What’s Eating You? Caterpillars
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.
Reactions to Adult Organisms—Although it is more common for the caterpillar form of these organisms to cause an adverse reaction, the adult moth also might be capable of causing a similar reaction by retaining setae from the cocoon or by their own spines. The most notable example of this is female moths of the genus Hylesia, which possess spines attached to glands on the abdomen. The poison in these spines—a mixture of proteases and chitinase—causes a dermatitis known as Caripito itch—the name derived from a river port in Venezuela where this moth caused a memorable epidemic of moth-induced dermatitis.7,15 Caripito itch is known for intense pruritus that most commonly lasts days or weeks, possibly longer than 1 year.
Diagnostic Difficulties
The challenge of diagnosing a caterpillar- or moth-induced reaction in humans arises from (1) the lack of clinical history (the caterpillar might not be seen at all by the patient or the examiner) and (2) the similarity of these reactions to those with more common triggers.
When setae remain embedded in the skin or mucous membranes, skin scrapings allow accelerated diagnosis. On a skin scraping prepared with 20% potassium hydroxide, setae appear as tapered and barbed hairlike structures, which allows them to be distinguished from other similar-appearing but differently shaped structures, such as glass fibers.
When setae do not remain embedded in the skin or when the cause of the reaction is due to spines, the physician is left with a nonspecific histologic picture and a large differential diagnosis to be narrowed down based on the history and occasionally the pattern of the skin lesion.
A challenge in sting diagnosis is differentiating a caterpillar or moth sting from that of another organism. In certain cases, such as those of the family Megalopygidae, specific patterns of stings might assist in making the diagnosis. Hypersensitivity reactions are associated with a wider differential diagnosis, including irritant or allergic dermatitis from other causes, scabies, eczema, lichen planus, lichen simplex chronicus, seborrheic dermatitis, and tinea corporis, to name a few.6 Skin scrapings can be examined for other features, such as burrows in the case of scabies, to further narrow the differential.
