Not a whip, but a beetle: A striking case of flagellate dermatitis

An 18-year-old female presented with linear, red, raw areas on her upper and lower limbs for the past 3 days. On examination, there were multiple linear, parallel, erythematous erosions over both thighs and forearms. Linear healed pigmentation was evident over the left thigh, while others were healing with crust formation [Figures 1a-c]. There was no history of mushroom ingestion, drug intake, or garden exposure before the episode. There was no history of any rheumatological disease or any injury before the episode. Her hematological and biochemical investigations were normal, and serology for human immunodeficiency virus, hepatitis B surface antigen, and hepatitis C virus tests were negative. She was treated with a short course of oral corticosteroids for 5 days, along with antihistamines, and was also advised to keep doors closed in the evening and to clean her bedding before sleep.

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Figure 1: (a) Multiple linear erythematous erosions over bilateral lower limbs arranged in a whip-like pattern (black arrows), linear hyperpigmentation over the left inner thigh (blue arrow). (b) Forearms showing discrete, parallel excoriated lesions of different lengths (black arrows) (c) Close-up view demonstrating erythema, superficial crusting, and early healing (white arrow).

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Flagellate dermatoses are uncommon but striking skin conditions that present as parallel linear or gently curving streaks, often resembling marks left by a whip.^[1] The term flagellate dermatitis was originally used to describe skin reactions caused by bleomycin. One of the more familiar causes in tropical settings is contact with rove beetles, particularly the genus Paederus. These beetles do not bite or sting; instead, when accidentally crushed on the skin, they release a potent irritant toxin called pederin. This leads to a localized reaction, beginning with redness or a wheal at the site, followed by blister formation within 12–24 hours, as in our case. In our case, one helpful clue was that most of the lesions were seen on the inner sides of both thighs, lying in an apposed pattern. The lesions often appear in linear or streaky patterns, giving rise to the classic “whiplash” appearance.^[2] Several other conditions can also produce a similar flagellate pattern, and their triggers, timing of onset, clinical appearance, and distinguishing features have been summarized in Table 1. Although the underlying causes vary, from insect exposure to drugs and systemic diseases, the shared morphology can be both intriguing and alarming to patients. Patients with flagellate beetle dermatitis should be advised to gently brush insects off the skin instead of crushing them, keep doors closed at night, use clean bedding, and avoid scratching the lesions, as this condition can recur. Recognizing this characteristic pattern and understanding its different causes are important for accurate diagnosis, avoiding unnecessary investigations, and guiding appropriate management.