Hypericum perforatum (perforated St John's wort, common St John's wort, millepertuis). Related species: H. crispum, H. calycinum (rose of Sharon, St John's wort). A member of the plant family HYPERICACEAE. Herbaceous plant with dense foliage; the leaves and stems contain small secretory glands which produce a translucent chlorophyll-free oil with a balsamic odour. The plant has large conspicuous, cup-shaped yellow flowers with prominent stamens; the fruit are ovoid capsules containing black seeds.

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Animals most affected
Sheep (cattle, horses).

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Etiology
Ingestion through grazing or consumption of fresh cuttings or contaminated hay or fodder.

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Toxicity
Contains hypericine, a red fluorescent pigment responsible for causing primary photosensitization in animals. The entire plant is toxic, particularly the flowers. Drying the plant does not modify significantly its toxicity. Toxic doses not known.

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Clinical features
There is a latent period between ingestion of the plant and manifestation of clinical signs, varying from several hours to several days. The effects occur exclusively when the animal is exposed to sunlight.

General effects

• anorexia, restlessness, prostration, occasionally convulsions.

Gastrointestinal effects

• jaundice (intermittent);
• constipation or mild diarrhoea;
• colic.

Local effects

• late onset (from several hours to 1-3 days) of photodermatitis in fine, unpigmented skin causing a red blotchy discoloration (like red wine sediment), severe pruritus, oedema with serous exudate and formation of black sticky scabs resulting in hard cracked skin;
• affected animals become very sensitive to touch, with hyperirritability, and extreme hyperaesthesia;
• possibility of stomatitis, keratitis, kerato-conjunctivitis, opacity of the cornea, risk of secondary infection, necrosis in and gangrene of cutaneous tissue;
• if treatment is given, mortality is rare.

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Lesions

• principally cutaneous: exudative dermatitis;
• degeneration of the liver, inflammation of the gall bladder.

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Treatment
No antidote. Symptomatic and supportive care only:

• place animal in shade for several days;
• treat cutaneous sores with topical antiseptics and antibiotics;
• administer desensitizing drugs (to reduce hyperallergenicity) and pain killers;
• anti-inflammatory drugs;
• nicotmamide (factor PP);
• antihistamines appear to be ineffective.