The Southern Flannel Moth (Megalopygidae family) is best known by its larval stage, the Puss Caterpillar, which is the source of its notoriety. While the adult moth is harmless, the caterpillar is considered the most venomous in the United States. Contact results in intense pain, often described as excruciating. The caterpillar is venomous because it injects toxins, causing an immediate and intense reaction upon contact, rather than being poisonous (harmful only if ingested).
Identifying the Flannel Moth and Puss Caterpillar
The adult flannel moth has a chunky, fluffy appearance, covered in long, dense, fur-like scales. Its wingspan typically measures between one and one-and-a-half inches. Colors range from drab gray or brown to cream with rusty-orange accents near the thorax. The adult moth is often found in the Southeastern United States, along the Gulf Coast, and as far north as Maryland.
The larval form, the Puss Caterpillar (Megalopyge opercularis), is the stage to be avoided, appearing in late spring and again in the fall. It is teardrop-shaped, measuring about one to one-and-a-half inches long, and is completely covered in dense, silky hair. This covering can be grayish-white, golden brown, or dark charcoal gray, often resembling a miniature Persian cat. Puss caterpillars primarily inhabit deciduous trees and shrubs, feeding on the leaves of oaks, elms, pecans, and citrus plants.
The Mechanism of Danger: Venom Delivery
The caterpillar’s soft appearance conceals its defense system, which consists of hundreds of sharp, hollow, venomous spines known as setae. These spines are nestled among the harmless outer hairs, making them nearly impossible to see. Each seta is connected to a small poison sac at its base that holds the complex venom.
Envenomation occurs instantly upon brushing against or touching the caterpillar. The spines easily break off and become embedded in the skin, simultaneously squeezing the venom from the sac into the wound. The venom is a complex mix of toxins, including proteins that trigger an immediate and intense pain response in the nervous system. These toxins work by punching tiny holes in the membranes of cells.
Symptoms Following Contact
The immediate reaction to a Puss Caterpillar sting is intense, throbbing, or burning pain that develops within minutes of contact. This localized discomfort often radiates outward from the contact site, sometimes traveling up an entire limb. The skin at the sting site typically develops an erythematous rash, presenting as swollen, itchy, red blotches.
In severe cases, the reaction can progress beyond the immediate skin area to include systemic symptoms. These broader reactions may include persistent headaches, nausea, vomiting, and fever. The venom can also affect the lymphatic system, leading to lymphadenopathy, which is the swelling of lymph nodes near the site of contact. While rare, more serious reactions involving intense abdominal distress, muscle cramps, and symptoms of shock necessitate prompt medical intervention.
Immediate First Aid and When to Seek Medical Help
The first step following a sting is to immediately remove the embedded venomous spines from the skin. This is best accomplished by repeatedly applying and stripping off a piece of adhesive tape, such as cellophane or duct tape, over the affected area. After spine removal, gently wash the site with soap and water to cleanse the area and remove residual venom or hairs.
To manage the intense pain and localized swelling, apply an ice pack or cold compress to the sting site for 15 to 20 minutes at a time. Over-the-counter pain relievers and oral antihistamines may be used to help reduce discomfort, itching, and swelling. Applying a paste made from baking soda and water, or a corticosteroid cream, can also soothe the local skin reaction.
Medical attention should be sought immediately if systemic symptoms develop, such as difficulty breathing, wheezing, intense abdominal pain, widespread swelling, or signs of shock. A physician should also be contacted if the sting occurred on a sensitive area like the eye or mouth, or if the pain remains unmanageable despite first aid. Individuals with a history of severe allergic reactions should seek emergency care promptly.