What's Your Diagnosis?

A Mystery

A patient presented to an urgent care facility 15 minutes after contact with the organism depicted in Figure 1. He described throbbing pain at the site but denied generalized pruritus, edema, fever, chills, and difficulty breathing. All vital signs were normal. Clinical examination revealed wheals surrounded by an erythematous macular halo (Figure 2). Approximately 40 minutes after envenomation, the wheals resolved and petechiae developed at the site of envenomation (Figure 3).

Figure 1

The offending organism.

Figure 2

Site of envenomation approximately 20 minutes after contact.

Figure 3

Site of envenomation approximately 2 hours after contact.

What is the diagnosis and treatment?

Diagnosis

Buck moth caterpillar (Hemileuca maia) dermatitis (erucism). The buck moth caterpillar is prevalent in the southern United States and 1 of more than 50 species of caterpillars in the United States known to inflict a painful sting. ¹ Other venomous species in the United States include the puss (Megalopyge opercularis), saddleback (Sibine stimulea), io moth (Automeris io), gypsy moth (Lymantria dispar), browntail moth (Euproctis chrysorrhoea), hag moth (Phobetron pithecium), and tree processionary caterpillars (subfamily: Thaumetopoeidae). Only the larval form (caterpillar) of the organism is capable of envenomation; moths are considered harmless. Caterpillars transmit their venom through urticating hairs, spines, or setae distributed over their bodies. ² The spines of the buck moth caterpillar are branched, hollow, and connected to underlying venom glands. ³ The pathogenic components of caterpillar venom are incompletely characterized but may include histamine, a histamine-releasing substance, proteases, and esterases.^4,5 Caterpillar stings typically consist of urticarial dermatitis but in some cases can progress to a systemic illness, known as lepidopterism, involving headache, conjunctivitis, and difficulty breathing. Atypical symptoms include radiating pain, lymphadenitis, muscle spasms, and abdominal pain. ⁵ The pathophysiology is likely a combination of mechanical, toxic, and allergic reactions. Treatment is supportive and includes stripping the area with adhesive tape to remove embedded spines, then washing the site with soap and water, applying ice, and administering topical or oral antihistamines or corticosteroids. In the rare cases where patients manifest systemic clinical effects consistent with an allergic reaction, oral or parenteral antihistamines, corticosteroids, or parenteral epinephrine may be indicated, depending on the severity of the reaction. ¹

In this patient the few remaining spines were removed with adhesive tape and he was treated with topical hydrocortisone (0.5 % cream); symptoms resolved within 5 days.