Phoradendron species: Mistletoe

Family

Viscaceae (Loranthaceae)

Common Name

Mistletoe

Plant Description

Consisting of over 400 species in 11 genera, the mistletoes are distributed worldwide, and are parasitic plants whose roots penetrate the branches and trunks of trees as a means of acquiring nutrients. The leaves are simple, evergreen, leathery, and oval or scale-like depending upon the species. Plants can be monoecious or dioecious, with the rather inconspicuous, greenish-white flowers being produced in leaf axils. Fruits are one-seeded berries, the flesh being mucilaginous, and the color varying from white to pink or red depending upon the species (Fig. 309).

The most common species of mistletoe found traditionally in households at Christmas time is Phoradendron serotinum, while the European counterpart is Viscum album.

Figure 309. Mistletoe berries.

Toxic Principle and Mechanism of Action

Numerous glycoprotein lectins are present in the leaves and fruits. The lectins inhibit protein synthesis in a similar manner to ricin and abrin, but are far less potent. Additionally, there are alkaloids found in the Phoradendron species that have vasoactive properties. Considerable research has been done to investigate the reported anticancer benefits of different mistletoe species [1]. The toxicity of mistletoe varies depending upon the species and may be associated with the host tree it is parasitizing [2].

Risk Assessment

Historically the ingestion of mistletoe berries has been associated with poisoning and fatalities in humans. Mistletoe berries are one of the most frequently ingested berries found in people’s homes [3]. However evaluation of mistletoe poisoning cases indicate that the ingestion of the berries has little or no effect and are rarely fatal unless eaten in large numbers [4,5]. It is reasonable to assume therefore, that household pets that eat only a few mistletoe berries will be minimally affected.

Clinical Signs

The most common signs of poisoning in humans include vomiting, abdominal pain, and diarrhea. Incoordination, hypotension, and cardiovascular collapse are occasionally reported. Treatment should be symptomatic and include fluids and electrolytes.