Bites and Stings

Geoffrey K Isbister and Peter I Whelan

We report the first case of envenomation by the billygoat plum stinging caterpillar, Thosea penthima Turner (Limacodidae). The sting, on the forearm, caused immediate burning pain and local wheal formation. Pain radiated up the arm and there was severe "crushing" chest pain lasting four hours. The local pain persisted for 10 hours and required opiate analgesia. (MJA 2000; 173: 654-655)

Introduction

The clinical effects of lepidopterism can range from mild, itchy reactions to localised burning pain, occasionally with regional and systemic involvement. The extent of the clinical effects depends on the caterpillar species, the type of urticating hair, the site of exposure and individual sensitivity.

There are two major types of urticating caterpillar hairs:

• Itchy caterpillars have non-venomous hairs that cause localised itchy reactions and dermatitis^3,4 produced by mechanical irritation and foreign-body reactions.⁵

• Stinging caterpillars have hollow spines with a poison-secreting cell at their base. The spines enter the skin and break off, injecting venom and causing severe local reactions, occasionally with systemic effects.^1,5

Here we describe a significant sting by the billygoat plum stinging caterpillar, Thosea penthima Turner (family Limacodidae) (Box 1), from northern Australia. Reports of lepidopterism in Australia are rare,^1,2,6-8 and there have been no previous reports of stings by this caterpillar.

There was localised reaction at the site, but the size and appearance of the lesion were not documented. The patient's blood pressure was 110/70 mmHg and heart rate 48 beats/min. An electrocardiogram (ECG) showed sinus rhythm and no abnormalities. She was given 20 mg of oral promethazine, two paracetamol/codeine tablets (1 g/60 mg) and ice was applied locally. Over the next four hours the chest pain resolved, but the local pain radiating up the arm to the shoulder continued. A repeat ECG showed no change. The patient was then transferred to Royal Darwin Hospital with continuing pain.

On arrival, six hours after the sting, the patient had continuing severe pain at the sting site radiating to the right shoulder. A 5 cm x 8 cm raised area on her right forearm (Box 2) was centrally pale with surrounding erythema and tender to touch. There was no axillary lymphadenopathy. The blood pressure was 95/60 mmHg and heart rate 60 beats/min.

The patient was given two doses of 2.5 mg morphine intravenously over one hour and ice was applied. The pain gradually resolved over three hours and the local swelling and surrounding erythema decreased. She was discharged 10 hours after the sting. Followed up by telephone, she reported an irritating, red rash at the sting site that lasted a week. There was complete resolution.

There are few records of the effects of Australian stinging caterpillars.^1,2,6-8 The only records of stings by billygoat plum stinging caterpillars are by us.^2,6 Most people with stings experienced sharp pain and had a raised white wheal that resolved over 5-30 minutes.⁶ In one case, a sting on the thigh caused a painful burning sensation that increased over 15 minutes, with associated muscle ache, and lasted for 90 minutes.

Most stings from other Australian caterpillars have occurred in entomologists collecting larvae or in investigators who have stung themselves experimentally.^1,6-8 Three species of the genus Doratifera (Limacodidae) have been reported to cause stings similar to T. penthima.^1,6-8 A sting from Uraba lugens (the eucalyptus leaf "skeletoniser") (Nolidae) causes local pain and a wheal, sometimes for up to three weeks.¹

Most reports of stinging caterpillar envenomations involve the North American caterpillar Megalopyge opercularis (Megalopygidae).^11-13 Stings by this caterpillar cause severe local pain, erythema and swelling, often with pain radiating up the limb;^11,12 systemic effects have been reported less commonly.¹³ One study of caterpillar envenomations showed that radiating pain and systemic effects were more common with M. opercularis (35.2%) or Simine stimulea (45.5%), the saddleback caterpillar (Limacodidae).¹¹

The sting in our patient caused more severe acute effects than previously reported in Australia.^1,2,6-8 The initial severity of the chest pain prompted investigation for angina. Although pain radiating from the sting site has been described previously,¹¹ chest pain has not previously been associated with caterpillar envenomation.

The severe regional pain and chest pain in our patient suggest a systemic toxin effect. Chest and muscle pain is well described in stings by the jellyfish Carukia barnesi (Irukandji syndrome)¹⁴ and it is postulated that these effects are due to toxin-mediated catecholamine release. Studies have shown that components of some caterpillar venoms can cause endogenous mediator release,⁵ which may be the mechanism of the reaction seen in our patient.

Although most of the stinging caterpillars in Australia, including T. penthima,⁶ usually cause only minor local effects, our patient's experience shows that the more severe reactions seen with caterpillar envenomation in other parts of the world can also occur here.¹¹ Prevention of stings relies on recognition and avoidance of plants that these caterpillars feed on.

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(Received 7 Jun, accepted 14 Sep, 2000)

Territory Health Services, Darwin, NT.
Peter I Whelan, AM, BAgSc, Senior Medical Entomologist.

Reprints will not be available from the authors.
Correspondence: Dr G K Isbister, Department of Clinical Toxicology and Pharmacology, Newcastle Mater Misericordiae Hospital, Locked Bag 7, Hunter Region Mail Centre, NSW 2310.
gsbiteATbigpond.com

©MJA 2000
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