Blister Mystery

Case Report ^⁎

A patient in Sri Lanka presented with a blistering rash in September (Figure 1). What is the diagnosis and how would you treat it?

Figure 1

Mystery blisters. (Note: Not a photo of the fictional case.) Source: Ajao et al. ⁶ Used with permission.

Diagnosis

Dermatitis due to blister beetle.

Blister beetles (Mylabris pustulata) (Figure 2) are common in South Asia. They are considered pests. Blister beetles are often seen in the vicinity of farmlands during August and September. ¹ They are very sluggish during early morning, becoming more active after sunrise. Blister beetles secrete a yellow-colored oily substance from the leg joints. ^{1 –}3 This substance contains canthadrine, a skin irritant with high blistering capacity. Blister beetles do not sting or bite, but they release the blistering agent if they are pressed, brushed, or crushed. ⁴ Skin contact causes a burning, itching, or tingling sensation followed by the appearance of linear or irregular areas of erythema that later form vesicles (Figure 1). ^{4 –}6 The vesicles rupture in a few days and form crusts. Secondary infection may occur. Lesions usually heal in a few days when treated with topical antibiotics.^4,5 Early cleansing of the contact site with soap or alcohol helps to remove the canthadrine and may prevent the dermatitis. ⁴

Figure 2

The blister beetle Mylabris pustulata (∼25 mm length). Photograph by BTB Wijerathne while on expedition in a forest near Sigiriya, Sri Lanka.

Acknowledgments: I thank the editorial team of the Wilderness & Environmental Medicine journal for their continuous support to improve this manuscript. I acknowledge Dr OA Oluwasogo and Dr AM Ajao for the permission to use Figure 1 for this article.