British Poisonous Plants & Shrubs

The toxin in brackenfern is thiaminase, an enzyme that destroys thiamine (vitamin B1). The horse then essentially suffers from a vitamin deficiency of thiamine, which causes myelin degeneration of peripheral nerves ( a loss of the fatty insulation layer to nerves that primarily control muscles). Poisoning can occur at any time of year, but is more likely in the late summer between August and October when other forages are scarce and the level of thiaminase is at its peak. Bracken is not considered palatable, but horses will eat it if no other forage is available, or they will consume it in hay or bedding, where it remains toxic. Some horses are believed to acquire a taste for it, and these horses will consume it even if other forages are available.

Horses need to consume bracken for one to two months prior to manifesting clinical signs. After this time horses may then be fed bracken-free forage and yet still develop clinical signs within 2 to 3 weeks. The first signs in horses is weight loss after a few days on bracken. Later, weakness and gait abnormalities are present, which progress to staggering, hence "bracken staggers". Affected horses may stand with their legs widely placed and their back arched. Muscle tremors and weakness is apparent when the horses are forced to move. Early in the course of the syndrome, a slow heart rate and abnormalities of the heart rhythm may be noted. Near the end of the clinical course, the heart rate and temperature rise, and the animals cannot get up and may have spasms and an upward arching of the head and neck. The syndrome runs its course, with death occurring within 2 to 10 days of the onset of signs.
First signs include:

• weight loss
• staring coat
• constipation
• slow pulse
• tucked up and/or hollow appearence

• incoordination
• restlessness
• severe muscle tremors
• gait abnormalities
• difficulty in moving and turning during excercise
• staggering (bracken staggers)
• raised pulse and heart rate
• convulsions
• unable to rise

Poisoning by oak is usually seasonal, being most common in spring when the young buds or leaves are eaten and the autumn when the acorns are eaten. Oak leaves and acorns contain tannic acid which is poisonous to horses and though eating a small number of leaves or acorns is almost certainly harmless, they can also be addictive, and once a horse has acquired a taste for them they can actively search them out. Also some animals seem to be more suseptible to oak poisoning than others with individual animals having different levels of tolerance.
Oak poisoning causes gastroenteritis and kidney damage.
Symptoms include:

• lack of appetite
• staring coat
• constipation followed by diarrhoea which may be bloodstained
• abdominal pain
• depression
• blood in urine

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Yew (Taxus baccata)
All parts of the yew tree are very poisonous, probably the most poisonous tree in Britain. The poison is not reduced by wilting or drying, so that clippings and fallen leaves are as toxic as the fresh plant. The poison is the alkaloid taxine, which affects the heart. In many cases the symptoms of yew poisoning are never seen, as the animal dies a few hours after eating it - one mouthful is enough to kill !
Symptoms include:

• muscular tremors
• staggering
• convulsions
• difficulty in breathing
• collapse
• death from heart failure