July 21, 2014: What can sting but cannot fly … yet?

“ … Out of the egg came a tiny and very hungry caterpillar … “   Eric Carle’s beloved children’s book depicts a colorful, sweet caterpillar. Fiction aside, beautiful and inviting does not always equal friendly.


There are thousands of caterpillar species in the United States.  Many are armed with defensive “setae” or fragile, hollow spines that provide their ability to induce various degrees of dermatitis or urticaria.  Of these species, only about 50 possess setae that contain a toxin, allowing their ability to “sting”.


The Southern United States is home to at least 6 known “stinging” caterpillars that are of medical significance.  The Hickory Tussock, Hag or Monkey Slug, Saddleback, Buck, Lo Moth and Puss or Asp.  The Lo and Puss are the most problematic.


Despite ubiquity, encounters with harmful caterpillars are infrequent and can usually be handled at home.


“Erucism” is the term for localized, pruritic, contact dermatitis or urticaria that can follow contact with caterpillar setae, with or without a sting.  When a sting is also experienced, venom is injected intra-dermally via each individual setae. The setae will typically break off upon contact allowing venom to also spill to the surrounding skin.  The dermal reaction to follow often displays a perfect outline of the offender. 


The onset of Erucism is typically immediate, but can have up to an 8-hour delay when not associated with an acute sting. Sting reactions vary widely and can include localized and/or radiating pain, numbness, erythematous raised rash, ecchymosis and clear or blood filled blisters or pustules.  In some cases, dermal symptoms may last for days.


Standard dermal treatment includes applying and gently removing tape over the area, dislodging any embedded setae.  Wash the area with soap and warm water.  Use cold compresses, elevation and over-the-counter NSAIDS.  H1 and H2 blockers may also be of some benefit.


As always with any exposure, tetanus prophylaxis should be considered as well as monitoring for infection.


Systemic symptoms are uncommon. When seen, medical evaluation is typically recommended. Effects may produce nausea, vomiting, headache, dizziness, tachycardia, local lymphadenopathy, fever and rarely altered mental status, hypotension and seizures.  Systemic symptoms can be delayed for up to 2 hours and usually resolve within 24 hours with appropriate symptomatic care.


Lepidopterism is a rare systemic illness that can follow moderate-severe Erucism. The onset is delayed 24hrs after the initial exposure and is typified by diffuse urticaria, upper airway inflammation, nausea, vomiting, fever, headache and bronchospasm. Treatment is also symptomatic.


Ocular exposures should receive medical evaluation. Treatment should include prompt topical anesthetic, irrigation, and an Ophthalmology consult to remove retained setae.


Differentiating between “stinging” and “harmless” caterpillars can be challenging, especially for children. Unfortunately, handling them is foolproof. Remind them, look but do not touch. 


This question prepared by:  Donna Taylor, RN, CSPI (Certified Specialist in Poison Information)


I am interested in any questions you would like answered in the Question of the Week.  Please email me with any suggestion at donna.seger@vanderbilt.edu


Donna Seger, MD

Medical Director

Tennessee Poison Center


Poison Help Hotline: 1-800-222-1222