Diffuse scaling dermatitis in an athymic nude mouse

This report describes the clinicopathological features of a case of diffuse scaling dermatitis that occurred in a 16-week-old female athymic nude (CrTac:NCr-Foxn1(nu)) mouse. Gross presentation was suggestive of Corynebacterium bovis infection (scaly skin disease). However, C. bovis was not isolated from the skin of the affected animal or from the skin of unaffected CrTac:NCr-Foxn1(nu) mice housed in the same cage or room. Staphylococcus xylosus was instead isolated in high numbers from the skin lesion, whereas only a few colonies were recovered from the skin of unaffected mice. Microscopically, the affected skin was characterized by chronic hyperplastic and hyperkeratotic dermatitis with focal ulcerations, extensive serocellular crusts, and intralesional clusters of Gram-positive coccoid bacteria. Although gross presentation of the reported case was suggestive of C. bovis infection, epidemiological, histopathological, and bacteriological findings definitively ruled out an outbreak of scaly skin disease. A diagnostic hypothesis of hyperplastic and hyperkeratotic dermatitis associated with opportunistic S. xylosus infection was formulated based on increased bacterial burden and presence of intralesional Gram-positive coccoid bacteria.