modified from: CSIRO. (1991). Insects of Australia. 2nd ed. Melbourne University Press, Victoria.
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Australian Lepidoptera (moths and butterflies) contain several species of caterpillars that are armed with stinging hairs and/or fragile spines. These modified hairs serve as an effective means of protection against predators intertested in the soft bodied caterpillars. Skin contact with these specially equipped caterpillars can produce severe irritation and inflammation which is often referred to as lepidopterism.
The body surface of various urticating (irritating) caterpillars are adorned with microscopic dart hairs, or rigid bristles, or long and flexible tapering hairs. Hairs may be arranged in a pattern or tubercle on the side or dorsal surface of the caterpillar, depending on the species. The location and structure of the hairs, or group of hairs on the insect, can be used as a diagnostic tool to identify the species of caterpillar.
Urticating hairs can be of two distinct types. The first are envenomating hairs, which are tubular or porous spines capable of holding a venom or irritant produced by a gland at the base. On contact, the tips of the hair break under pressure and release their fluid contents, which is generally a mixture of histamines. There are only two families of lepidoptera within Australia that have caterpillars which possess these stinging hairs; they are the Limacodidae ("cup moths" or "Chinese junk" caterpillars) and the Nolidae (gumleaf skeletonisers). Other hair types on caterpillars are referred to as non-envenomating hairs and these produce a mechanical irritation on contact. These hairs are fragile and easily dislodged from the caterpillar, they adhere to the surface of skin when the caterpiller is contacted, or they become airborne and on settling the barbed or dart hairs easily fragment and penetrate clothing or skin. Hairs that are air-borne can drift and settle on nearby washing or other surfaces which humans will contact. Accidental disturbance or handling of old larval skins and spent cocoons, deposited under leaf litter, bark, wood piles, timber or any other material that caterpillars have had contact with, can result in irritation. These hairs retain their urticating properties long after the caterpillars have pupated. The families of Lepidoptera that contain these special hairs include, the Arctiidae (tiger moths), Anthelidae (white stemmed gum moth), Eupterotidae (bag shelter moths), Lymantriidae, the tussock moths (mistletoe brown-tail moth and the white cedar moth) and Notodontidae (bag shelter moth and processionary caterpillars).
The intensity of the irritation, whether it be caused by "venomous" or "irritating" hairs, will be dependent on the species of caterpillar and the sensitivity of the patient. Patients that come in contact with urticating hairs usually develop wheals and widespread rashes which can be accompanied by a burning sensation. Other symptoms include dermatitis, papules, pain, itching and swelling of the infected area. This inflammation can persist for days, although in most cases the symptoms are transitory. If mucous membranes have been affected there may be some swelling and irritation. Detached hairs can also be inhaled and the upper respiratory tract can be affected producing dyspnoea or laboured breathing. Injuries to the eye have been recorded, resulting in conditions such as nodular conjunctivitis and, less commonly, permanent damage to the cornea. In the case of a mistletoe browntail caterpillar infestation, large numbers of school age children have been known to become affected after sitting under infested Eucalyptus trees, or as a result of disturbing leaf litter and bark at the base of the trees where the caterpillars have rested or pupated.
Accurate identification is essential in treating and controlling caterpillar infestations. This should be undertaken by experienced staff, with the use of light microscopy and taxonomic keys.
Treatment and Control
To treat irritations as a result of urticating caterpillars, remove all affected clothing and apply a piece of adhesive tape to each of the affected areas, then pull the tape off immediately. This will remove some of the hairs and irritants and reduce the full impact of the irritation. The use of analgesics, creams, antihistamines and lotions with steroids will also assist in relieving the symptoms.
Avoid handling any hairy caterpillars or material with which they have been in contact. Suitable protective clothing, including eyewear and gloves should always be worn when handling these insects. Most caterpillar infestations are usually short lived and should be left undisturbed, unless they are causing a problem, when intervention by a reputable pest control officer would be recommended. Many infestations will die out either through predation or when all food sources are exhausted. However, some preventative control measures should remain in place to guard against Mistletoe browntail infestations especially in or near school yards. The removal of all mistletoe from Eucalypt trees within school yards or nearby areas before spring will prevent colonisation. Bare soil or mulching with pebbles at the base of existing eucalypt trunks will help deter pupation and resting of the caterpillar stage. Planting tree species that do not attract infestations, when shade is required in school yards or backyards, is another means of avoiding the problem.
Confirmation and Enquiries
Identification of caterpillars and all other medically important arthropods is preformed through the Medical Entomology Department at ICPMR, Westmead Hospital.
See 'Contacts' for further information.
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