We report an outbreak of caterpillar dermatitis related to an infestation of the caterpillar Euproctis edwardsi (Family: Lymantriidae), commonly known as the mistletoe browntail moth. The infestation occurred at a community centre, where staff and visitors experienced dermatitis and skin irritation for several months before caterpillar dermatitis was diagnosed. To our knowledge, these are the first clear cases of dermatitis caused by airborne caterpillar hairs in an indoor environment.

Clinical records

The centre had been sprayed against a variety of pests on several occasions, without benefit. Each time, the pest control company reported no visible evidence of insects in the centre. Many staff sought medical advice, and various treatments were prescribed, including topical corticosteroids and oral antihistamines, with no benefit.

The site was inspected by the Environmental Health Officer of the Population Health Unit (H C P). In front of the centre was an ironbark tree (Eucalyptus sideroxylon) which contained a mistletoe plant and, at its base, a large clump of caterpillars (pictured above). The tree was growing adjacent to the mail box, about 3 m from the front door of the centre and the window of the upstairs lunchroom, and 10 m from the intake vent of the air-conditioning system.

Caterpillar samples were identified at the Department of Medical Entomology, Westmead Hospital, Sydney, as E. edwardsi, commonly known as the mistletoe browntail moth. Caterpillar-like hairs were also identified in samples of dust from inside the centre. Based on reports of successful use of sticky tape to sample affected areas for nematocysts in jellyfish stings,¹¹ this method was tried on three affected individuals. Transparent sticky tape was applied to the affected area and then placed onto a glass slide. Caterpillar-like hairs were identified from two of the three people thus tested (Box 3).

The caterpillars were treated with insecticide and removed along with the mistletoe, according to the recommendations of the environmental health officer. Although the officer also recommended re-inspection of the tree and removal of any mistletoe regrowth the following spring, the centre eventually removed the tree completely. The building was thoroughly cleaned to remove all caterpillar hairs. Follow-up of people at the centre a month later showed significant improvement in their conditions. Occasional episodes of skin irritation in two people were successfully treated by immediate application of sticky tape to the affected areas. Follow-up after six months revealed no further reports of irritation.

Discussion

E. edwardsi is reported to be the most important cause of caterpillar dermatitis in Australia.¹ It occurs from Queensland to South Australia and is widely distributed in south-eastern Australia.¹² Its food source is usually Amyema species of mistletoes.¹ The incidence of caterpillar dermatitis peaks between December and March,³ with two generations of the moth each year, in early summer and autumn.¹ The fully grown caterpillar is about 4 cm long and has golden tufts of spicules on its back. These spicules easily separate from the caterpillar, causing irritation on skin contact. The most common reaction is a papulourticarial rash, usually on exposed skin, but possibly more extensive if clothing is contaminated.¹ The hairs are small enough to become airborne and affect people without direct contact with the caterpillar.

In patients presenting with skin reactions and dermatitis of unknown cause, particularly recurrent rash, a careful history of location and seasonality is required. Caterpillar dermatitis should be considered in the differential diagnosis and can be confirmed by identifying the source of the caterpillar or its hairs. Microscopy of sticky-tape samples from the affected area may aid diagnosis. The use of sticky tape has been reported previously as a treatment option in caterpillar exposures, to remove fine hairs that may cause ongoing symptoms.⁵ This is the first report demonstrating the use of sticky tape as a simple and effective diagnostic tool.

Once the source of exposure has been removed, treatment of caterpillar dermatitis is essentially symptomatic and supportive. Patient 1 allowed several treatment options to be assessed. Most effective was topical aspirin paste, with improvement within hours. Topical aspirin has previously been reported to be effective for histamine-induced rash.¹³ The paste is made by adding a few drops of water to a soluble aspirin tablet and applying it to the affected area. A topical preparation containing lignocaine offered some symptomatic relief but did not shorten the duration of symptoms. A topical hydrocortisone cream produced no noticeable change.

Outbreaks of caterpillar dermatitis are not uncommon and may be difficult to diagnose. Sticky-tape sampling of the affected area may aid diagnosis. Topical aspirin paste appears an effective treatment, and topical lignocaine preparations and oral antihistamines may provide partial symptomatic relief.

Acknowledgements

We thank Dr James Isbister (Royal North Shore Hospital, Sydney, NSW) for providing digital images of the sticky tape slides and Mr Stephen Doggett (Department of Medical Entomology, ICPMR, Westmead Hospital) for taking the clinical photograph. We also thank Judith Kirby and all the staff at the NSW Poisons Information Centre for their support and assistance.

References

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8. Cleland JB. Papulo-urticarial rashes caused by the hairlets of caterpillars of the moth (Euproctis edwardsi Newm.). Med J Aust 1920; 1: 169-170.
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Authors' details

Wentworth Population Health Unit, Sydney, NSW.
Helen C Ptolemy, BAppSci, Environmental Health Officer.

Department of Medical Entomology, ICPMR, Westmead Hospital, Sydney, NSW.
Merilyn J Geary, DipAppSci, PestContCert, Laboratory Manager;
Richard C Russell, MSc, PhD, Director, and Associate Professor, University of Sydney, NSW.

Department of Clinical Toxicology and Pharmacology, Newcastle Mater Misericordiae Hospital, Newcastle, NSW.
Geoffrey K Isbister, BSc, MB BS, Toxicology Registrar.

Reprints will not be available from the authors.
Correspondence: Ms Corrine R Balit, NSW Poisons Information Centre, The Children's Hospital, Locked Bag 4001, Westmead, NSW 2145.
CorrineBATchw.edu.au

©MJA 2001
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