A wide variety of plants grow in the pastures that our horses graze and whilst most are not likely to cause any harm, there are a number of plants that do pose a risk to equine health. For this reason it is important to check paddocks on a regular basis throughout the year and remove or prevent access to any potentially poisonous plants. To give you an idea of what to look for I have included some of the more commonly toxic plants found in this country below.
Ragwort – This common yellow weed is the bane of many a horse owner as all parts of the plant are toxic with the alkaloid toxin found within it acting on the liver to cause irreparable damage. The bitter taste of it prevents most horses from consuming much, but if cut and left in the field or baled up in hay it becomes far more palatable as well as far more toxic. Unfortunately not much is required to cause significant harm and ingesting multiple small amounts can be just as serious as one large dose due to the cumulative effects of the toxin. Detection of ragwort poisoning is a real challenge as up to 80% of the liver may be damaged before horses start to show obvious signs. The things to watch for include weight loss, reduced appetite, diarrhoea or constipation, colic, photosensitisation of unpigmented skin, fluid collection under the skin and jaundice. In advanced cases there may be incoordination, circling, head pressing against solid objects, blindness, manic behaviour and convulsions. Unfortunately at this stage there is little that can be done as the liver is too damaged to recover, however treatment may be possible in the earlier stages.
Bracken – This whole plant is poisonous though the roots are the worst part and younger plants are more toxic than older ones. The toxic element is thiaminase which causes Vitamin B1deficiency and consequently degeneration of the nerve linings. It requires ingestion of significant amounts of the toxin for these effects to be observed which means it may take weeks or months for your horse to show signs of poisoning. Weight loss is usually the first indicator of bracken poisoning, but as the nerves supplying the muscles are affected weakness and gait abnormalities will develop and eventually progress to staggering. The heart rate may be unusually slow with an abnormal rhythm as well. As the syndrome runs its course it will ultimately lead to convulsions and death within 10 days of the initial signs being seen, but fortunately it is treatable if caught in the early stages.
Yew – Whether fresh or dried all parts of this tree are extremely toxic, with as little as half a pound sufficient to kill a horse. The alkaloid taxine is the poisonous component which causes depression of the heart rate and respiratory centre. Unfortunately death is very rapid, so rarely are these horses seen before they succumb to the poison. Signs that may be seen include trembling, slow heart rate, breathing difficulties and collapse.
Laburnum – Second only to yew as the most poisonous tree grown in the UK. The toxic element is the alkaloid cytosine to which horses are particularly susceptible. Look out for excitement, incoordination, sweating, convulsions and eventually death from an inability to breathe. Sadly there is no known treatment.
Oak – This common tree contains toxins known as tannins which are particularly concentrated in young leaves and acorns. Hence poisoning is most often seen at springtime or in the autumn. In small amounts little harm is done, but unfortunately once sampled horses can develop an addiction to the taste and consequently will actively consume more if they are able. The gastrointestinal system and kidneys are the main areas affected by oak toxicity, with signs including depression, constipation, passing red/ brown urine, diarrhoea, incoordination and reluctance to eat. Although again there is no counter to the poison, supportive treatment is possible if the signs are not too severe.
Watch for a reluctance to eat, colic, diarrhoea, tremors and convulsions. Occasionally ingestion of these plants may cause sudden death, but treatment is possible if discovered in time.
Buttercups – Often abundantly present in pastures and seemingly unavoidable this year in particular; these yellow flowers harbor a strongly irritant compound, protoanemonin. This can lead to blistering and ulceration inside the mouth, which may present as increased salivation, discomfort when eating and occasionally colic. Due to the pain inside their mouths most horses will stop eating before they consume enough buttercups to be toxic.
Privet Hedging – This strong ornamental shrub is frequently used to create borders, but should not be used to edge horse paddocks due to its extreme toxicity. The leaves and fruit contain several toxic glycosides that have an irritant effect on the gastrointestinal tract. Symptoms include diarrhea, abdominal pain, incoordination, partial paralysis, hypothermia, convulsions, and sometimes death.
St John’s Wort – Another yellow flowering plant, this inhabitant of hedges and woodland causes photosensitation. This means that patches of unpigmented (pink) skin react under exposure to sunlight, becoming very inflamed which in turn may lead to rubbing and self trauma. Though of much reduced toxicity when dried, St John’s Wort may still be dangerous when baled in hay.
Deadly Nightshade – This resident of hedges and woodland clearings is entirely poisonous, with signs of consumption that include pupil dilation and incapacity to stand. Fortunately death rarely occurs.
Potato – All of the parts of this plant that are found above ground are poisonous as well as tubers that have rotted, sprouted or turned green through sun exposure.
Above are some of the more likely poisonous plants that your horse might encounter. Other plants to be aware of if consumed in large quantities include: MORNING GLORY, AVOCADO LEAVES, BULB FLOWERS (e.g. Tulips/Daffodils), WILTED RED MAPLE LEAVES. And a few other plants to be avoided at all costs include: Rhubarb leaves/stem, Tomato, Hemlock, Blue Flax, Mustard, Larkspur.
If you suspect that your horse may have ingested a toxic plant or may be suffering from the effects of poisoning please do not hesitate to contact your vet for advice.
If you have any questions about any aspects of your horses’s health, or indeed your pet, farm or smallholding animals, please do not hesitate to contact us on 01892 835456. Putlands Veterinary Surgery is based in Paddock Wood in Kent, and offers a friendly, proffessional and personal service for all species of animal. We have four dedicated large animal vets with a wealth of equine experience between us, and are always happy to help