Japanese Spotted Fever with Severe Myositis: A Diagnostic Challenge.

A woman in her late 70s presented with a fever, rashes, and marked proximal muscle weakness. Noninfectious conditions, including myositis and vasculitis, were initially considered. Treatment with cephalosporins was ineffective, but the symptoms improved with minocycline, indicating possible Japanese spotted fever (JSF) despite no apparent history of tick exposure. The diagnosis was confirmed by serological tests. A skin biopsy revealed leukocytoclastic vasculitis with immune complex deposition, suggesting muscle damage due to immune complexes. This case underscores the need to consider JSF in patients with atypical symptoms and initiate timely treatment to prevent severe complications.