There are three species of white-marked tussock moths, Orgyia detrita, Orgyia leucostigma, and Orgyia definita, found across the eastern United States. Orgyia is Greek for a unit of measurement “the length between outstretched arms.” Their life cycle begins in the winter as eggs, from which caterpillars hatch from April to June. These mature over 30 days and create cocoons. Adults emerge two weeks later and mate; new eggs are laid on the surface of old cocoons by the flightless females.

All three species have similar characteristics, and while their adult form is gray and drab, their larval or caterpillar form is striking. Orgyia leucostigma (illustrated here) has a deep red-colored head and matching red dots (glands) on its back, coupled with clumps of short, vertical yellow hairs (called dorsal tussocks) on the first four abdominal segments, all surrounded by an inviting covering of fluffy long black ‘plume hairs’.

They are most commonly encountered by people in spring and early summer. Symptoms usually begin between 30 minutes and three hours.

They look friendly and fuzzy, especially when they raise the front of their body to search for their next path forward, the “hair pencils” looking like arms reaching out (hence the name Orgyia; see image at right. They are most commonly encountered by people in spring and early summer; however, your reaction of “how cute, I must pick it up” should be tempered by the knowledge that looks can be deceiving. Those hairs and warning colors are a clear announcement that they should be treated with avoidance, not petting, as these structures can cause injury to skin, eyes, and mucous membranes. There are 52 species of moths spanning 10 families of Lepidoptera that are known to have structures that can cause injury to humans—including Orgyia.³

Although they look benign, those attractive hairlike structures covering the caterpillar, especially the tussocks, are specially designed to act as passive armor to protect the caterpillar from predators. They cause injury, and therefore avoidance, among animals intending to eat them—or cause pain and problems in those curious enough to touch them.

The Hairs (Setae)

The brittle tussock hairs from the clumps on the caterpillar’s back (called urticating hairs) have backwards-angled barbs along their length, which, once they have poked into your skin or landed in your eyes, break off and travel deep into your tissues in a linear fashion, resisting withdrawal once imbedded.^1,5

As an additional defense, these urticating hairs have a venom gland at their base that can cause significant erythema and pronounced swelling.⁴ The toxin is not well characterized but can cause a local release of histamine and a broad itchy rash in sensitive individuals.

These hairs (and fragments) are also found in the cocoon matrix—the egg-laying female rubs her setae on the structure to embed them as protection for the maturing insects. These hairs have also been found floating in the air of forests with high caterpillar concentration.

Injury

The eye is the organ most sensitive to caterpillar hairs. These hairs cause severe local discomfort and pain, foreign body sensation, photophobia, lacrimation, lid edema, conjunctival injection, and blepharospasm; conjunctival irritation and congestion; chemosis; linear corneal abrasions; and punctate corneal lesions.

There is a well-described ocular disease process from this injury—ophthalmia nodosa. Schön first described it as an inflammatory reaction of the eyes to the mechanical and toxicologic attributes of caterpillar hairs that causes a characteristic immunologically mediated granulomatous nodular response primarily of the conjunctiva.⁸ These appear as small yellowish-red conjunctival nodules surrounding an embedded hair. When these fine, pointed hairs contact the surface of the eye, they can travel more deeply due to movement of the globe, blinking, and eye rubbing, which leads to further injury.

Symptoms usually begin between 30 minutes and three hours.³

These injuries are so common, a five-degree ocular injury scale, designed to measure injury caused by caterpillars, was developed and ranges from Type 1 (acute inflammation: chemosis, inflammation, epiphora, foreign body sensation) to Type 5 (vitreoretinal involvement with migration of the hair into the anterior chamber, iris, or lens).²

Skin and Mucous Membrane Exposures

Contact with the caterpillar hairs can result in imbedding, causing mild to severe local urticaria. This is made worse with rubbing, which further embeds the spines.

Treatment

Initial care is copious irrigation of the eyes or exposed skin and an attempt to locate the hairs and remove them. Further care depends on the extent of the injury; because the hairs do not dissolve with time, the setae will require physical removal from the eye—often using fine needle techniques.

Topical medications for erythema, irritation, and swelling include ice, topical steroids, and topical antibiotics if superinfection is suspected.

Most eye injuries will need ophthalmologic referral for a second examination to search for retained hairs and due to the risk of migration of hairs causing intra-ophthalmologic insults evidenced by cells and flare, such as vitritis and lens injury, which may require invasive interventions.^6,9,7

There have been cases of ocular caterpillar injury requiring enucleation.¹

Dr. Hack is chief of the division of medical toxicology and vice chair for research at East Carolina University in Greenville, North Carolina.

References

1. Agarwal M, Acharya MC, Majumdar S, et al. Managing multiple caterpillar hair in the eye. Indian J Ophthalmol. 2017;65(3):248-250.
2. Cadera W, Pachtman MA, Fountain JA, et al. Ocular lesions caused by caterpillar hairs (ophthalmia nodosa). Can J Ophthalmol. 1984;19(1):40-44.
3. Doshi PY, Usgaonkar U, Kamat P. A hairy affair: ophthalmia nodosa due to caterpillar hairs. Ocul Immunol Inflamm. 2018;26(1):136-141.
4. Gilmer PM. A comparative study of the poison apparatus of certain lepidopterous larvæ.” Ann Entomol Soc Am. 1925;18(2):203-239.
5. Jullienne R, He Z, Manoli P, et al. In vivo confocal microscopy of pine processionary caterpillar hairinduced keratitis. Cornea. 2015;34(3):350-352.
6. Levy S, Posa M, Kelly M, et al. Ophthalmia nodosa secondary to intraocular, white-marked tussock caterpillar setae (Orgyia leucostigma) in a 15-year-old. SAGE Open Med Case Rep. 2023;11:2050313X231172563.
7. Sahay P, Bari A, Maharana PK, et al. Missed caterpillar cilia in the eye: cause for ongoing ocular inflammation. BMJ Case Rep. 2019;12(4):e230275.
8. Schön J. Bieträge zur Praktische Augenheilkunde. Hamburg, Germany: Hoffman & Campe; 1861.
9. Watson PG, Sevel D. Ophthalmia nodosa. Br J Ophthalmol. 1966;50(4):209-217.