Mistletoe: The Poisonous Plant We Love To Hang At Christmas

European or Common mistletoe, Viscum album

This season many of us will be hanging mistletoe as part of a long-standing tradition. And while kissing under its evergreen branches is a holiday ritual, the plant doesn’t necessarily have our best interests in mind. Why?  Because mistletoe contains a Christmas cocktail of toxins that can be harmful to human and pet health. Best to keep it out of reach if it’s going to be part of your seasonal decorations.

What it is

For all its romantic associations, mistletoe is in fact no loving plant. In nature, it is known as parasitic. That means it specializes in attaching to the branches of a tree or shrub and penetrating it with its roots to steal water and other nutrients.

And while its deep green, ball-like form adds a touch of ‘life’ to bare branches, once mistletoe gets its roots into a plant it starts to destroy it. This usually requires the removal of all infested limbs and in some cases entire trees in which there are large-scale invasions.

Attractive but parasitic bright green clusters of European mistletoe

Complicating things is the fact that mistletoe seeds are easily spread. Those pretty white berries that add a frosty touch to the sprig? Birds love them. And as they’re carried away, the berries’ sticky pulp drops onto the upper branches of shrubs and trees, effectively ‘planting’ seeds on other species.

The white berries of mistletoe

All told, it can take up to two years for a mistletoe to fully develop within a plant. Once it has firmly taken root, it finishes off the job by sending out clumps of aerial shoots that typically weaken and distort the host, sometimes even killing it over time.

What happens if I eat mistletoe?

There are two main species of mistletoe; Viscum album, known as European or Common mistletoe, and Phoradendron, known as American or Oak mistletoe. Both contain a mix of toxic compounds in their stems, leaves and berries that can be harmful to humans and pets if ingested.

The more toxic of the two is Viscum album. Native to Europe and western and southern Asia, it has pairs of smooth, oval shaped leaves and small clusters of sticky white berries. Viscum album contains a mix of chemicals that includes poisonous amounts of the alkaloid tyramine, which can cause stomach upset, nausea and vomiting, blurred vision, blood pressure changes and in rare cases even death.

The oval leaves and white berries of Viscum album, Common mistletoe

Not to worry (too much), though. In North America, viscum album is a rarity unless it has been purposely transplanted (California being the exception.) Instead, a similar species populates our forests. Native to the U.S. and Mexico, Phoradendron has shorter and broader leaves than the European species and larger clusters of white berries. It secretes a toxin called phoratoxin, which causes the same symptoms as Viscum album, but to a lesser extent.

The paddle-shaped leaves of Phoradendron, American mistletoe

The good news is that although until recently American mistletoe has been widely considered to be as poisonous as the European species, downing a few berries is likely to lead to no more than a stomachache. Studies have shown that you’d have to eat a whole lot of berries to experience these reactions. This according to the National Capital Poison Center’s recent studies describing American mistletoe exposures (mainly by young children at Christmas). The vast majority of patients who had eaten parts of the plant had no symptoms and there were no fatalities, even among those who had swallowed mistletoe on purpose.

Mistletoe poisoning in dogs and cats

In small amounts, mistletoe most likely will cause no more than mild gastrointestinal distress to your pet. But if your cat or dog accidentally consumes large amounts of the plant, it could lead to abnormal heart rate, collapse or even seizures. If you suspect your pet has eaten mistletoe, contact your veterinarian or Pet Poison Helpline for treatment recommendations.

Mistletoe is most harmful to small children and pets/Photo credit: Michael Pettigrew

Mistletoe as medicine

Perhaps due to its toxicity, the European species, Viscum album, has been used by herbalists for centuries to improve circulatory and respiratory problems and to treat a variety of conditions including seizures, hypertension, headaches and arthritis. More recently, mistletoe extract has shown promise in stimulating the immune system in some limited laboratory studies. Today in Europe it is also being used as a cancer treatment.

(Although the United States FDA has not approved mistletoe as a treatment for cancer or any other medical condition, it is nonetheless being studied in clinical trials.)

Mistletoe is currently being harvested in Europe for its cancer-fighting properties

The take-away

Used safely, mistletoe may do a lot more for humankind than just providing a romantic canopy. As we learn more and more about what plants can do, mistletoe’s powerful medicinal qualities are something to celebrate in addition to its decorative properties. Something to think about next time someone reaches in for a kiss under its bough.

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