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Fig 5: Hypericum perforatum (St John’s wort). Photograph by C. Cortinovis.
et al. 2008; Vanschandevijl et al. 2010). Abnormalities in heart rate and/or rhythm have also been described (Vanschandevijl et al. 2010). Although death is not frequent (Anadón et al. 2012), fatal poisoning of horses following the ingestion of black locust have been reported in Belgium (Vandenbroucke et al. 2010). Ponies seem very susceptible, showing severe, reversible CNS signs attributable to hyperammonaemia caused by gastrointestinal disease (Vanschandevijl et al. 2010).
Other toxic plants
Hypericum perforatum (St John’s wort) St John’s wort (Fig 5) is a herbaceous perennial plant from the Hypericaceae family that produces golden yellow flowers. The plant is widely distributed in Europe and often grows as weed in woods, pastures, rangeland and along roadsides (Anadón et al. 2012). The toxic principle is hypericin, a primary photodynamic agent. Hypericin is contained in dark glands present on the surface of flowers, leaves and to some extent the stems of the plant (Anadón et al. 2012). St John’s wort is poisonous at all stages of growth. Young tender shoots may attract horses in the spring and hay contaminated with it can cause poisoning in the winter (Stegelmeier 2002). Clinical signs usually appear 24 h to 21 days after ingestion depending on the time required for hypericin to reach a critical concentration in the skin and the intensity and duration of the horse exposure to sunlight (Stegelmeier 2002; Anadón et al. 2012). In the skin hypericin reacts with ultraviolet rays and induces primary photosensitisation (Stegelmeier 2002). Clinical signs include photophobia, severe pruritus manifested as scratching and rubbing of the ears, eyelids and muzzle and dermatitis with erythema followed by oedema, serous exudation, scab formation and skin necrosis (Stegelmeier 2002; Anadón et al. 2012). The areas of skin most severely affected are those with minimal pigmentation and little hair protection (muzzle, ears, eyelids, face, tail, vulva and coronary bands) (Stegelmeier 2002). In Europe, cases of photodynamic dermatitis after ingestion of St John’s wort have been reported in episodes involving grazing horses in the Czech Republic (Modrá and Svobodová 2009).
Pteridium aquilinum (bracken fern) Bracken fern (Fig 6) is one of the 5 most abundant plants in the world (Vetter 2009). It belongs to the Dennstaedtiaceae family
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Fig 6: Pteridium aquilinum (bracken fern). Photograph by C. Cortinovis.
(formerly to Polypodiaceae) and is commonly found in woods, old fields, waste locations and roadsides throughout Europe (Vetter 2009; Anadón et al. 2012). Since bracken fern contains the enzyme thiaminase, which inactivates thiamine (vitamin B1), the signs of bracken fern toxicity in horses are essentially the same as those related to thiamine deficiency (Vetter 2009; Anadón et al. 2012). Horses commonly consume this perennial fern for several weeks or months prior to developing clinical signs including anorexia, weakness, rapid and weak pulse, tremor, staggering and motor incoordination. Fever as high as 40°C may also be present. Death is preceded by tonic-clonic spasms with backward inflection of the head and neck (Vetter 2009; Anadón et al. 2012). Necropsy reveals congestion of the brain, a swollen and oedematous cerebrum and, microscopically, necrosis of some neurons (Panter et al. 2012). Bracken fern is widely distributed in all European countries and the percentage of land covered by this fern is increasing, particularly in the UK where it accounts for 7% of the country’s surface area (Vetter 2009; Gil da Costa et al. 2012). However, to our knowledge, no horse poisoning episode in Europe has been reported in the literature.
Thuja occidentalis (white cedar) White cedar is an evergreen coniferous tree which belongs to the Cupressaceae family and is grown in Europe as an ornamental tree. White cedar proved to be the second most commonly involved plant in cases of horse poisoning in France (Chandes 2002; Berny et al. 2010). Poisoning usually occurs from the consumption of branches on the ground after being cut down or following a storm (Anadón et al. 2012). The toxic principle is the monoterpene thujone which occurs in nature as a mixture of α- and β-isomers. Clinical signs of poisoning are essentially related to the gastrointestinal tract and include hypersalivation, colic, bloating and bloody diarrhoea (Coenen 2010; Anadón et al. 2012).
Conclusions
In Europe the number of plants poisonous to horses is considerable. As a consequence, the possibility of plant poisoning should always be considered in differential diagnoses. Accurate identification of the plants is essential and this may necessitate recognition of the scientific and
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