Mistletoe

Mistletoe is the English common name for most obligate hemiparasitic plants in the order Santalales. They are attached to their host tree or shrub by a structure called the haustorium, through which they extract water and nutrients from the host plant.

Mistletoe is especially interesting botanically because it is a partial parasite (a "hemiparasite"). As a parasitic plant, it grows on the branches or trunk of a tree and actually sends out roots that penetrate into the tree and take up nutrients. But mistletoe is also capable for growing on its own; like other plants it can produce its own food by photosynthesis. Mistletoe, however, is more commonly found growing as a parasitic plant. 

There are two types of mistletoe. 

The mistletoe that is commonly used as a Christmas decoration (Phoradendron flavescens) is native to North America and grows as a parasite on trees from New Jersey to Florida.

The other type of mistletoe, Viscum album, is of European origin. 

The European mistletoe is a green shrub with small, yellow flowers and white, sticky berries which are considered poisonous. It commonly seen on apple but only rarely on oak trees. The rarer oak mistletoe was greatly venerated by the ancient Celts and Germans and used as a ceremonial plant by early Europeans. The Greeks and earlier peoples thought that it had mystical powers and down through the centuries it became associated with many folklore customs.

The risk of poisoning depends on the type of mistletoe and what part of the plant is eaten. There are several species of mistletoe. All are hemiparasitic plants that grow on host trees, such as oak and pine. 

The Phoradendron species contain a toxin called phoratoxin, which can cause blurred vision, nausea, abdominal pain, diarrhea, blood pressure changes, and even death. The Viscum species of mistletoe contain a slightly different cocktail of chemicals, including the poisonous alkaloid tyramine, which produce essentially the same symptoms.

The leaves and berries contain the highest concentration of toxic chemicals. Alternatively, drinking a tea from the plant can result in sickness and possibly death. That being said, the average healthy adult can tolerate a few berries. The risk of poisoning is higher for children and particularly for pets. Most of the risk comes from the effect the proteins in the plan have on the cardiovascular system.

European mistletoe ingestion has caused cases of poisonings and sometimes deaths. However, American mistletoe isn't as toxic. A study of 1754 American mistletoe exposures revealed none resulted in death, even though 92% of the cases involved children. Another study of 92 cases reported to poison control centers revealed no cases of death, even though up to 20 berries and 5 leaves were eaten. In one case, a child suffered a seizure, but researchers were unable to definitively link it to mistletoe consumption.

Eating one or a few berries is unlikely to cause sickness or death. However, anaphylactic reactions are known, so it's important to watch for indications of a reaction to the plant. Consumption of a large number of berries is extremely dangerous

Although mistletoe can be dangerous, it also has therapeutic uses. The plant has been used medicinally in Europe for hundreds of years to treat arthritis, high blood pressure, epilepsy, and infertility. However, it's important to remember the species in Europe (Viscum album) is less toxic than the species found in America (Phoradendron serotinum). 

Some studies indicate mistletoe may be useful in treating cancer, although further evidence is needed. According to the National Cancer Institute, mistletoe extract has been demonstrated to affect the immune system and kill cancer cells in the laboratory. It may also decrease side effects of radiation and chemotherapy. However, its use is not approved by the FDA.