Skin problems due to caterpillars and moths

Author: Marie Hartley, Staff Writer, 2010.

Background

Moths and butterflies, and their larvae, caterpillars, belong to the order Lepidoptera. Although there are an estimated 125,000 to 150,000 different species in this order, very few are capable of causing adverse reactions in humans. Caterpillars are responsible for the vast majority of adverse reactions. Adult moths are less commonly implicated, and adult butterflies do not appear to be responsible for any adverse reactions in humans.

Bamboo caterpillars, which caused a rash on a gardener

How do caterpillars and moths injure the skin?

In order to defend themselves against predators, some caterpillars and moths have developed hairs and/or sharp spines that may be directly irritating or capable of transmitting various toxins.
In most cases direct contact with the offending species is necessary to provoke a reaction. Occasionally the irritating hairs can be detached and dispersed by winds, causing large outbreaks of reactions in humans. These hairs may also surround cocoons, eggs (transferred from the abdomen of female moths), or other environmental objects. Hairs from some species, such as the oak processionary caterpillar, are stable in the environment for at least one year.
Some individuals may develop allergic reactions to particular species of caterpillars and moths.
In very rare cases, spurs on the legs of large moths can penetrate human skin and cause stings, dermatitis, or urticaria.
Rare species of moths, from the genus Calyptra, are able to bite human skin in order to feed on blood.

How common are skin complaints due to caterpillars and moths?

Caterpillars and moths are a rare cause of skin disease, particularly in New Zealand.
Worldwide it is difficult to quantify the number of people affected, as many of the reactions are mild and are not reported to health authorities. Furthermore, in many cases the source of the reaction may not be correctly identified.
Some species of Lepidoptera become prolific in summer leading to “epidemics” of adverse reactions, for example summertime outbreaks of dermatitis from gypsy moth caterpillars in parts of the United States. Caterpillar and moth exposures are reported more frequently in children.

What are the reactions to caterpillars and moths?

In most cases, adverse reactions to caterpillars and moths are mild and resolve spontaneously. Reactions from contact with caterpillars or moths can be classified based on the predominant symptoms.

Reactions to caterpillars and moths
Type of reaction	Clinical features
Localised stinging reaction	The majority are caused by caterpillars. In NZ, the gum leaf skeletonizer sting can cause pain, redness, and weal formation (swollen small red bumps or large flat areas). Internationally stings from other caterpillar species can cause varying degrees of pain, itchiness, weal or blister formation, and rarely systemic symptoms such as dizziness, sweating, and abdominal pain. The most severe sting is from the puss caterpillar from the US and Central America. Severe pain occurs in all patients, and 1 in 3 develop systemic reactions such as headache, muscle spasms, breathing difficulty, and convulsions.
Papular urticaria and dermatitis	Usually caused by hairs from caterpillars or moths. Reactions range from mildly itchy, papular urticaria (small red bumps and swelling) that resolves within an hour; to moderately itchy, urticarial, scaly, blistering, or widespread eczema-like reactions that can persist for weeks. No Lepidoptera species in NZ cause this type of reaction.
Urticarial weals	Three species of caterpillars cause urticaria or angioedema. These species are all from the genus Thaumetopoea and are not found in NZ. Appears to be due to an allergic reaction. Systemic symptoms such as vomiting, abdominal pain, or anaphylaxis are rare.
Widespread haemorrhage (bleeding)	Some species of Lonomia caterpillars, found in South America, cause localised stings that may progress to a severe haemorrhagic illness. The sting transmits venom which causes burning pain, headache, nausea, or vomiting. Over the next few days widespread bleeding occurs into the skin, mucous membranes, lungs, brain, or kidneys. This can be fatal, but antivenom is available.
Biting moths	Symptoms have been described as painless and transient to intensely painful with swelling that persists for up to 24 hours. Biting moths, from the genus Calyptra, are found only in southern and Southeast Asia and eastern Russia.
Ophthalmia nodosa	This is a toxic or allergic eye irritation caused by caterpillar hairs. The hairs may be windblown, transferred to the eye with a finger or other object, or the caterpillar may contact the eye directly. Upper eyelid contact dermatitis usually occurs. Immediately after exposure, chemosis (swelling of the conjunctiva) develops. Patients may feel the sensation of a foreign body. Structures in the eye can become inflamed and granulomas (ball-like collections of immune cells) may form. Caterpillar hairs are frequently found within these granulomas. Any hairy caterpillar can cause this reaction.
Oral exposure	Rare. Most cases occur in children. Crying, drooling, and lip irritation occurs immediately. Redness, pain, swelling, and itch develop commonly around the tongue and lips. Occasionally the palate, airways, or oesophagus may be affected.
Dendrolimiasis and pararamose	These refer to itchy skin rashes associated with joint pain or inflammation. Cartilage may also be involved in dendrolimiasis. Joint destruction and potentially deforming arthritis can result. Caterpillars and moths that cause these conditions are found in China and Brazil.

Irritation due to contact with caterpillars

How are reactions to caterpillars and moths diagnosed?

A diagnosis of a reaction to a caterpillar or moth is very difficult to make. The clinical features are non-specific and mimic those of many other insect bites, allergic reactions, and dermatological conditions. Features which may make the diagnosis more likely include:

A reliable history of exposure in an at-risk geographic location.
Features consistent with a reaction to a caterpillar or moth such as symptoms on areas of uncovered skin, or occurrence of symptoms in summer months.

Laboratory tests are generally not helpful.

What is the treatment for reactions to caterpillars and moths?

General first aid measures include:

Remove the caterpillar carefully with forceps or tweezers (bare hands should not be used to remove or squash the insect).
Sticky tape (especially duct tape) can be used to strip off any remaining hairs from the affected area. Then immediately wash the area with soap and water. Contaminated clothing should be removed and laundered thoroughly.

There is no specific treatment for most caterpillar and moth reactions (the only exception is the widespread haemorrhagic illness due to Lonomia caterpillars, for which there is an antivenom).

Stinging reactions may be relieved with the application of ice, oral pain relief such as paracetamol, or topical anaesthetics.
Eczematous reactions can be treated with topical steroids or oral antihistamines, but these treatments are not always effective. Oral or intramuscular steroids have also had mixed success.
Ophthalmia nodosa and oral exposure require management in hospital to irrigate the area and remove the hairs. Steroids, antihistamines, and surgical removal of granulomas may be necessary.
Dendrolimiasis can be treated with antihistamines and oral pain relief; surgery may also be required.

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Related information

References

eMedicine Specialties, Emergency Medicine, Environmental – Caterpillar Envenomation
Hossler EW. Caterpillars and moths: Part I. Dermatologic manifestations of encounters with Lepidoptera. J Am Acad Dermatol. 2010 Jan;62(1):1-10; quiz 11-2.
Hossler EW. Caterpillars and moths: Part II. Dermatologic manifestations of encounters with Lepidoptera. J Am Acad Dermatol. 2010 Jan;62(1):13-28; quiz 29-30.