Cardiotoxic Plant Ingestion in Horses

There are different plants known to be cardiotoxic in horses. In all cases, symptoms of poisoning are an emergency requiring urgent veterinary care. Common examples of cardiotoxic plants include:

Oleander: Oleandrin is the cardiotoxin found in the oleander plant.

English Yew : Taxine is a highly toxic compound contained in the evergreen tree Taxus bachata or yew . All parts, except for the red berry, are highly toxic to horses.

White snakeroot: White snakeroot is a perennial herb native to central and eastern North America, containing the toxin tremetol. Tremetol can be passed through mare's milk, causing poisoning in foals.

Rhododendron, Laurel, and Azalea : These plants are common sources of grayanotoxins, a subclass of cardiotoxins.

Cardiotoxic plant ingestion is uncommon in horses, but depends on the prevalence of the plant species in a particular geographic area. Of the cardiotoxic plants, oleander ingestion is the most common toxicosis in horses, and is typically associated with contaminated hay.

The ingestion of cardiotoxic plants is an emergency. Immediate veterinarian attention is highly recommended as, in most cases, early treatment is the best way to increase the probability of a positive outcome.

A variety of plants contain toxins that affect the heart of horses. Toxic plant species include:

• Oleander
• Summer pheasant’s eye
• Foxglove
• Lily of the valley
• Dogbane
• Some species of milkweed
• Yew
• Avocado
• Death camas
• Cheeseweed mallow
• Rhododendron
• Laurel
• Azalea
• White snakeroot

Of these, the most commonly ingested plant is oleander. Most cases of oleander ingestion occur from eating contaminated hay or hay cubes.

Diagnosis is generally based on symptoms and history of exposure. Further diagnostic tools for horses presenting with cardiotoxic plant ingestion symptoms include:

• Physical examination
• Electrocardiogram
• Blood work
• Analysis of stomach contents for plant material

The first step of treatment involves removing the contaminated feed or toxic plants from the horse’s diet. Early treatment, typically before symptoms arise, focuses on gastrointestinal decontamination and administration of activated charcoal to reduce toxin absorption.

Note: always consult a veterinarian before administering medication, including activated charcoal, to horses at home.

Once symptoms appear, treatment is generally of a supportive care nature and includes:

• IV fluid therapy
• Medications to prevent heart arrhythmias
• Antibiotics
• Supplemental oxygen

In most cases of severe poisoning, once symptoms arise, it is too late for treatment to be effective. Many horses are euthanized due to a poor prognosis.

Prognosis ranges from guarded to extremely poor, depending on the amount and kind of toxic plant ingested. Horses that survive cardiotoxic plant ingestion often have long-term heart damage that prevents return to athletic performance.

Heart muscle damage caused by cardiotoxic plants can be completely avoided by preventing ingestion. Strategies include:

• Proper disposal of lawn and paddock clippings
• Removal of poisonous plants from pastures and paddocks
• Regular inspection of paddocks and lawns for weeds
• Researching plants prior to planting in gardens or areas where horses may have access
• Inspecting hay carefully for weed contamination
• Providing adequate forage or hay to reduce cardiotoxic plant consumption

Cardiotoxic plant ingestion is uncommon in horses, however the risk depends on the geographic location. Horses with adequate access to hay or forage are less likely to ingest toxic plants.

• Supportive care