing (IDT) of urticaria and atopy also can be equivocal as
Evans et al (1992) had found that positive results were also obtained from horses not
showing signs of atopy. Further studies (Lorch et al 2001) have assisted in the
elucidation of more of the quandary associated with IDT
Many causes of urticaria have been suggested. (Scott & Miller 2003)
1. Degranulation of mast cells and basophils is presumed to be the basic
pathogenesis. Liberation of chemical mediators, which cause increased vascular
permeability leads to wheal formation.
ACVSC Proceedings Dermatology Chapter Science Week 2005 41