Caterpillar dermatitis

More than 165,000 species of caterpillars in the order Lepidoptera (phylum Arthropoda, class Insecta) exist. About 150 are of medical importance. This importance lies in the ability of many species to induce an irritant or toxic dermatitis in humans and in the ability of some species to sting. Caterpillars are the larval forms of moths and butterflies. After hatching from their eggs, caterpillars pass through 4-5 instars (stages between molts) before they pupate in a cocoon. The adult moth or butterfly emerges from the cocoon to reproduce the next generation. More than 50 species in the United States alone are capable of inflicting a painful sting. Seasonal epidemics of dermatitis can occur when caterpillars are numerous.

The most dangerous caterpillar in the United States is the puss caterpillar or asp (Megalopyge opercularis; shown in the image below), the larval form of the flannel moth. It is found throughout the Southeast, from Maryland to Mexico. Stings from this species are common from June through September.

Human disease from caterpillars or moths usually arises from direct contact, exposure to substances or animals that have been infested with caterpillars or their webs, or contact with airborne caterpillar debris.

Diaz classifies the diseases caused by caterpillars into 5 groups: erucism, lepidopterism, dendrolimiasis, ophthalmia nodosa, and consumptive coagulopathy with secondary fibrinolysis.

Erucism (caterpillar dermatitis) is characterized by a localized, pruritic, maculopapular contact dermatitis and urticaria, and follows contact with toxic hairs, spines, or hemolymph, either directly or following aerosolization.

Lepidopterism is a systemic illness that occurs following such contact, and it is typified by diffuse urticaria, upper airway inflammation, nausea, vomiting, headache, and bronchospasm.

Dendrolimiasis is a more chronic illness that follows contact with the Asian Dendorlimus pini caterpillar. Patients with this disorder demonstrate a pruritic maculopapular rash and migratory polyarthritis/polychondritis, which can progress to chronic osteoarthritis. Occasionally, acute scleritis occurs as well.

Ophthalmia nodosa presents with acute conjunctivitis, progressing to panophthalmitis, following penetration of the cornea by urticating hairs.

Consumptive coagulopathy with secondary fibrinolysis occurs most commonly following stings by the South American Lonomia caterpillar whose venom activates factor X and prothrombin. Patients can demonstrate bleeding from almost any anatomic site and may develop acute renal failure.

Caterpillar venoms are produced by glandular cells in the epithelium and are stored in and injected by urticating hairs and spines (setae). Some species produce toxic hemolymph, which can cause human disease.

In some patients, immunoglobulin E (IgE) antibodies are produced following contact, resulting in a hypersensitive state and the production of generalized urticaria on subsequent re-contact. A few caterpillars lacking urticating hairs are capable of inducing a contact dermatitis (type IV hypersensitivity).

Some caterpillars and moths release their toxic hairs into the environment, where the hairs can be inhaled. Pets or contaminated objects also can carry venomous hairs. In any of these situations, contact with the hairs can cause rhinitis or respiratory disease. Seasonal epidemics of respiratory disease have occurred in Latin America because of this phenomenon. The pathologic response leading to erucism or lepidopterism consists of acute inflammation and cellular infiltration around hairs that have penetrated the skin or conjunctiva or have been inhaled into the respiratory tract.

In the skin, diffuse vascular dilatation occurs, with subsequent edema formation in the superficial dermis and ballooning of keratinocytes within the epidermis that can lead to vesiculation. In the eye, hairs have a remarkable penetrating capacity and may work their way into the cornea, anterior chamber, or lens, where an intense inflammatory response occurs secondary to the nature of the foreign material and direct toxic effects.

Caterpillar venom-filled spines, found only in the larval forms (adult moths and butterflies do not sting), are hollow structures with a single basal poison cell that produces toxin. When pressed into the skin, the tip of the spine fractures, and the venom is injected under pressure. Toxicity declines significantly after the creature's death, but irritant or toxic hairs may retain the ability to cause dermatitis for years. Likewise, the toxic hemolymph of some caterpillars retains its potency for prolonged periods after the animal's death.

Caterpillar venoms are poorly studied but may contain peptides, hyaluronidase, phospholipase A, and biogenic amines such as histamine or histamine-releasing substances. Some, such as the South American Lonomia species, contain fibrinolytic proteases and coagulation activators that can stimulate a consumptive coagulopathy and renal failure in victims.

• While the history of exposure related temporally to the onset of signs and symptoms may be clear, the diagnosis can be challenging. Maintain a high index of suspicion when patients present with unexplained acute dermatitis, rhinitis, conjunctivitis, or wheezing during months when caterpillars are numerous. In some regions of the world, the diagnosis becomes clear when clusters of seemingly unrelated cases start to present.
• Onset of acute dermatitis due to hair exposure may immediately follow exposure or may be delayed 8-12 hours and can be manifested by the following:
  • Intense pruritus, local pain or soreness (less common), and erythematous raised rash, blisters, and bruising at the site may occur. Skin necrosis has occurred following prolonged exposure to toxic hairs. With most toxic caterpillars, systemic symptoms are unusual in the absence of respiratory exposure.
  • Respiratory exposure may precipitate acute rhinitis, tearing, cough, dyspnea, respiratory distress, wheezing, and chest pain. This syndrome may need to be differentiated from the rare case of anaphylaxis.
  • Ocular exposure may initiate an acute conjunctivitis with severe pain, tearing, and redness. Visual acuity can be reduced as inflammation progresses.
  • Stings by venomous caterpillars result in immediate, localized, severe, burning pain, which can radiate proximally and be severe enough to inhibit movement of the extremity (pseudoparalysis). Redness and swelling with slight bruising may occur at the site. Over the first few days, the patient may note the development of small blisters, which can become hemorrhagic. Local findings, including pain, may persist for several days.
  • Systemic complaints may occur within minutes of the sting, although they usually occur within 2 hours. These complaints include headache, dizziness, restlessness, nausea and vomiting, malaise, swollen or tender lymph nodes, muscle spasms, rapid heart rate, and, in exceptional cases, altered mental status. Systemic symptoms usually resolve in 24 hours.
  • Following stings by some of the New World caterpillars in the family Saturniidae, which contain a fibrinolytic component to their venoms, patients may note scattered bruising (onset in 8-72 h) and bleeding from any of a number of sites (eg, gingival bleeding, hematuria). Intracranial bleeding and acute renal failure are possible complications. Coagulopathy can last 2-5 weeks.

• Local findings include erythematous papules that tend to congregate around the face, neck, trunk, arms, wrists, and hands and may become confluent; vesicles; local purpura; or ecchymosis.
• Systemic signs include a low-grade fever.
• Findings of respiratory exposure include acute rhinitis, tearing, respiratory distress, and wheezing.
• Ocular exposure presents as follows:
  • Findings include acute conjunctivitis.
  • Following penetration of the cornea, findings include keratitis, acute uveitis, and retinochoroiditis.
• Stings present as follows:
  • Local findings include erythema, edema, small petechiae or hemorrhagic papules, vesicles, bullae, and pseudoparalysis of the extremity because of pain. Following M opercularis stings, the site may take on a gridlike pattern matching the distribution of the creature's spines.
  • Systemic findings include restlessness, lymphangitis, lymphadenopathy, muscle spasms, tachycardia, altered mental status (unusual), seizures, and hypotension

  Laboratory Studies

  Laboratory studies are generally not required for caterpillar stings unless evidence of coagulopathy is present.

  • CBC count
  • Coagulation studies
    • Prothrombin time (PT)
    • Activated partial thromboplastin time (aPTT)
    • Fibrinogen level
    • Fibrin degradation products
    • D-dimer assay
    • Urine bedside test for blood

  A chest radiograph is reasonable if the patient has significant respiratory symptoms.

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  Prehospital Care

  • The involved skin should be immediately washed with soap and water, and dried without contacting the skin (eg, use a hair dryer).
  • Local cooling measures can be applied to reduce pain. This may be enhanced by applying topical isopropyl alcohol or ammonia.
  • Following ocular exposure, the eyes should be irrigated immediately with copious water.
  • Following dermal exposure to irritant or toxic hairs or setae of caterpillars or moths, sticky tape (especially duct tape) can be applied to the site in an effort to remove retained setae. Alternative effective methods of removal include use of rubber cement, clear fingernail polish, or facial peels (each applied, allowed to dry, then peeled away).
  • If acute symptoms follow respiratory exposure, supportive care is in order as necessary, including oxygen, antihistamines, and beta-agonist inhalers, if available.
  • Anaphylaxis should be treated in standard fashion.
  • Following caterpillar stings, the extremity should be splinted and elevated, and ice should be applied to reduce pain.
  • Any potentially constrictive jewelry should be removed before swelling progresses.

  Emergency Department Care

  • Wash the skin with soap and water as mentioned above if this has not already been done in the field.
  • Ensure appropriate tetanus immunization status.
  • Treat skin exposure as follows:
    • Apply sticky adhesive tape (especially duct tape) to the site to remove all remaining hairs or spines possible. Other measures of removal as described previously for prehospital care can also be tried.
    • Acute dermatitis can be treated with antihistamines (H1 and/or H2 blockers), although their efficacy is controversial. Additionally, topical steroids may be employed. Systemic steroids may be necessary in patients with severe or persistent cutaneous symptoms. Application of antipruritic products containing menthol may be soothing.
    • Prostaglandin-synthetase inhibitors, such as aspirin or indomethacin, have been reported to reduce associated discomfort, but should be avoided if any evidence of coagulopathy is present.
  • Treat respiratory exposure as follows:
    • Symptoms can be managed with antihistamines (H1 and/or H2 blockers) and beta agonist aerosols/inhalers if wheezing is present.
    • If significant symptoms occur, supplemental oxygen administration may be needed, and systemic steroids may be useful.
  • Treat ocular exposure as follows:
    • Instill a topical anesthetic and irrigate the eyes immediately with copious saline.
    • Perform a slit lamp examination with fluorescein. The patient should receive close ophthalmologic follow-up care to rule out retained setae or hairs.
    • Eye complications resulting from a retained migrating hair can be severe, and surgical removal may be necessary.
  • Treat stings as follows:
    • Management is primarily symptomatic and supportive. Splint and elevate the involved extremity; ice can be applied to reduce pain and swelling. Efforts, as outlined above, should be instituted to remove any retained spines or hairs.
    • Narcotic analgesics may be required for pain relief. Anecdotal reports exist of the successful use of calcium gluconate (eg, 10 mL of a 10% solution by slow intravenous [IV] administration) to relieve muscle pain following M opercularis stings. Antihistamines (H1 and/or H2 blockers) may reduce concomitant pruritus.
  • Treat rare cases of caterpillar or moth-related anaphylaxis in standard, aggressive fashion, including airway management, epinephrine, oxygen, antihistamines, steroids, IV fluids, and vasopressors as needed.

  Epinephrine and systemic antihistamines (eg, diphenhydramine, cimetidine), topical or systemic steroids, menthol-containing creams, and prostaglandin-synthetase inhibitors, such as aspirin and indomethacin, all may be beneficial in treating dermatitis. Rhinitis resulting from respiratory exposure may respond to antihistamines and systemic steroids. These are also useful for lower respiratory symptoms. Beta-agonist aerosols or inhalers (eg, albuterol) may be beneficial for wheezing. Analgesics may be required for caterpillar stings. The choice of agent should depend on the severity of symptoms. Mild cases may be treated adequately with oral opiates such as hydrocodone or oxycodone, while more severe pain initially may require parenteral agents such as morphine sulfate.

  Stings by the South American Lonomia species, which can cause consumptive coagulopathy with hemorrhagic diathesis and acute renal failure, may be treated with antifibrinolytics. If blood products are required, they must be given cautiously to avoid feeding fuel to an on-going consumptive coagulopathy. An antivenom against this species has been produced in Brazil.¹