The changes are those of a viral infection with both intranuclear and intracytoplasmic viral inclusions. The affected keratinocytes show the cytopathic changes of a viral infection with ballooning degeneration. Many of the affected keratinocytes show only shadowy remnants of nuclei. In areas, the intracytoplasmic inclusions were numerous and in various stages of development. The clinical and histologic findings are compatible with milker’s nodule. Histologically orf cannot be excluded.

The organism is a pox virus. In some lesions in this category, there is a marked lymphoid hyperplasia with a high component of large, transformed lymphoid cells. Such lesions might be misdiagnosed as a lymphoid neoplasm, if careful attention is not given to the changes in the keratinocytes. The basic reaction is lichenoid and focally there is lysis of the basal unit of the epidermis. A lichenoid reaction with a high component of histiocytes is a common response to viral infections of the skin. In ballooning degeneration, some of the affected keratinocytes rupture and the resulting defects have a tendency to coalesce and to produce reticulated vesicles (a characteristic feature of a viral infection affecting the epidermis).
 
 

Fig. 1 The epidermis shows the features of acral skin. There is acanthosis with irregular elongation and erosion of rete ridges. Inflammatory infiltrates are perivascular in the reticular dermis and band-like in the widened papillary dermis. The infiltrates hug the epidermis. The combination of features provides a lichenoid quality. To the right of the center of the field, an inspissated vesicle is present between the epidermis and the keratin layer.

 

Fig. 2 The epidermis is hyperplastic. Many of the  keratinocytes are individually enlarged with abundant cytoplasm. Some of the keratinocytes have clear cytoplasm (ballooning degeneration). Many of the ballooned keratinocytes are anuclear (dead). At the dermal-epidermal interface on the left an elongated thin rete ridge is represented; it is a marker for a remnant of the basal unit of the epidermis. Lymphoid cells in band-like infiltrates hug the epidermis and have migrated into it to produce a lichenoid pattern. To the right of this area the basal unit has been destroyed by the cytopathic changes of the virus infection and by the lichenoid reaction. The papillary dermis is widened and fibrotic.

 

Fig. 3 The altered epidermis shows ballooning degeneration of keratinocytes. Focally keratinocytes show condensation of cytoplasm (increased acidophilia) and pyknosis of nuclei. Centrally a cell shows a poorly defined intranuclear inclusion. Several cells contain intracytoplasmic viral inclusions. A cell to the right of the center of the field contains a well developed, acidophilic inclusion.

 

Fig. 4 Ballooning degeneration is a prominent feature. Near the center of the field an affected cell contains a prominent cytoplasmic inclusion.

 

Fig. 5 The infiltrate of the papillary dermis is band-like and composed of lymphocytes and histiocytes. There is not a significant component of transformed lymphocytes. Vessels have swollen endothelium.

 

Fig. 6 An intranuclear inclusion is at the tip of the blue arrow. An  area of early reticular degeneration is represented at the tip of the green (top) arrow.

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