Reassessment of the toxicity of Hypericum perforatum (St John's wort) for cattle

Abstract

Objective: To investigate the clinical effect of administering sufficient Hypericum perforatum to cattle to deliver quadruple the reported oral toxic dose. Animals Thirty-six yearling Hereford (n = 18) and Angus (n = 18) steers. Design: A series of six experiments was conducted, each using 12 animals in a 2 × 2 factorial design, with two breeds of cattle (Hereford, Angus) and two dose levels of hypericin, 1.5 mg/kg (treated group) and 0 mg/kg (control group). Each set of 12 steers was used in duplicate experiments, with all animals alternated between treated and control groups. Procedures: Treated groups received finely milled H perforatum administered orally in gelatin capsules to provide 1.5 mg hypericin/kg body weight. All cattle were then exposed to direct sunlight for 5 h per day for 5 successive days. Rectal temperatures were measured immediately before and at the end of each sunlight exposure session. Rectal temperature above 40°C, together with some other clinical sign of hypericin poisoning, was considered indicative of intoxication. Results: No animals developed a rectal temperature above 40°C or other clinical signs of hypericin poisoning. Conclusions While the reported bovine oral toxic dose of 3 g dried plant/kg body weight, for flowering stage, presumed narrow leaved biotype, H perforatum, is probably correct, the corresponding dose for hypericin of 0.37 mg/kg is incorrect. Based on its known concentration in this plant the toxic dose of hypericin for partially pigmented Hereford-cross cattle is estimated at about 10.5 mg/kg body weight and more than this for fully pigmented cattle. This would imply that cattle of the former type should be about three and a half times better protected against H perforatum toxicity than are unpigmented, wool protected, Merino sheep. Cattle, particularly if fully pigmented, may have a role in grazing management to control H perforatum.