This section is from the book "The Gardener's Monthly And Horticulturist V23", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
I have never seen a case of poisoning from Kalmia latifolia; but when I lived in Massachusetts, sheep were occasionally killed by eating the leaves when late snow storms covered up the grass after they were turned out to pasture in spring. Some relatives of mine had quite a number of sheep poisoned in this way at one time, and among them a cosset, but the latter was saved by giving it milk which it drank freely. Gaultheria procumbens (Teaberry, of this section, Chequerberry, of the N. E. states) was said to be a remedy. I have the impression that the leaves of Kalmia contain Prussic acid; cannot now give any authority for it.
A correspondent says on looking over something from the pen of the editor many years ago, he notes the opinion that the prevalent idea that the leaves of the Kalmia are poisonous was doubted, and he inquires whether we are yet of the same opinion. He firmly believes it is. All we can say is, that we know of no well authenticated case that it is, but of course should be glad to have anything made known that is conclusive on the subject.