Friday, 9 November 2012

A Curious Australian Plant - Gympie Gympie

Recently I happened across an interesting article on the Australian Geographic website, detailing the existence of a rather curious plant native to northeastern Australia, Indonesia and India. Known by its scientific name dendrocnide moroides, or more affectionately as Gympie Gympie, this plant is alleged to be one of the most poisonous plants in the world.

Relatively common in Queensland and growing to between 1-2 metres in height, Gympie Gympie has heart-shaped leaves. More to the point, Gympie Gympie has heart-shaped leaves that are covered in tiny, hollow, silica-tipped hairs that are rather adept at penetrating skin. Contact with the stems and leaves of this plant is considered to be a bad idea, with the aforementioned hairs penetrating the skin and releasing a potent toxin known as moroidin. Unfortunately, physical contact is not the only method of falling victim to this frightful flora. Indeed, merely being in the presence of this plant is sufficient to cause itching, rashes, sneezing and a fair bit of pain.

The leaves of Gympie Gympie
(photo courtesy of Wikipedia)
Well, that doesn't look too scary, does it? For a plant, one may even say it looks downright boring. A closer look, however, paints a very different picture of this plant:

Tiny, painful needles, filled with the toxin moroidin
(Photo courtesy of Alan's Wildlife Blog)
These are the hairs that cover Gympie Gympie, and are readily shed into the air, to the discomfort and pain of those that pass by. The pain caused by contact with these hairs is described in some detail by M. Hurley (2000). It is mentioned that slight contact with these hairs produces "an extremely painful, burning sensation", and that severe contact causes "excruciating pain often requiring urgent medical attention and can cause death in humans... and dogs and horses". The Society for Growing Australian Plants (SGAP) also describes the pain caused by Gympie Gympie in one of their leaflets:
"The pain starts as a tingling sensation and develops into stabbing or radiating pain...being referred to other parts of the body"
In research undertaken by Robertson and MacFarlane (1957), references to several instances where the sting of this plant has been described in earlier articles. Indeed, as early as 1860, the sting of this plant has required horses to rest for several days prior to being suitable for use. In relation to horses, it is also noted that the sting of Gympie Gympie has been sufficient to kill, and alternately, causes horses to "become violent and have had to be shot".

In the Australian Geographic article mentioned previously, humans who have encountered Gympie Gympie have not fared much better. This article recalls a serviceman falling into the tree during military training in World War II, who spent three weeks in hospital. It also makes mention of a tale that I had encountered previously on the internet, where an unwitting fellow who committed suicide after using the leaf of a stinging tree for "toilet purposes".

Also curious is that the pain from the sting of this plant may persist for weeks and months, and may recur at points some years after the original exposure. It appears as though this recurrence may be related to mechanical trauma to the previously affected area, as well as exposure to the cold. I was unable to find any mention of whether this recurrence of pain is mediated by attempts at removing the hairs from the skin.

Aside from pain-inducing, hair-covered leaves, Gympie Gympie also produces pink/purple fruit, as seen below:

The fruit of Gympie Gympie

The principle compound responsible for the extreme pain caused by the sting of Gympie Gympie is known as moroidin. A bicyclic octapeptide, it is also found in the hollow, stinging trichromes of the Stinging Nettle (urtica dioica), and its structure is shown below:

One significant research project studying the sting of Gympie Gympie and the search for specific pain-producing substances present in this plant was undertaken by Robertson and MacFarlane. Indeed, as no specific pharmacologically active compound had been isolated from Gympie Gympie and related species of stinging trees, they selected laportea moroides (also known as dendrocnide moroides, Gympie Gympie) for study, as it was reported to give "the most vicious sting in both intensity and duration".

Of particular note in Robertson's and MacFarlane's research is their method in testing extracts of the leaves of the Gympie Gympie plant:
"Since pain appeared to be the primary effect of L. moroides action, the essential test was the reaction of human skin to the stinging hairs or extracts of the hairs. Other pharmacological actions were sought on a variety of tissues and preparations"
Not only were hairs and extracts of the hairs from L. moroides tested on humans to understand the underlying cause of the painful sting of this plant, a few other methods were used. Indeed, the stinging potential of this plant was also tested on:

  • Guinea pigs and rabbits: both isolated sections of intestine from freshly killed subjects, but also the eye of what would appear to be live test subjects;
  • Rats: in particular, the uterus of young, adult, female, virgin rats, which appear to have also been freshly killed;
  • Anesthetised cats, where the effect of the sting on blood pressure was recorded

I offer no commentary on this manner of experimentation, except to say that bioethics requirements have evolved significantly over time.

Now, back to the chemistry. As mentioned previously, moroidin is a peptide composed of eight amino acids. These are, in alphabetical order:
  1. Arginine
  2. β-leucine: The (3R)-β-isomer of leucine
  3. Glycine
  4. Histidine
  5. Leucine
  6. Pyroglutamic acid: An amino acid derivative and lactam formed by the cyclisation of glutamic acid
  7. Tryptophan
  8. Valine
Above, we have seen the structure of moroidin. Now, let us have a second look, with the eight amino acids (or seven amino acids + one amino acid derivative, if you prefer) highlighted. It is also worth noting the carbon-nitrogen bond between tryptophan and histidine.

Another look at the structure of moroidin, with amino acids/derivatives highlighted

Now that we have an understanding of the excruciating pain that may be caused by contact with Gympie Gympie, it seems reasonable to know how to treat the sting of this plant. One method that is reported to be effective is to use a sticking plaster or hair waxing strip to remove the hairs from the skin. Otherwise, general analgesics are recommended for minor exposures to the plant, though in all instances it may be best to seek medical attention, regardless of the severity of the sting.

So there you have it. A bit of background on a rather curious Australian plant, the horrible pain that it causes, and the search for the compound responsible. In closing, I would proffer one last comment, in that it was exquisite fun to type (and mentally hear) the name "Gympie Gympie" again and again.

Off to stock up on hair waxing strips,

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