Which plants are most toxic to horses? In this series of posts, I continue to describe the most common plants that are toxic to horses.
Foxglove (Digitalis Purpurea) is a biennial plant that reaches about 1.5m in height. Its flowers are bell or helmet-shaped and they may be either purple or white.
What the herbalist says:
The herbalist Culpeper tells us that: This hath many long and broad Leaves lying upon the Ground dented about the edges, a little soft or woolly, and of a hoary green colour among which rise up somtimes sundry Stalks, but one very often bearing such Leavs thereon from the bottom to the middle, from whence to the top it is stored with large and long hollow reddish Purple Flowers, a little more long and eminent at the lower edg, with some white Spots within them, one above another, with small green Leaves at every one, but all of them turning their Heads one way and hanging downwards, having some threds also in the middle, from whence rise round Heads pointed sharp at the ends, wherein smal brown Seed lieth. The Roots are many smal Huskie Fibres, and some greater strings among them; The Flower hath no scent; but the Leavs have a bitter hot taste. Vertues and Use. This Herb is familiarly and frequently used by the Italians to heal any fresh or green Wound, the Leavs being but bruised and bound thereon; and the Juyce therof is also used in old Sores, to clens, dry, and heal them. The Decoction hereof made up with some Sugar or Honey is.
The active agents:
The poisonous agents in Foxglove are several. They are known as cardiac glycosides. These include digitalin, digitoxin, and several others. If ingested, abdominal pain, irregular pulse, tremors and convulsions may follow. Foxglove is not usually ingested by animals. It is easily avoided when growing in pasture, but more dangerous when it is incorporated in hay.
In herbal medicine, Digitalis has been used in the treatment of internal bleeding, in inflammatory diseases, in epilepsy, and delerium tremens. Digitoxin is commonly used in heart medication for humans and horses but in tightly controlled doses.
Poisoning with Foxglove?
According to Simon Constables Equine Vets, these plants are thankfully unpalatable to horses and are rarely eaten as a fresh flower. If they are caught up in hay or haylage they can however be accidentally eaten. The effects on the heart increase the strength of heart contraction but slow conduction between the top (atrium) and bottom (ventricle) parts of the heart. This shift can lead to a fatal arrhythmia where the heartbeat becomes irregular. When the dose is sufficient, it may cause cardiac arrest. Treatments involve binding the digitoxin with activated charcoal. Oral liquid paraffin and fluids are given to reduce toxin absorption. If caught early enough, suitable drugs include atropine and lignocaine which help to reduce arrhythmia.
Whilst it is not suggested that the plant be removed from pastures (unless in great quantity), it is important to inspect hay/haylage. In this way, we can ensure that traces of Digitalis have not contaminated our horse’s staple.
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Keith Allison: A guide to Plants Poisonous to Horses. J.A. Allen. Revised edition, 2011.
John Gerarde: The Herball or General Historie of Plantes, 1636.
Culpeper: Index Of Herbs By Nich. Culpeper, Gent. Student in Physick and Astrologie, 1616, 1654