St John’s wort was introduced into Australia in 1875 as a garden plant. It is now widespread with over 250,000 hectares in NSW. It appears to be suited to areas receiving >600mm of rain at altitudes from 600 — 1500masl. It causes primary photosensitization in ruminants and horses.
All parts of the plant contain high levels of hypericin which is photodynamic. If enough hypericin accumulates in the skin photosensitization occurs with resultant skin damage. St John’s wort is not very palatable and large quantities need to be consumed for photosensitive dermatitis to occur. Most outbreaks occur when St John’s wort is in the young stage and dominates the pasture. The narrow leafed variety contains 2 or 3 times as much hypericin as the broad leafed variety and the flowers contain 6 to 8 times more toxin as other parts of the plant. Hypericin is not completely destroyed in hay contaminated with St John’s wort.
Non-pigmented areas of the skin with little hair or wool cover will be affected if exposed to sunlight. In sheep the face ears and muzzle are the worst affected with extensive lesions in recently shorn sheep. In cattle skin damage is commonly limited to the white areas especially in Hereford and Friesian cattle. The muzzle and white markings on the legs in horses are commonly affected.
The history of access to St John’s wort, the location and appearance of the skin damage accompanied by shade-seeking behaviour means the condition is usually diagnosed on clinical signs. If a differential diagnosis is required liver function tests can usually confirm hepatogenous photosensitisation.
Animals should be moved to a shaded area or a darkened shed for 5 to 7 days to prevent further skin damage and allow the accumulated hypericin to be excreted.
They must also have continual access to water. Valuable animals may require treatments to prevent secondary skin infection and restore fluid and electrolyte levels.
If possible, and if alternative clean grazing is available, move the unaffected remainder of the mob when the first clinical signs of photosensitisation are seen. St John’s wort may be grazed in the low hypericin period of July to September for the narrow-leaved variety and May to October for the broad-leaved variety. There is extensive information on control in Agfact P7.6.1 fourth edition 2001 DPI NSW, St John’s wort control. Recent research suggests cattle are 3 to 6 times more tolerant of hypericin compared with Merino sheep and thus could remain on St John’s wort for longer periods in spring.
Agfact A0.9.38 (second edition) NSW DPI. Photosensitisation in livestock
Agfact P7.6.1 (fourth edition 2001) NSW DPI. St John’s wort control
Bourke, C.A. (2000) Sunlight associated hyperthermia as a consistent and rapidly developing, clinical sign in sheep intoxicated by St John’s wort (Hypericum perforatum). Australian Veterinary Journal 78: 483 - 488