Botanical Briefs: Australian Stinging Tree (Dendrocnide moroides)
Dendrocnide moroides (also known as gympie-gympie, mulberrylike stinging tree, or stinger) is arguably the most brutal of stinging plants, even leading to death in dogs, horses, and humans in rare cases. They can be recognized as shrubs with heart-shaped, serrated, dark green leaves that are covered in what appears to be soft downy fur with red to dark purple raspberries growing on long stems. After contact, there is immediate piloerection and local swelling, which may disappear after 1 hour or last as long as 24 hours, but the subjective pain, pruritus, and burning can persist for months. One can only treat conservatively with symptom management, and the most successful method of removing plant hair is hair removal wax strips, which are considered an essential component of a first aid kit where D moroides is found.
Practice Points
- Dendrocnide moroides is arguably the most brutal of stinging plants, even leading to death in dogs, horses, and humans in rare cases.
- Clinical observations after contact reveal immediate piloerection and local swelling, which may disappear after 1 hour or last as long as 24 hours, but subjective pain, pruritus, and burning can persist for months.
- The most successful method of removing plant hair is hair removal wax strips, which are considered an essential component of a first aid kit where D moroides is found.
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
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.
