Pustules and crusts: What else can it be besides pyoderma? (Proceedings)

Pustules, crusts and epidermal collarettes are superficial skin lesions which are often seen in association with superficial pyoderma. Although in such situations a pyoderma is the most likely diagnosis and antibiotic therapy is justified, other diseases should be considered, especially after poor response to initial therapy.

Pustules

Pustules are macroscopic subcorneal/intraepidermal cavities filled with predominantly neutrophils. They result from the breakdown in the integrity keratinocytes in the epidermis. Pustules can be discrete or poorly defined, and may contain besides neutrophils, eosinophils, and apoptotic keratinocytes. Pustules are very fragile and tend to rupture easily.

Crusts

Crusts are composed of keratin, cellular debris and often microorganisms. Crusts are usually the result of previous exudation and so are not considered to be a good source for making a specific diagnosis. However, it is important to examine these crusts for bacteria, fungal spores (Malassezia or dermatophytes) and unusual cells such as clusters of acantholytic cells, which would indicate a possible Pemphigus foliaceus.

Crusts can be occasionally mistaken with hyperkeratotic conditions, which are associated with some form of crusting, but usually don't show concurrent pustule formation unless a secondary pyoderma is present. This kind of clinical presentation could be associated with diseases such as other auto-immune diseases, Vit A deficiency, Zinc-responsive dermatosis, hepatocutanous syndrome, canine primary seborrhea, sebaceous adenitis, etc.

Diagnostic Tests

Cytology is the easiest in house test to obtain a preliminary diagnosis. Although not 100% reliable, as any other test, it will provide enough information to come up with a preliminary diagnosis. If pustules are large enough samples should be obtained through fine-needle aspiration to prevent surface contamination. If no bacteria can be detected or if unusual cells are present (e.g. clusters of acantholytic cells), skin scrapings (e.g. Demodex), fungal cultures (e.g. Dermatophytes), and biopsies (e.g. Pemphigus foliaceus) may be necessary to rule out other diseases which have been associated with pustule formation.

In this lecture a series of clinical cases with a variation of pustular and crusty lesions will be presented and discussed.