Mistletoe Poisoning in Dogs

Mistletoe is a parasitic plant that grows in the crowns of certain types of trees. Both the American mistletoe of the Phoradendron genus and the European mistletoe of the Viscum genus have leathery yellowish green leaves with clusters of small white, yellow, or red berries. They may be encountered growing wild in many parts of Europe, North America, and Mexico. The American mistletoe has an affinity for oak trees, which is where it gets the nickname of oak mistletoe. Mistletoe is often used as an indoor decoration, particularly around the winter holidays. Both varieties of mistletoe contain a cytotoxin by the name of viscumin, which can cause gastrointestinal distress, breathing difficulties, and erratic behavior. In rare cases, it can lead to a slowed heart rate and eventual cardiovascular collapse.

The parasitic shrub mistletoe is moderately toxic to canines and humans as it produces a cytotoxin called viscumin. If your pet has consumed mistletoe, you should contact your veterinarian right away.

Mistletoe plants, both American and European, contain toxins that are damaging to the cell structure. The toxins responsible can be found in the leaves and berries of the plant. In humans, the poisoning symptoms also include hallucinations, which may account for the erratic behavior in other animals as well.

• Abdominal pain
• Breathing difficulties
• Cardiovascular collapse
• Diarrhea
• Erratic behavior
• Low blood pressure
• Slowed heart rate
• Vomiting 

Types

There are actually two different parasitic shrubs that go by the name mistletoe, and although they are not in the same family, they are similar in both appearance and toxic properties. They are found growing in the crowns of broad-leaved trees. 

The most common types of mistletoe native to Europe are Viscum album and Viscum cruciatum. They have yellowish-green pairs of leaves with a leathery texture. The Viscum album has small white or yellow berries in clusters of two to six that enclose a single seed and on the Viscum cruciatum, the berries are colored red. 

The most familiar mistletoe that is native to North America and Mexico is the Phoradendron leucarpum, commonly known as American mistletoe. It is remarkably similar in appearance but has slightly more rounded leaves and larger clusters of ten or more of white berries.

The lectin by the name of viscumin is responsible for the toxicity in both varieties. A cytotoxin, it attacks the cell structure of the animal that ingests it. Viscumin is similar to other plant toxins such as abrin and ricin in structure and in mechanism. Although similar to abrin and ricin, it is not as damaging, approximately 30 times less effective in attacking the cells than ricin. Viscumin is found throughout the plant but is somewhat more concentrated in the berries. This toxin is dangerous to canines and humans alike. Mistletoe is used as a decoration during winter holidays and also grows wild in many parts of the United States, frequently taking up residence in oak trees.

If you believe your pet has ingested any leaves or berries from a mistletoe plant, do not hesitate to contact your veterinarian. Mistletoe poisoning is occasionally fatal, and early treatment is the best course of action for a positive outcome. If you see your pet consuming plant material, the identification of the plant is often all that is required for an initial assessment of the origin of the toxin. If you didn’t happen to witness the ingestion of the plant, biochemistry profile, complete blood count, and urinalysis tests are likely to be prepared at this time to check for toxins or imbalances. 

Your veterinarian may also ask you about any opportunistic eating that may have been witnessed or suspected such as ingestion of compost, garbage, or household products that may produce illness, in addition to any prescriptions or dietary supplements being administered to your canine. This information may help in the attempt to determine what toxin is causing the symptoms.

If the time since ingestion has only been a few hours, your veterinarian will probably start by either inducing vomiting or performing a gastric irrigation, which is usually done with the use of general anesthetic. Activated charcoal may also be administered at this point to soak up as much of the toxin as possible before it can be released into the bloodstream. Supportive measures are often offered at this time, such as IV fluids for dehydration and oxygen therapy in the event that your pet is having difficulty breathing.

Due to the possibility of cardiac collapse, the heart will be closely monitored until the toxins have completely cleared the system. Gastroprotective medications may be recommended to counteract any symptoms of nausea and vomiting. Viscumin is also believed to have a detrimental effect on the liver so hepatoprotective medications may be warranted, especially in situations when large quantities of the plant were eaten.

Prognosis for ingestion of plant material from the mistletoe plant is usually fairly good. The size of the animal, the amount ingested, and duration between exposure and the start of treatment are all significant factors in the overall outcome. If your pet required a gastric lavage, which usually requires general anesthesia, he may have coordination difficulties when he first returns home, and he could be quite disoriented. Keeping your pet isolated from children and other pets may be a good idea until your companion’s system has fully dispelled the toxin and the anesthesia. Your veterinarian may also recommend more frequent monitoring of the blood chemistry in relation to liver impairment levels.