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 dermat-
itis. 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 hyper-
sensitivity.
Primary lesions
Macules
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 hyperadreno-
corticism. 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.
Papule
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 pyoderma.
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).
Nodule
A well-defined, solid elevation (Figure 6), greater
than 1 cm in diameter. Nodules are typically,
although not exclusively, associated with neoplasia.
Pustule
Pustules (Figure 7) are the primary lesions of
superficial pyoderma, although often outnumbered
by papules, crusted papules (Figure 8) and epi-
dermal 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.
Vol 18 No 1 / / 2008 / / Veterinary Focus / / 47
CUT-OUT AND KEEP GUIDE
Understanding
primary skin lesions
By Richard Harvey
Figure 2. More or
less symmetrical
crusting and linear
alopecia in a
Rottweiler with flea
bite hypersensitivity.
Figures 1 a and 1 b. Almost total alopecia and crusting on a Cocker Spaniel with a 6
month history. Careful examination of the dog revealed a few pustules (Figure 1b).
ab
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