Multiple follicular abnormalities in a 1-year old cat consistent with basaloid follicular hamartomas

Background In humans, basaloid follicular hamartomas are benign follicular tumours, that can be solitary or multiple, in which case they show autosomal dominant inheritance. Hypothesis/Objectives This study describes clinical and histopathological findings observed in a young cat, which could be consistent with basaloid follicular hamartomas. Case description Multiple follicular abnormalities, consistent with cutaneous diffuse basaloid follicular hamartomas, were observed in skin samples from a one-year old neutered domestic short hair cat. Clinical signs were diffuse symmetrical alopecia with exaggerated skin markings (ventral abdomen, thorax and medial aspects of the limbs) and intense follicular-centred thickening (face and feet). Microscopic lesions were characterised by multiple proliferative follicular abnormalities in all samples. The epidermis showed a very irregular surface with the follicles filled with variably pigmented keratin. The epithelial walls of the follicles had multiple small hyperplastic basaloid cells foci. In the superficial dermis under the epidermis and around the follicles, fibroblastic spindle-shaped mesenchymal cells with a homogeneous moderate density were present in the collagenous connective tissue. The interfollicular epidermis was also abnormal with multiple small proliferating trichoblastic foci originating from the basal layer. RNAscope testing for feline papillomavirus was negative. Conclusions and clinical relevance This case report provides the first evidence of clinical and histopathological findings of multiple follicular abnormalities, consistent with cutaneous diffuse basaloid follicular hamartomas in a cat.