What’s Eating You? Caterpillars

Cutis. 2021 December;108(6):346-351,E2 | doi:10.12788/cutis.0406

December 6, 2021|Dermatology

Carter Reid Ellis, MD;Dirk M. Elston, MD;Eric W. Hossler, MD;Shawn E. Cowper, MD;Ronald P. Rapini, MD

Caterpillar envenomation is a worldwide problem, with manifestations ranging from dermatitis to iridocyclitis and a fatal hemorrhagic diathesis. This article focuses on the diagnosis and management of dermatoses related to caterpillars.

Practice Points

Lepidopterism describes adverse reactions caused by the stings, hypersensitivity reactions, and lonomism (a hemorrhagic diathesis) of caterpillars, moths, and butterflies.
Caterpillars can induce an adverse reaction by injecting venom stored in their bristles, inducing a foreign-body reaction to embedded bristles, or a combination of these mechanisms.
A thorough history, skin scrapings, relevant examination of affected body parts (such as slit-lamp examination, in the case of eyes), and laboratory testing should be conducted to narrow the wide differential diagnosis associated with lepidopterism.

One of the most characteristic sting patterns is that of the caterpillar of family Megalopygidae (flannel moth)(eFigures 2 and 3). The stings of these caterpillars create a unique tram-track pattern of hemorrhagic macules or papules (Figure 4).⁴ A study found that 90% of reported M opercularis envenomations consist primarily of cutaneous symptoms, with 84% of those symptoms being irritation or pain; 45% a puncture or wound; 29% erythema; and 15% edema.³ Systemic findings can include headache, fever, adenopathy, nausea, vomiting, abdominal pain, and chest pain.⁴ Symptoms normally are self-limited, though they can last minutes or hours.

**eFIGURE 2.** *Megalopyge opercularis* (southern flannel moth) caterpillar, a member of a family of caterpillars (Megalopygidae) known for causing a sting with a characteristic pattern. Reproduced with permission of Dirk Elston, MD (Charleston, South Carolina). This image is in the public domain.

**eFIGURE 3.** Caterpillar belonging to the Megalopygidae family, which is known for causing a sting with a characteristic pattern. Reproduced with permission of Ronald P. Rapini, MD (Houston, Texas).

**FIGURE 4.** Tram-track pattern of the sting of family Megalopygidae caterpillars. Reproduced with permission of Dirk Elston, MD (Charleston, South Carolina). This image is in the public domain.

Hypersensitivity Reaction—Studies demonstrate that the symptoms of this reaction are a mixture of type I hypersensitivity, type IV hypersensitivity, and a foreign-body response.² The specific hypersensitivity reaction depends on the venom and the exposed individual—most commonly including a combination of pruritic papules, urticarial wheals, flares, and dermatitis.² A reaction that is a result of direct contact with the caterpillar or moth will appear on exposed areas; however, because setae embed in linens and clothing, they might cause a reaction anywhere on the body. Although usually self-limited, a hypersensitivity reaction might develop within minutes and can last for days or weeks.

Stings and hypersensitivity reactions to caterpillars and moths tend to lead to a nonspecific histologic presentation characterized by epidermal edema and a superficial perivascular lymphocytic infiltrate, often with eosinophils.⁶ After approximately 1 week, a foreign-body response to setae can lead to tuberculoid granulomas accompanied by neutrophils in the dermis and occasionally in subcutaneous tissues (Figures 5 and 6).⁸ If setae have not yet been removed, they also might be visible in skin scrapings.

**FIGURE 5.** Foreign-body response to embedded caterpillar seta, characterized by granuloma formation (H&E, original magnification ×400). Reproduced with permission of Shawn E. Cowper, MD (New Haven, Connecticut).

**FIGURE 6.** Caterpillar seta embedded in skin and surrounded by granuloma (H&E, original magnification ×600). Reproduced with permission of Shawn E. Cowper, MD (New Haven, Connecticut).

Additional complications can accompany the hypersensitivity reaction to setae or spines. Type I hypersensitivity reactions can lead to severe reactions on second contact due to previously sensitized IgE antibodies. Although the first reaction appears mild, second contact might result in angioedema, wheezing, dyspnea, or anaphylaxis, or a combination of these findings.⁹ In addition, some patients who come in contact with Dendrolimus caterpillars might develop a condition known as dendrolimiasis, characterized by dermatitis in addition to arthritis or chondritis.⁶ The arthritis is normally monoarticular and can result in complete destruction of the joint. Pararamose, a condition with a similar presentation, is caused by the Brazilian moth Premolis semirufa.⁶

Contact of setae or spines with mucous membranes or inhalation of setae also might result in edema, dysphagia, dyspnea, drooling, rhinitis, or conjunctivitis, or a combination of these findings.⁶ In addition, setae can embed in the eye and cause an inflammatory reaction—ophthalmia nodosa—most commonly caused by caterpillars of the pine processionary moth (Thaumetopoea pityocampa) and characterized by immediate chemosis, which can progress to liquefactive necrosis and hypopyon, later developing into a granulomatous foreign-body response.^2,10 The process is thought to be the result of a combination of the thaumetopoein toxin in the setae and an IgE-mediated response to other proteins.¹⁰

References

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