Poisonous Plant Quiz

St. John's-WortThat's right!  St. John's-Wort occurs throughout Ontario in pastures, edges of woodlots, roadsides, abandoned fields, water areas and occasionally in lawns and lower borders.

If eaten by horses, St. John's-Wort may cause photosensitization, since this weed contains black dots composed by hypercin, a pigment that is absorbed by the body and activated by exposure to sunlight. Ingestion can result in a condition in which patches of white or light-coloured skin become seriously sunburned under normal exposure to sunlight.


  • photosensitivity
  • inflammation of the unpigmented portion of the skin
  • affected area becomes sore and reddened and may peel
  • tongue and mouth may be affected

Treatment: Avoid pasturing horses where this weed is abundant or keep animals under shade during normal sunny days.

A veterinarian should be consulted immediately for supportive treatment to reduce the damage caused by ingestion of poisonous plants.

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For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca

Author: Penny Lawlis - Animal Care Specialist/OMAFRA
Creation Date: Not Available
Last Reviewed: November 2003