BY RYAN LYNCH, MD, & MARK SUPINO, MD
A 2-year-old girl was playing outside when she spotted a furry caterpillar. Like her stuffed animals, it was fluffy and had long hair. She let the insect crawl on her legs, but then suddenly screamed in pain. Her parents brought her to the ED, though she was no longer in distress by the time she arrived.
The patient had a large urticarial lesion on her inner right thigh and a smaller one on her left thigh. (Photo.) There were no puncture wounds, bite marks, or barbs visible or palpable. Her vital signs and physical exam were within normal limits. The patient's father had captured the culprit in an empty pill bottle and brought it to the ED for identification. (Photo.)
Urticarial lesions prompting ED presentation.
Megalopyge opercularis, known as the puss caterpillar, woolly slug, possum bug, or perrito (Spanish for little dog), is a moth found all over the United States. (United States Department of Agriculture. Department Circular 288. 1923; http://bit.ly/2Bdx6WG.) The larval caterpillar form has venomous long hair-like setae protruding from its body. (van Nieukerken EJ, et al. Order Lepidoptera Linnaeus, 1758. In: Animal Biodiversity: An Outline of Higher-Level Classification and Survey of Taxonomic Richness, Zhang ZQ (Ed), Magnolia Press: Auckland 2011. Vol 3148, p. 212.) Its fluffy, stuffed animal-like appearance is often confused, especially by children, for an innocuous caterpillar.
Adverse reactions after contact with poisonous or barbed caterpillars, moths, or butterflies are classified as lepidopterism. (MMWR Morb Mortal Wkly Rep 1990;39:219; http://bit.ly/2Tu5WlD.) This encompasses a group of symptoms varying in severity that includes sting reactions, hypersensitivity reactions, and lonomism, the most severe form. Most stinging and hypersensitivity reactions are self-limiting and require only minor supportive care with analgesics and antihistamines, but lonomism is a bleeding diathesis that has a much more serious presentation.
This bleeding diathesis activates prothrombin with simultaneous degradation of several important clotting factors, including fibrin, fibrinogen, and factor XIII. (Thromb Res 2004;113:147; Thromb Res 2001;102:437.) Patients with lonomism present with excruciating localized pain at the envenomation site, vague neurological symptoms such as dizziness or altered mental status, generalized abdominal pain, and evidence of bleeding. It can be lethal in up to four percent of patients and requires inpatient supportive care, hematology consultation, and possibly serum immunoglobulin and dialysis if symptoms of acute renal failure develop. (Thromb Res 2004;113:147.)
Megalopyge opercularis, fortunately for our patient, is only capable of sting reactions and has not been reported to cause hypersensitivity reactions or lonomism. (JAMA 1961;175:1155.) The identity of the organism responsible for a bite or sting can dictate expectations, outcome, and therapeutic management. The mild, self-resolving symptoms, relatively benign physical exam, and correct identification of the organism reassured the patient and her family. Minimal supportive care, such as removing any potentially retained barbs with adhesive tape and giving NSAIDs, is sufficient treatment. A final take-home point: It is best to stay away from unknown insects, even ones that may have a friendly appearance.
Dr. Lynch is a junior resident, and Dr. Supino is the associate program director of the emergency medicine residency at Jackson Health System/University of Miami.