Cutaneous Histopathology of Rocky Mountain Spotted Fever: A Case Report with Literature Review

Summary: A 67-year-old woman with rheumatoid arthritis, hypothyroidism, and asthma presented with recurrent fevers, headache, worsening joint pain, elevated liver enzymes, and mild thrombocytopenia. She developed a limit extent of morbilliform, blanchable rash, later evolving into a macular rash on the trunk without mucosal involvement or skin sloughing. Skin punch biopsy of abdomen rash showed purpuric, slightly spongiotic, lichenoid, and perivascular dermatitis with neutrophils and karyorrhectic debris, suggesting possible acute collagen vascular disease. Differential diagnoses include lupus, dermatomyositis, Still disease, unusual drug reaction, and unusual infection. Despite no clear exposure, her Rickettsia rickettsii IgG antibody came back positive. This serological finding, combined with her clinical presentations and histopathological findings, confirmed the diagnosis of Rocky Mountain Spotted Fever (RMSF). Her condition improved significantly following initiation of doxycycline treatment. Cutaneous histopathology of RMSF shows lymphocytic vasculitis with major lymphohistiocytic and minor polymorphonuclear leukocytic infiltrates, evolving into leukocytoclastic vasculitis. Additional findings include fibrin deposition, vascular necrosis, and intraluminal thrombi, contributing to ischemia. Immunohistochemistry for Rickettsial antigens can aid in diagnosis, particularly in ambiguous cases. A literature review indicates that while skin biopsies are commonly performed to confirm the diagnosis, the full spectrum of RMSF cutaneous histopathology remains inadequately described and warrants further investigation.