here - Australian College of Veterinary Scientists

Clinical Disease
The clinical result of disruption to blood supply from cutaneous vasculitis is lesions
that range in severity from wheals, papules, purpura, erythema and oedema if acute;
to alopecia and scarring if mild and chronic; to areas of well-demarcated necrosis,
ulceration and crusting if severe. Lesions occur most commonly on the distal limbs,
pinnae, lips and periocular areas. Diascopy confirms purpura as erythemic lesions
that will not blanch. Signs of systemic illness, including pyrexia, lethargy, decreased
appetite and weight loss may accompany cutaneous lesions. Forms of cutaneous
vasculitis recognised in horses include:
1. Purpura Haemorrhagica - is reported as the most common form of
cutaneous vasculitis in the horse. It is an acute disease characterised by extensive
oedematous swelling of the distal limbs, ventral abdomen, and sometimes face and
muzzle, and typically concurrent signs of systemic illness. Incidence is low and
sporadic, with most affected horses greater than 2 years of age, and no apparent
breed or sex predispositions. This syndrome may occur within 2-4 weeks of a
respiratory infection (Streptococcus equi or zooepidemicus), or other potential triggering
factors or disease. Urticaria may occur initially, and progress to oedema that is
usually well-demarcated, painful, symmetrical, and involving all four limbs. Severely
affected areas may have surface serum exudation and progress to necrosis and
ulceration. Petechiae and ecchymoses may be present on mucous membranes. Most
affected horses have obvious signs of depression and are reluctant to move,
anorexic, tachycardic, tachypnoeic, and often pyrexic. There is a wide variation in
severity and clinical course.
2. Subacute Pastern Vasculitis – is proposed to be a relatively common
although poorly characterised entity in the horse. Lesions primarily occur on nonpigmented
lower limbs, and may be more prevalent in animals on pasture. Sunlight
exposure and photactivation are proposed aetiological factors in some cases,
however appear uninvolved in others. Early lesions include erythema and
oedematous swelling, which progress to serum exudation, necrosis, crusting and
ulceration. Pain on palpation is often present, and pruritus absent. Chronic lesions
may be alopecic, scaling, lichenified, and proliferative.
Diagnosis
Definitive diagnosis of cutaneous vasculitis requires consistent history, clinical
lesions, and histopathological changes. In acute disease varying degrees of specific
vessel wall changes may be identified, including disproportionately more leukocytes
within vessel walls than surrounding perivascular areas, leukocytoclasis within or
adjacent to walls, fibrinoid necrosis, and vessel thrombosis. Less specific supportive
changes include intense oedema, red cell extravasation, and endothelial cell swelling.
There may be evidence of cutaneous infarction if severe, and follicular atrophy
(“faded follicles”) with chronic disease. Clear histological support for a diagnosis of
ACVSC Proceedings Dermatology Chapter Science Week 2005 113