Identification of rickettsiae in cutaneous biopsy specimens from dogs with experimental Rocky Mountain spotted fever.

Direct immunofluorescence reaction for Rickettsia rickettsii was performed on formalin-fixed, paraffin-embedded cutaneous biopsy specimens collected from dogs with experimental Rocky Mountain spotted fever (RMSF). A technique of trypsin digestion of deparaffinized, rehydrated sections was successful in demonstrating discrete, immunofluorescent organisms in endothelia and adjacent vessel walls in the dermis. R rickettsii was identified only in grossly evident dermal lesions (macular rash or oral vesicles) and was not apparent in randomly collected biopsy specimens from clinically normal inguinal skin. These results suggest that clinical application of this technique for diagnosis of RMSF may be limited in dogs without cutaneous lesions.