Wednesday, May 1, 2013

Venomous Plants – A Hairy Situation

Biology concepts – venom, toxin, poison, nettle, urticating hairs, trichomes, defense behavior

The cobalt blue tarantula is a beautiful old world
tarantula, but not very cleverly named. They are
popular as pets, even though they are fast,
aggressive and have a potent venom. Fortunately,
they don’t have urticating hairs.
A tarantula, a jellyfish, and an ongaonga tree walk into a bar – O.K., maybe not the best start. But these three organisms do have something in common, something that has been recognized since the time of their classification and naming. Follow along.

Tarantula spiders are a popular example of venomous arthropods, arachnids to be exact. “Tarantula” is a vague term as it is used in the general population. The name comes from Taranto, Italy and came to mean any unknown, hairy, long-legged spider. In scientific taxonomy, tarantulas belong to the family Theraphodsidae, a group containing at least a dozen subfamilies and more than 900 species.

Many tarantulas have impressive fangs that deliver potent toxins to their victims. The fringed ornamental tarantula (Poecilotheria ornate) has produced a coma in a human; however, no known tarantula possesses venom that is acutely lethal to people.

But biting isn’t the only way tarantulas can defend themselves. Besides giving you the heebie-jeebies, two subfamilies of tarantula spiders have defenses called urticating hairs. These hairs are easily lost from their hairy backs or legs when the spider is touched by a predator. These small hairs can lodge in the eyes or skin of predators and cause significant physical irritation, enough to ward off a predator.

There are at least four types of urticating hairs, each differing in size and in the type of predator against which they are most effective. The old world tarantulas have type II urticating hairs that are dislodged by touch, but some tarantulas from the Americas can go one step further. They can fire their urticating hairs from a distance (types I, III or IV).

Urticating hairs often cause urticaria (hives), but
sometimes the red bumps will coelesce and form a
rash. And if you allergic, as seen here, the rash will
become big, ugly, and painful. You can see blister
development at the bottom. Explain to me why he is
affected on his belly?!
Species like the chilean rose and the mexican red-knee tarantulas have urticating hairs that can be fired by kicking their back legs against the back of their abdomen. When threatened, the tarantulas turn and rise up on their legs – ready! They point their abdomen at the threat – aim! And then they rub their legs against their abdomen and release a cloud of hairs toward the target – fire! This leaves a bald patch on their back and a very annoyed predator.

Most urticating hairs are mildly irritating to humans, unless you hold the spider up to your face. This is what happened in 2013 to a three year old boy at his birthday party. He held a rose tarantula up to his face to get a good look, and got two eyes worth of uritcating hairs! He cried for days, as they are so small as to become completely buried in the cornea and cannot be removed. He has made several subsequent trips to the hospital for care.

Other tarantulas have more damaging urticating hairs.  The Goliath Birdeater has larger hairs that can cause very bad rashes, and feel like fiberglass shards embedded in the skin. Some people will become allergic to the hairs, and the rash and reaction will be even worse (see the picture above).
So what has tarantula hair got to do with jellyfish or the ongaonga tree? Urtica is the Latin word for “nettle,” and the ongaonga tree is also known as the tree nettle or Urtica ferox. And the Greek word for nettle is “cnida,” as in cnidarians – like the jellyfish and coral we talked about two weeks ago. All three of these types of organisms use stinging cells for defense or offense.
Cnidarians use nematocysts to envenomate their prey, shooting toxin filled harpoons at the target. Tarantulas (and some caterpillars) use urticating hairs, not to poison but to irritate their predators. And there are some plants, the nettles, which use urticating hairs as venom delivery systems – the best of both worlds.
The nettles (genus Urtica, approximately 80 species) have hollow uricating hairs that can deliver toxins when they are broken off and embedded in an unfortunate victim. The hairs are actually modified trichomes, epithelial structures found in many plants that are merely raised areas on the plant surface.
Trichomes evolved many variations, those termed “hairs” can be thick or thin, long or short, fuzzy or smooth. Some may be used for water absorption or evaporation, while others will physically impede the movement of insects along the plant, or act as sensors. Venus flytraps (Dionaea muscipula) have three different kinds of trichomes; two secrete digestive juices and one is the sensitive trip wire for closing the trap.
These are the trichomes (stingers) of the ongaonga
nettle. Most nettles have smaller hairs, but this makes
for a more ominous picture. Remember that it isn’t
just their sharp points, they contain venom too.

Typical toxins included in nettle tricomes are formic acid, like in many ant species, and neurotransmitters like serotonin, and histamine. The pain or itch goes away in a few hours. They raise red welts that itch, called hives. In scientific terms, all hive-producing reactions are called urticaria. Get the connection? Most nettle trichome envenomations, like those from Urtica dioica (common nettle) are irritating, but little else.

However, the ongaonga tree (Uritca ferox) is the exception. There has been at least one death associated with just brushing against it. The ongaonga has unusually large spines; the lightest touch brings pain for more than five days.  Its neurotoxins also include an acetylcholine (Ach)-like chemical, yet another neurotransmitter.

The late symptoms can include breathing problems, blindness and paralysis. A 21 year old student developed a paralysis after a brush with the ongaonga. The neurotoxin caused her motor nerves to malfunction, firing too slowly and without pattern. It took weeks for her to recover.

But the news isn’t all bad. Nettle toxins may be used to in medicine, including diabetes, infection and even liver damage. A 2013 study in India treated rats with common nettle oil before performing a partial liver removal. The oils helped promote liver regeneration and decreased cell death after surgery. They also reduced the amount of oxidative damage in the surviving cells. So if you plan on destroying your liver, go run through a nettle patch first. However, I couldn’t find any studies using ongaonga oils – it is just too toxic. So be sure of your nettle patch species prior to your liver-protecting frolic.
A strange picture to see here, but follow along. You can
have part of your liver removed if it is damaged and live
just fine. A partial removal is called a hepatectomy. Some
parts can even regenerate after you have them removed.
Hepatectomy is important, as it makes it possible to have
living liver donors – you give someone part of your liver,
and you grow it back. This is where the nettle medicine
could be useful.

Our king of venomous plants comes from a different genus of the same family of plants as the nettles. You would think a plant that could kill you by touch would have a tough name, but it turns out to be just another insult added to the injury. You have tell your best buddies that you are laid up for weeks by a plant; and when asked, you have to tell them it was the “gympie gympie!” I can hear the laughter now.

The gympie gympie (Dendrocnidae moroides) lives in Australia, the land of painful deaths. The Australian Geographic website says that being envenomated by the gympie gympie is like, “being burnt with hot acid and electrocuted at the same time.” It has killed people, horses, and dogs.
Minor stings can last for hours to days with increased heart rate and sweating. The gympie’s trichomes seem to be silica based, like glass. You can heat them with a flame until they glow red, but they will still hold their shape. Add being stabbed with glass shards to the description of the gympie's sting.

Severe encounters can bring pain for months, with symptoms waning and then brought back by hot or cold air, water, or rubbing. Some people have shot themselves to relieve the pain, while others have had to be strapped to the bed.

There isn’t much you can do to treat the pain, but you might be able to shorten the length of your torture. The best first aid is to immediately apply hair removal wax and yank out the trichomes. You go for a hike and end up with silky, smooth skin and a pain that won’t stop – oh, wait, that could be just be describing the waxing.
The gympie gympie has huge leaves, like it is trying to ruin
your day. You can’t even see the hairs here, they are too
small. But you know it if you touch it. Did this guy lose a bet? 
Just being this close is a very bad idea.

Usually the pain comes from rubbing against the leaves, stem, or twigs. But the gympie wants to reach out and touch you, even if you don’t reach out and touch it. It sheds its urticating hairs all the time, so if you hang around a tree long enough, you will get a nosebleed and start to sneeze painfully. And you can’t wax the inside of your nose ….. I hope.

Fortunately, few deaths have been associated with the gympie gympie. It grows in the rainforests of northeast Australia where the population is very low, about 5-10 people per 2.5 sq. mile. The aborigines live here, and they actually eat the berries of the gympie gympie. Since all its trichomes point one direction, the natives know how to move along the stems and leaves in the right direction to harvest dinner. Apparently the berries aren’t poisonous.

D. moroides toxins include those said to act as neurotransmitters Ach, serotonin,  and histamine, but their chemical structures are different. They also include moroidin, a short peptide toxin that was first isolated from the leaves and stalks of the gympie.

No, this isn’t a picture of some electrical spark experiment
gone wrong. The green spines are the mitotic spindle and
the red blobs are the chromatids being pulled apart during
mitosis. More mitoses, more cell divisions. More divisions,
more cells. Too many more cells = cancer. It would be nice
to stop the spindles in that case.
Moroidin is a mitotic inhibitor; it interrupts the polymerization of tubulin during the formation of the mitotic spindle. If no spindle forms, then there is no alignment or segregation of chromatids during mitosis, so no cell division. Moroidin is supposed to be the factor that makes the sting pain last a long time, but not enough research has been done in this area. No one can even tell me specific chemicals the gympie possesses or how it causes pain! How can we make use of it in medicine if we don’t know how it works? I would think that a mitosis inhibitor might work well against cancer – let’s get to work people!

School is winding down, so why don't we start our summer posts. Each week will be a separate question in biology, from misconceptions to things that make you wonder, to weirdness galore. Next week - how good are different species at going without oxygen, and who can hold their breath the longest?

Oguz, S., Kanter, M., Erboga, M., Toydemir, T., Sayhan, M., & Onur, H. (2013). Effects of Urtica dioica on oxidative stress, proliferation and apoptosis after partial hepatectomy in rats Toxicology and Industrial Health DOI: 10.1177/0748233713480211

Hammond-Tooke, G., Taylor, P., Punchihewa, S., & Beasley, M. (2007). Urtica ferox neuropathy Muscle & Nerve, 35 (6), 804-807 DOI: 10.1002/mus.20730


For more information, see:

Tarantula urticating hairs –

Nettles –

Gympie gympie –