In this post I'd like to talk about a harmful reaction that may occur when Hypericum perforatum, commonly known as St. John's wort, is exposed to strong sunlight.
St. John's wort is used in modern medicine to treat depression and anxiety as well as other ailments. When rubbed on skin it may cause photosensitivity and lead to burn-like damage of the exposed skin. Individuals with lighter skin tone are more likely to experience this reaction.
Horses know not to graze this plant but don't avoid walking or rolling in it. Most vulnerable to the harmful reaction are young foals with white socks. In our stud it has happened to three, all piebalds younger than 3 months. The wounds are shallow, only skin deep, and scab over almost immediately. The scabbed spots tend to grow if left unchecked.
What to do about it?
Step one is to remove the mother and foal from the pasture and place them somewhere shady without St. John's wort. Step two is to wash the affected spots with a solution of povidone-iodine. Then use the solution to soak the scabs until soft and rub them off. Hair will fall out with the scabs! Do not be alarmed by this. Soaking and removing the scabs may take up to several days of frequent "baths" depending on the severity of the initial damage - sometimes scabs are not ready to come off yet and one must wait to avoid causing new open wounds. It is recommended to cover such sensitive areas with some healing balm (if you have any) to protect them from dirt and flies. The end result is healthy new pink skin. The hair will grow back in a couple of weeks.