EQUINE VETERINARY EDUCATION / AE / MAY 2015
269
Review Article Plants poisonous to horses in Europe
F. Caloni* and C. Cortinovis Department of Health, Animal Science and Food Safety (VESPA), Università degli Studi di Milano, Milan, Italy. *Corresponding author email:
francesca.caloni@
unimi.it
Keywords: horse; Europe; plant; poisoning; toxicity
Summary Poisoning of horses by plants is a relatively common veterinary problem and may occur when the fresh plant is ingested in pasture or when the plant contaminates hay, silage and feed. In Europe, horses are particularly at risk of poisoning by Cynoglossum officinale (houndstongue), Datura stramonium (jimson weed), Hypericum perforatum (St. John’s wort), Nerium oleander (oleander), Pteridium aquilinum (bracken fern), Robinia pseudoacacia (black locust), Senecio jacobaea (tansy ragwort), Taxus baccata (European yew) and Thuja occidentalis (white cedar). For each of these poisonous plants the information given in this review includes plant distribution, conditions of poisoning, toxic active principles and clinical signs.
Introduction
Based on toxicological data, poisoning of livestock and companion animals by plants is a relatively common occurrence in Europe (Berny et al. 2010; Caloni et al. 2012, 2013; Cortinovis and Caloni 2013). Horses appear to be very susceptible to many different plant species (Botha and Naudé 2002; Cortinovis and Caloni 2013) and accurate identification of the plant involved, which often requires recognition of the scientific and common names by a qualified person, is needed (Anadón et al. 2012). The susceptibility of horses to a specific poisonous plant may also be related to the anatomical and physiological peculiarity of the gastrointestinal tract. Poisoning may occur when the fresh plant is ingested in pasture, especially when horses are exposed to poor grazing or when the dried plant parts contaminate hay or silage (Panter et al. 2012). Clinical signs may vary from mild gastrointestinal irritation to sudden death and a diagnosis can rarely be made by the clinical syndrome alone (Anadón et al. 2012). Clinical signs accompanied by a history of exposure to the plant are usually needed. Unfortunately, most cases are not diagnosed or suspected before necropsy and identification of the plant parts in the stomach (Anadón et al. 2012). The present review focuses on the common plants involved in the poisoning of horses in Europe and provides an overview of the poisoning episodes occurring in European countries.
Cardiac glycoside-containing plants
Nerium oleander (oleander) Oleander (Fig 1) is a flowering evergreen shrub from the Apocynaceae family commonly used for ornamental landscaping in Mediterranean Europe. The compounds responsible for the toxicity of oleander are several cardiac glycosides (oleandrin, neriin, digitoxigenin, folinerin and rosagenin) that are found in all, either fresh or dried, parts of
the plant (Anadón et al. 2012; Renier et al. 2013). Cardiac glycosides inhibit the cellular membrane Na+/K+-ATPase, resulting in an electrolytic disturbance that affects the electrical conductivity of the heart (Joubert 1989). Sudden death is the most common result of horse poisoning by oleander and it has been estimated that the ingestion of as little as 10–20 leaves may be lethal for an adult horse (Renier et al. 2013). When clinical signs are observed, they are seen to appear within a few hours of ingestion and may include abdominal pain, diarrhoea, bradycardia or tachycardia, a variety of cardiac arrhythmia, cold extremities, sweating, weakness and inappetance (Hughes et al. 2002; Anadón et al. 2012). Renal failure and neurological abnormalities may also develop (Renier et al. 2013). In Belgium, after the ingestion of oleander, 3 male horses exhibited signs of anorexia, depression and weakness (Durie et al. 2008). Thoracic and abdominal auscultation revealed tachycardia and reduced gastrointestinal motility. Haematological investigations showed haemoconcentration in all affected horses. Increased haematocrit is generally associated with a poor prognosis for horses with oleander poisoning (Renier et al. 2013). In the described study, one horse died of ventricular fibrillation, whereas the others recovered and were discharged after 8–9 days (Durie et al. 2008).
Pyrrolizidine alkaloid-containing plants
Cynoglossum officinale (houndstongue) Houndstongue is a biennial weed from the Boraginaceae family that invades many pastures and fields in Europe. It is generally unpalatable to horses and poisoning usually occurs due to ingestion of hay or feed contaminated with it
Fig 1: Nerium oleander (oleander). Photograph by C. Cortinovis. © 2015 EVJ Ltd
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