Primary Skin Lesions

Primary Skin Lesions

Understanding primary skin lesions

Most often we clinicians are not presented with a dog in the early stages of skin disease. Rather, the disease has been present for some time. Thus, the effects of self trauma, and the ongoing nature of the disease, results in secondary lesions such as crust and scale, alopecia and self trauma. Consider the Cocker Spaniel in Figure 1a. This dog had a 6 month history of a moderately pruritic progressive dermatitis. At presentation the most obvious signs were almost total alopecia and crusting-both secondary lesions and of little diagnostic help. Careful search, however, revealed a few pustules, as Figure 1b. These pustules were the primary lesions of the
dermatitis, in this case a drug eruption.

In some cases the pattern of the secondary lesions may suggest a diagnosis, while not being in themselves of definitive diagnosis value. Thus the bilateral excoriations on the Rottweiler in Figure 2 were a consequence of flea bite hypersensitivity.

Primary lesions

Areas of discoloration of the skin, less than 1 cm in diameter. Typically erythematous, but they may be hyperpigmented. Figure 3 shows conjoining erythematous macules in a dog with hyperadrenocorticism. Macules may precede papules in cases of pyoderma. In atopic dogs erythematous macules may be found in the groins or in the palmer and plantar interdigitae.

Papules are defined as small, discrete, palpably solid elevations on the skin surface (Figure 4a). Some papules may be neoplastic (Figure 4b). Most commonly, and particularly in the groins, they precede pustules. Crusted papules may follow vesicles or pustules. In the dog crusted papules outnumber pustules in superficial hyoderma. Superficial pyoderma is uncommon in the cat and in this species the most common underlying
cause of a crusted papular dermatitis is flea bite hypersensitivity (Figure 5).
A well-defined, solid elevation (Figure 6), greater than 1 cm in diameter. Nodules are typically, although not exclusively, associated with neoplasia.

Pustules (Figure 7) are the primary lesions of superficial pyoderma, although often outnumbered by papules, crusted papules (Figure 8) and epidermal collarettes (Figure 9). Pustules are the primary lesions of pemphigus foliaceus but are rarely accompanied by epidermal collarettes in this disease. Pustules are also found in association with other conditions associated with secondary infection such as demodicosis.

Comedones (Figure 10) result from blockage of a follicular orifice by debris and sebaceous material. Typically black in appearance they are associated with feline acne, demodicosis and hyperadrenocorticism in particular.

Erosion and ulcer
Erosions are considered to be superficial, whereas ulcers erode beneath the basement membrane, exposing the dermis. Acral granulomas (Figure 11) typically present as erosions on the distal limb.

A sinus (Figure 12) is a manifestation of a deep infection that has broken through the basement membrane, or of a dermal lesion (such as panniculitis) that has broken through the skin.