Environmental Dermatology

What’s Eating You? Megalopyge opercularis

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Caterpillars—the larval form of butterflies and moths—belong to the order Lepidoptera. Some identified caterpillars from this order have developed venomous defense mechanisms such as irritating hairs and spines to protect themselves from predators. Direct contact to allergenic caterpillars’ hairs can cause human disease. Cutaneous lesions following caterpillar envenomation can range from mild localized inflammatory signs and pain to a worsening appearance of vesicles, bullae, erosions, petechiae, necrosis, and ulceration. This review addresses the varied general manifestations caused by venomous lepidopteran exposure with special focus on the adverse cutaneous manifestations and therapeutic options of Megalopyge opercularis, the caterpillar of the flannel moth.

Practice Points

  • Megalopyge opercularis is the most widely distributed caterpillar species in the Americas, and envenomation by it can occur year-round.
  • Skin reactions to M opercularis stings can present as maculopapular dermatitis, eczematous eruptions, or urticarial reactions.
  • During the initial presentation, patients experience intense throbbing pain, yet the severity of symptoms depends on the caterpillar’s size and the extent of contact.
  • A history of caterpillar exposure helps with diagnosis, and treatment remains empiric.



Lepidoptera is the second largest order of the class Insecta and comprises approximately 160,000 species of butterflies and moths classified among approximately 124 families and subfamilies. Venomous properties have been identified in 12 of these families, posing a serious threat to human health. 1

The clinical manifestations from Lepidoptera envenomation can range from general systemic symptoms such as fever and abdominal distress; to more complex focal affections including hemorrhage, ophthalmologic lesions, and irritation of the respiratory tracts; to less severe reactions of the skin, which are the most common presentation.1


Lepidopterism is the term used to address a clinical spectrum of systemic manifestations from direct contact with venomous butterflies or moths and/or their products.2 Conversely, erucism is a term used to describe localized cutaneous reactions after direct contact with toxins from caterpillars.

Lepidopterism is derived from the Greek roots lepis, meaning scale, and pteron, meaning wing. The term erucism stems from the Latin word eruca, which means larva.2

Ideally, lepidopterism should refer solely to reactions from butterflies and moths—adult forms of insects with scaly wings—while erucism should refer to reactions from contact with caterpillars—the larval form of butterflies and moths.

In common use, lepidopterism can describe any reaction from caterpillars, moths, or adult butterflies, as well as any case of Lepidoptera exposure with only systemic manifestations, regardless of cutaneous findings. Concurrently, erucism has been defined as either any reaction from caterpillars or any skin reaction from contact with caterpillars or moths.2

Because caterpillars are the larval form of butterflies and moths, caterpillar-associated skin reactions also have been conveniently denominated caterpillar dermatitis.1 Henceforth in this article, both terms erucism and caterpillar dermatitis are used interchangeably.

Caterpillar Envenomation

Caterpillars cause the vast majority of adverse events from lepidopteran exposures.2 Envenomation by caterpillars might stand as the world’s most common envenomation given the larvae proximity to humans.3 Although involvement of internal organs (eg, renal failure), cerebral hemorrhage, and joint lesions can occur, skin manifestations are more predominant with the majority of species. Initial localized pain, edema, and erythema usually are present at the site of direct contact and subsequently progress toward maculopapular to bullous lesions, erosions, petechiae, necrosis, and ulceration depending on the offending species.1,4

Megalopyge opercularis

In the United States, more than 50 species of caterpillars have been identified as poisonous or venomous.5 Megalopyge opercularis (Figure 1), the larval form of the flannel moth, is an important cause of caterpillar-associated dermatitis in the southern United States.6,7 Megalopyge opercularis also is commonly known as the puss caterpillar, opossum bug, wooly slug, el perrito, tree asp, or Italian asp.6 This lepidopteran insect is mainly found in the southeastern and southcentral United States, with noted particular abundance in Texas, Louisiana, and Florida.6,8 The puss caterpillar has 2 generations per year; the first develops during the months of June to July, and the second develops from September to October, carrying seasonal health hazards.6,8

Figure 1. A and B, Larval stage of Megalopyge opercularis.


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