Mistletoe by Bluefuton (CC BY-NC-ND 2.0)

Ah, Christmas. The season of over-indulgence, crappy eggnog, and shooting your eye out with an official Red Ryder, carbine action, two-hundred shot model air rifle. I shouldn’t be so Grinchy. It’s also the season for giving. Giving a kiss under the mistletoe, that is.

We’ve all heard, and perhaps witnessed or experienced, the tradition of snogging under the mistletoe. The practice of hanging mistletoe probably originated more than a thousand years ago with the Druids, who believed it to bring good luck and ward off evil spirits. Later, the servant and working class of England hung mistletoe over doorways, and men were allowed a kiss from any woman standing underneath it, and it was considered bad luck if she refused. Other versions involve mistletoe berries, in which a berry was plucked off before each kiss, and when the berries were gone, so were the kisses.

It sounds like all fun and games, with mistletoe the harbinger of festive follies, but much like how oleander is kind of a dick, mistletoe is really a poisonous parasite. Sorry everyone, it’s true.

Mistletoe is the common name given to well over a thousand different species, a hundred genera, and half a dozen families, and grow just about everywhere. But the one traditionally associated with Christmas, and native to most of Europe, is Viscum album. This mistletoe has inch long, thick, green leaves and white berries, and is a common decorative in the holiday season. In the United States, mistletoes are found throughout, even in arid and desert locales. The ‘Desert Mistletoe’, Phoradendron californicum, I’ve seen while hiking in the Mojave Desert is leafless, and with pink berries that feed the local avian population.

Invasion of the Harlequin Mistletoe by Boobook48 (CC BY-NC-SA 2.0)

Remember the ‘parasite’ part earlier? We have birds to thank for that. Birds eat mistletoe berries, and when human targets are nowhere to be found, poop on trees instead, with the gooey seeds sticking to the trees. Once germinated, mistletoe grows into the bark of the infected tree and eventually makes its way into the tissue where it can take nutrients and water from its host. Mistletoe though has its own chlorophyll and performs its own photosynthesis, so in reality it’s only a hemiparasite, but it’s still pretty cool, no? (1)

Here in North Carolina we can see clumps of mistletoe growing in deciduous trees, though it’s much easier to see them in the fall and winter once the hickory, oak, and gum trees have shed their leaves. So no, those clumps aren’t birds nests. And the fact that they are alive during winter is likely a reason why early peoples thought it had magical qualities: when all else is dead and dying, mistletoe is thriving.

Want to hear about another cool way mistletoe infects? I thought so. Ballistics. Dwarf mistletoe fruit contains a single seed coated with viscin (a sticky sap), and when internal pressure of the exocarp reaches its limit, the fruit explodes, sending the seed hurling at 1370 cm/sec with an initial velocity of 5000g. The seed is perfectly shaped (or so I’m told) for a ballistic, and the tiny seed can travel several feet (2). I’m impressed. There are even photos of this genetic material being discharged along with “a violent release of viscous material that is ejected behind the seed (3),” but this is a family-friendly site, so I’ll show some restraint for once. Perv.

Mistletoe Infestation by Orin Zebest (CC BY 2.0)

Enough with the plant porn, you’re here for the poisons. Mistletoe is considered to be extremely toxic, but a lot of that has to do with the type of exposure and ingestion. Most data supports mistletoe being mildly toxic. In a 7-year analysis of the American Association of Poison Control Center data system, 1,754 exposures were identified. The bulk of these exposure were accidental (95%) and involving children (92%). Most ingestions were just a few berries or leaves, but the kicker is that only 4% became symptomatic, with GI upset, and no mortalities. And of particular interest to the clinicians out there, GI decontamination techniques did not affect patient outcome (4).

Viscotoxin structure from RCSB Protein Data Bank

That’s not to say, however, that mistletoe is completely safe. Mistletoes contain several peptide-based toxins, with the most notable being phoratoxin and viscotoxin. These toxins are hemolytic and depolarize cell membranes in skeletal and cardiac muscles, which could pose a risk if ingested, like in a concentrated tea (5). These same peptide toxins have also been shown to be cytotoxic to cancer cells (which really isn’t that rare, so don’t get excited) (6), but may help explain why there is a growing movement towards mistletoe as a “natural” and “holistic” treatment for cancer.

But this isn’t exactly unique either. An interesting letter to Lancet in 1904 from a British officer in the Indian Army Medical Corps wrote about Persians who used mistletoe concoctions to “purge away black bile and mucous humours” and “drawing out gross humours from the depths of the body” (7). And the doses used? A misgal, of course. What’s a misgal? Just the weight of 96 barleycorns. I don’t know how much one barleycorn weighs, let alone 96. And seriously, just round it up to 100. The metric system, it works.

So now that you’re all experts on everything mistletoe, from parasites, to exploding, flying streams of DNA, the only thing you have to worry about this holiday season is the creep with the mistletoe belt buckle.

Mistletoe by s3aphotography (CC BY-NC-ND 2.0)

Happy Holidays everyone!

** Homepage featured image of Mistletoe by Bluefuton (CC BY-NC-ND 2.0) **

References:
1. Glatzel, G., and B. W. Geils. “Mistletoe Ecophysiology: Host–parasite Interactions.” Botany 87.1 (2009): 10-15. [Free PDF]
2. Hawksworth, F. G. “Ballistics of Dwarf Mistletoe Seeds.” Science 130.3374 (1959): 504.
3. Hinds, T. E., F. G. Hawksworth, and W. J. Mcginnies. “Seed Discharge in Arceuthobium: A Photographic Study.” Science 140.3572 (1963): 1236-238.
4. Krenzelok, Edward P., T.d Jacobsen, and John Aronis. “American Mistletoe Exposures.” The American Journal of Emergency Medicine 15.5 (1997): 516-20.
5. Sauviat, Martin-Pierre. “Effect of Phoratoxin B, a Toxin Isolated from Mistletoe, on Frog Skeletal Muscle Fibres.” Toxicon 28.1 (1990): 83-89.
6. ohansson, S., J. Gullbo, P. Lindholm, B. Ek, E. Thunberg, G. Samuelsson, R. Larsson, L. Bohlin, and P. Claeson. “Small, Novel Proteins from the Mistletoe Phoradendron Tomentosum Exhibit Highly Selective Cytotoxicity to Human Breast Cancer Cells.” Cellular and Molecular Life Sciences (CMLS) 60.1 (2003): 165-75.
7. Ranking, George. “Mistletoe.” The Lancet 163.4202 (1904): 756.