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Primary lesions

Erythema refers to redness of the skin, diffuse or localised, which disappears when a piece of glass is pressed over it. Erythema results from vasodilatation in the superficial dermis^U1415. This common lesion has little diagnostic significance. Generalised erythema may suggest atopic dermatitis, food intolerance (Fig. 2:1), cutaneous drug reaction or epitheliotropic T celi lymphoma. Localised erythema should lead to suspicion of an infection or infestation (e.g. dermatophytosis (Fig. 2 : 2) or demodicosis), or, depending on its location, actinic or contact dermatitis.

Purpura refers to a dark red, non-palpable spot which does not disappear when a piece of glass is pressed over it. It results from extravasation of red blood cells from dermal vessels ^Ul4,15. Purpuric macules are called petechiae, when punctate or lenticular, and ecchymoses when morę extensive (Fig. 2 : 3). Purpura is rare in the cat and should be considered a sign of disordered blood clotting (e.g. peripheral or central thrombocytopaenia) or other systemie illness. Platelet purpura occurs in thrombocytopaenia and wascular purpura in necrotising vasculitis that accompanies infectious illnesses (e.g. feline infectious peritonitis).

A macułe is a fiat spot, with no infiltration, associated with a change in skin colour ^U1415. Hyperpigmented macules arise from an inerease in melanin pigmentation. Circumscribed areas of hypermelanosis may be congenital (e.g. lentigo) and caused by an inerease in the number of melanocytes in the epidermal basal layer. They may also be acąuired as in post-inflammatory melanin pigmentation (e.g. in pemphigus foliaceus or panereatie paraneoplastic alopecia (Fig. 2:4)), and associated with melanocytic hyperactivity. Hypopigmented macules relate to a reduction (hypomelanosis) or absence of (amelanosis) melanin pigments. In the cat, the congenital, circumscribed hypo- or amelanoses (vitiligo) are classified separately from the generalised conditions (piebaldism, Waardenburg Syndrome and albinism).

A \esicle is a smali (1-3 mm in diameter), translucent elevation containing elear fluid ^Ul4,15. Due to the extreme thinness of the epidermis, the vesicle is a fragile, transient lesion and therefore, rarely identified. Theoretically, epidermal vesicles are seen in for example, pemphigus vulgaris and poxvirus infection, whereas subepidermal vesicles are seen in epidermolysis bullosa and bullous pemphigoid.

A bulla is a large vesicle, greater than 3 mm in diameter²'³¹⁴¹⁵.

A pustule is a raised, purulent spot, associated with an accumulation of neutrophils and modified keratinocytes ^2AM>,S. Unlike in the dog, pustules are hard to identify, macroscopically, in the cat, as they are fragile and short-lived. Follicular pustules are centred around a hair and usually indicate bacterial infection (e.g. bacterial folliculitis (rare in the cat) and acne) (Figs 2 : 5,6), dermatophytosis or, very rarely, intra-follicular demodicosis. Non-follicular pustules are fiat and independent of hair follicles. Also very fleeting, they are seen mostly in superficial pemphigus conditions (foliaceus and erythematosus). Both follicular and non-follicular pustules containing eosinophils are sometimes seen in allergic dermatoses, eosinophilic granuloma complex and notoedric mange.

A papule is a smali, raised circumscribed swelling associated with thickening of the epidermis or superficial dermis²'³¹⁴¹⁵. Papules are classified according to their location: epidermal, follicular, dermal. Epidermal papules are found in sąuamous celi carcinoma in situ (Fig. 2 : 7). Follicular papules are common in cats with miliary dermatitis and allergic dermatitis, in which case the infiltrate is madę up mainly of eosinophils. In dermatophytosis and bacterial folliculitis, follicular papules often develop into intra-follicular pustules, progressing towards furunculosis and a granulomatous reaction. Dermal papules arise from localised infiltration in the dermis by inflammatory cells: eosinophils and mast cells in allergic dermatitis (Fig. 2 : 6), eosinophils in eosinophilic granuloma complex, and mast cells in urticaria pigmentosa. Papules sometimes join together to form plaques (eosinophilic plaques) (Fig. 2 : 8). Some dermal papules called dysmetabolic papules (Fig. 2 : 9) are associated with an intradermal excess of amorphous materiał which accumulates secondary to a systemie metabolic disorder. These papules are rare. They may be due to dermal accumulation of lipid (xanthoma), secondary to diabetes mellitus, or a primary lipid disorder (e.g. familial hypertriglyceridaemia).