Effects of Hypericum perforatum (St. John's wort) on animal health and production
C.A. Bourke, Agricultural Research and Veterinary Centre, Forest Road, Orange, New South Wales 2800, Australia.
Summary
Hypericum perforatum and related species contain a mixture of photodynamic compounds represented by hypericin. Visible wavelengths of light energize hypericin and make it toxic to cell membranes. This effect occurs in exposed non-pigmented skin areas on animals that eat wort. Photosensitization is the skin inflammation that follows. Hypericin can enter the brain and exert a stimulant effect upon behaviour, and an altered sense of temperature perception. In the peripheral nervous system hypericin alters heart, blood vessel, and intestinal function by inhibiting a catechol enzyme. A transient elevation of adrenaline and noradrenaline follows. Sucking young can ingest hypericin in their mother's milk.
Chronic hypericin ingestion causes weight loss, failure to gain weight, reduced milk and wool production, and reduced reproductive performance. Wort infested pastures can halve per hectare carrying capacity, and some livestock will die because of grazing wort. Horses are more susceptible to hypericin toxicity than cattle, cattle more than sheep, and sheep more than goats, probably because of differences in liver enzyme metabolizing systems. The presence of skin pigment or a thick wool covering will reduce the photosensitizing effect of hypericin. The other effects of hypericin exert themselves regardless of sunlight exposure. Annual animal production losses on wort infested pastures in New South Wales can approximate $22.5 million.
Plant Protection Quarterly (1997) 12 (2) 91-92.