Tierdermatologie Deisenhofen
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Papules may be present with or without pustules. Papules represent a collection of inflammatory cells within the dermis and it is these cells which result in the dermal swelling.
Some will be centred on a hair follicle => perifolliculitis is likely, so one must consider the follicular infections demodicosis, dermatophytosis and bacterial folliculitis.
Some papules will have the surface "excoriated" or - especially in the case of a spongiotic allergic reaction - will have formed a small vesicle on the top which rapidly ruptures. These are called crusted papules.
An interfollicular papule is frequently associated with insect bites (mosquito bite/ culicoides bite/ flea bite hypersensitivity).

Pustules are accumulations of cells and frequently serum within an intraepidermal space. Neutrophils and / or eosinophils give pustules their opaque colour. They are most frequently associated with infections and care should be taken (as in papules) to determine whether the pustule is centred on a hair follicle.
In the rare case of pemphigus foliaceus, the pustules may be large and contain numbers of - and groups of -
acantholytic cells. It is important to remember that acantholytic cells may also be obtained from pyoderma pustules, a diagnosis of pemphigus foliaceus requires a skin biopsy!

Care should be taken when sampling a pustule for cytology. So that the cells in the underlying dermis are not also collected, hold the needle tip parallel to the surface of the skin.

A follicular pustule
Impetiginous pustules
Papules and haemorrhagic bullae