A New Sign in UV Dermoscopy for Diagnosing Pthiriasis: The Glowing Crab Louse Sign

<i>Pthirus pubis</i>, commonly known as the pubic louse, is an obligate ectoparasite causing sexually transmitted infestation, typically presenting with intense pruritus in the pubic and genital regions. Atypical localizations, including eyelashes, axillae, chest, and scalp hair, have been described. Accurate diagnosis can be challenging in individuals with dense hair or low parasite burden. We report a case of a 27-year-old man presenting with two weeks of severe pubic pruritus. He reported multiple episodes of unprotected sexual intercourse, with no systemic illness or immunosuppression. Clinical examination was hindered by dense pubic hair. Dermoscopy revealed multiple <i>P. pubis</i> parasites and nits, exhibiting a characteristic "scorpion-like" appearance. Ultraviolet-induced fluorescence dermoscopy highlighted a novel feature, with bright blue fluorescence emitted by the entire parasite, termed the "glowing crab louse sign". The patient was successfully treated with a 10% dimeticone-based topical agent. Symptoms resolved within one week, and follow-up dermoscopy confirmed complete eradication. This report introduces the "glowing crab louse sign" as a novel dermoscopic marker for diagnosing pthiriasis. It may aid clinicians in challenging cases, enhance diagnostic accuracy, and could be incorporated into artificial intelligence-assisted diagnostic models. Further studies are needed to validate this feature and explore its underlying fluorescence mechanism.