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History: 49-year-old male with a 2-day history of a rapid onset intensely pruritic lesion on the left arm that began during the evening during a visit to the jungle. He initially noticed intense pruritus and approximately 2 hours later he noticed marked erythema and some small blisters. The lesion rapidly extended over the left forearm and was also accompanied by a burning sensation. The patient decided to come to our Institute for consultation after a local physician expressed concern about herpes zoster. He is on no medications and has no history of immunosuppressive conditions.

Epidemiology: Born and lives in Lima, with frequent business travel to Tarapoto in the jungle of Perú. No known TB exposure. No HIV risk factors.

Physical Examination: Afebrile with normal vital signs. No lymphadenopathy. Linear, superficial, erythematous lesion with some small blisters was seen on the left forearm [Image A]. Remainder of the exam was normal.

Laboratory Results: Hb: 12.6; WBC: 8800 (normal differential). Biochemistry and urine were normal.

Discussion: On further questioning the patient admitted to “removing” an insect during that night two days earlier while on a short visit to the jungle. He described the arthropod to be identical to the 1 cm long Paederus irritans [Image B], whose body fluids provoke the typical reaction seen in this patient when they are released onto the skin during a swiping and crushing motion as the person swats or brushes off the insect. The clinical differential diagnoses of paederus dermatitis include acute allergic or irritant contact dermatitis, thermal burns, herpes zoster, herpes simplex, bullous impetigo, and phytophotodermatitis. Two other patients with latigazo from our archives are shown [Images C & D].

There are three major families of blister beetles of the order Coleoptera: Meloidae, Oedemeridae, and Staphylinidae. The vesicant chemical in both Meloidae and Oedemeridae is cantharidin, whereas the third family, Staphylinidae (which includes the genus Paederus), contains pederin. Cantharidin dermatitis is characterized by non-inflammatory vesicles and bullae, whereas paederus dermatitis is characterized by vesicles and pustules arising from intensely inflamed skin. The condition is characterized by vesicles and pustules on an erythematous base, with sudden onset of stinging or burning sensation, and the skin often appears as “burned”.

The dermatitis is most frequently seen in regions with a hot, tropical climate; and the arthropods usually become active after the rains. Paederus dermatitis has been reported in many countries, including Turkey, Iran, Italy, Nigeria, Egypt, Tanzania, Australia, Sri Lanka, Malaysia, and Brazil. In Perú, an epidemic of Paederus irritans dermatitis occurred in 1999 after the unusually warm and wet year that resulted from the El Nino of 1998.

Our patient was treated with antihistamines initially but needed 20 mg of prednisone for 5 days to relieve the erythema and burning sensation.