Oleander contains a poisonous principle similar to digitalis in its effect on the heart, causing arrhythmia and cardiac arrest. A veterinarian should be consulted immediately for supportive treatment to reduce the damage caused by ingestion of poisonous plants. Leaves of bracken fern are poisonous to horses both when fresh and when dry in hay. Cattle are far more likely to be poisoned by bracken than horses. It would be necessary for a horse to ingest hundreds of pounds of bracken over a week or more in order for a problem to occur.
Symptoms are slow to develop.
Well-timed injections of thiamine can successfully reverse the damage caused by ingestion of bracken fern. A veterinarian should be consulted immediately for supportive treatment to reduce the damage caused by ingestion of poisonous plants. 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. Avoid pasturing horses where this weed is abundant or keep animals under shade during normal sunny days.