Etiology

• Viscum album is a partially parasitic, evergreen plant. It grows on tress or shrubs. It is often found on oak or apple trees in the UK .
• It is a woody plant with long stems and thick, dark green leaves .
• It produces small yellow flowers in the spring.
• The fruits (berries) are white and are found in clusters.  The berries are present on the plant throughout the winter period .
• The plant Viscum album contains a combination of toxic compounds including toxic proteins (alkaloids), polypeptides (viscotoxins) and lectin proteins (eg, viscumin).
• The toxic components are mainly found in the foliage (ie, leaves and stems) of Viscum album. Viscotoxins can also be found in dried foliage.
• Viscotoxins are not present in the berries.
• Small amounts of the plant can be eaten by cattle without incident, however cattle should not be allowed to eat large amounts.
  • In countries where Viscum album grows, ruminants may eat it in times of drought, or when other sources of food are scarce.  Studies have been undertaken to assess its nutritional value.

Predisposing factors

General

• Parts of the plant ingested (ie, foliage compared to berries).
• Species of host plant.
  • The toxic components present in Viscum album are governed by the particular species of plant the mistletoe is growing on.
• Availability of and access to other forage:
  • Mistleoe foliage may be eaten by cattle in times when access to other food is limited.
• Time of the year:
  • Forage may be sparse in wintertime and so animals may be more likely to ingest mistletoe and other less palatable plants.
  • It is reported that the viscotoxins in the plant are at higher levels in summer and lectins are at higher levels in winter.
• Individual animal grazing behaviours.

Pathophysiology

• The toxic compounds present in Viscum album include:
  • Alkaloid proteins.
  • Viscotoxins.
  • Lectins (eg, viscumin).
• Although the mistletoe plant does contains toxic compounds, the plant itself is considered to be of low toxicity following ingestion:
  • After the ingestion of Viscum album plant material, the protease enzymes involved in the digestion process act to remove the toxins.
• Experimental animal studies have shown that viscumin and the viscotoxins present in Viscum album are toxic when Viscum album extracts are given parenterally:
  • Viscotoxins are cardiotoxins; in experimental studies they are negative inotropes and can cause bradycardia when Viscum album extracts are given parenterally.
  • Experimentally lectin proteins (eg, viscumin) have been shown to inhibit protein synthesis and cell growth and are cytotoxic.
  • The toxins affect cells with rapid regeneration potential, such as the intestinal mucosa.
  • Viscumin has been reported to cause haemagglutination of human erythrocytes in vitro, it is not known whether this also occurs in-vivo.

Timecourse

• Gastrointestinal effects may be observed relatively promptly following ingestion of the plant, although effects may also be delayed.