Extramammary Paget's disease with superimposed herpes simplex virus infection: immunohistochemical comparison with cases of the two respective diseases

We describe an extremely rare case of genital Paget's disease with superimposed herpes simplex virus (HSV) infection. We also describe immunohistochemical comparison of this lesion with 19 cases of genital Paget's disease and 12 cases of skin lesions caused by HSV or varicella-zoster virus. The Paget cells expressed simple epithelial keratins (CK7 and CK19) and carcinoembryonic antigen (CEA), but did not express stratified epithelial keratins (CK1, CK2e, CK10, CK5/8, CK14). Conversely, the virus-infected keratinocytes were positive for stratified epithelial keratins but negative for simple epithelial keratins and CEA. In the present case, simple epithelial keratins, stratified epithelial keratins, CEA and HSV were heterogeneously expressed in the ballooning and multinucleated giant cells. These results suggest that these cells were derived from keratinocytes and Paget cells and that the production of many multinucleated giant cells resulted from the virus-mediated cell fusion between Paget cells and neighbouring keratinocytes.