St John’s Wort: Poisonous Plant Of The Month

St John’s Wort (Hypericum perforatum) has gained fame as a herbal remedy, but it is an important poison to livestock. Find out more

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The weed grows in open woods, dry meadows and fields and on grassy banks. It is tough and will tolerate acid or alkaline soil as well as heat and drought.

When the leaves are held up to the light, translucent dots can be seen which contain the photosensitising chemical hypericin. When the plant is eaten, hypericin is absorbed and migrates to the skin. In light coloured areas of skin, UV light reacts with the hypericin and damages the cell membranes of the skin cells – a reaction known as photosensitivity.

Which Animals Can It Affect?

Cattle, sheep and goats can all be affected – cattle when they consume 1% of their body weight, whereas sheep can tolerate up to 4% body weight. Signs of severe sunburn usually begin 2 days to 3 weeks following ingestion.

Due to the amount that must be consumed, there must be considerable amounts of wort on a pasture before problems are seen.When the plant is dried, 80% of the hypericin activity is lost, but it is still possible for animals to be poisoned by hay if enough wort is present.

Control is best achieved by keeping animals away from the plant. Once animals show signs of sensitisation, they should be kept out of direct sunlight. When the skin starts to peel, antibiotics and pain relief may be recommended under the direction of the vet.

Out of hours emergency

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