Puss moth caterpillars (Megalopyge opercularis), of the order Lepidoptera, are also known as asps. They are one of the most toxic caterpillars in North America. Puss moth caterpillars are endemic to the southern US and live in shade trees and shrubbery around homes and schools and in parks. The asp caterpillar produces 2 generations a year, leading to a bimodal peak in late spring and late fall. They are teardrop shaped and, because they have long silky hair, resemble a tuft of cotton or fur. Their color varies from yellow or gray to reddish brown. When a puss moth caterpillar rubs or is pressed against skin, venomous hairs become embedded.
Envenomation causes intense throbbing pain, burning, and a rash with erythematous spots. More susceptible patients can experience swelling, nausea, abdominal pain, headache, lymphadenopathy, lymphadenitis, shock, and respiratory distress. Wound pain usually subsides within an hour, and the erythematous spots disappear in a day.
Treatment for local reactions includes washing the skin with soap and water (using noncontact drying such as a hair dryer), local cooling measures such as an ice pack, or topical isopropyl alcohol, and putting tape on the site and pulling it off to remove embedded hairs. Applying a baking soda slurry or calamine lotion can be soothing. Treatment of systemic reactions is symptomatic. Treatment of severe reactions is like that for insect stings (see Bites and Stings: Treatment).
Last full review/revision February 2013 by Robert A. Barish, MD, MBA; Thomas Arnold, MD