Botanical Briefs: Australian Stinging Tree (Dendrocnide moroides)

Cutaneous Findings—Examination reveals immediate piloerection, erythema due to arteriolar dilation, and local swelling.² These findings may disappear after 1 hour or last as long as 24 hours.¹ Although objective signs may fade, subjective pain, pruritus, and burning can persist for months.³

Dermatitis-Inducing Plant Parts

After contact with the stems or leaves, the sharp trichomes become embedded in the skin, making them difficult to remove.¹ The toxins are contained in siliceous hairs that the human body cannot break down.³ Symptoms can be experienced for as long as 1 year after contact, especially when the skin is pressed firmly or washed with hot or cold water.^3,6 Because the plant’s hairs are shed continuously, being in close proximity to D moroides for longer than 20 minutes can lead to extreme sneezing, nosebleeds, and major respiratory damage from inhaling hairs.^1,6,9

The stinging hairs of D moroides differ from irritant hairs on other plants because they contain physiologically active substances. Stinging hairs are classified as either a hypodermic syringe, which expels liquid only, or as a tragia-type syringe, in which liquid and sharp crystals are injected.

The Australian stinging tree falls into the first of these 2 groups (Figure 2)¹; the sharp tip of the hair breaks on contact, leading to expulsion of the toxin into skin.^1,4 The hairs function as a defense against mammalian herbivores but typically have no impact on pests.¹ Nocturnal beetles and on occasion possums and red-legged pademelons dare to eat D moroides.^3,6

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FIGURE 2. Stinging hairs resembling hypodermic syringes of Dendrocnide moroides.

The Irritant

Initially, formic acid was proposed as the irritant chemical in D moroides¹; other candidates have included neurotransmitters, such as histamine, acetylcholine, and serotonin, as well as inorganic ions, such as potassium. These compounds may play a role but none explain the persistent sensory effects and years-long stable nature of the toxin.^1,4

The most likely culprit irritant is a member of a newly discovered family of neurotoxins, the gympietides. These knot-shaped chemicals, found in D moroides and some spider venoms, have the ability to activate voltage-gated sodium channels of cutaneous neurons and cause local cutaneous vasodilation by stimulating neurotransmitter release.⁴ These neurotoxins not only generate pain but also suppress the mechanism used to interrupt those pain signals.¹⁰ Synthesized gympietides can replicate the effects of natural contact, indicating that they are the primary active toxins. These toxins are ultrastable, thus producing lasting effects.¹

Although much is understood about the evolution and distribution of D moroides and the ecological role that it plays, there is still more to learn about the plant’s toxicology.