This section is from the book "A Guide To The Poisonous Plants And Weed Seeds Of Canada And The Northern United States", by Robert Boyd Thomson, H. B. Sifton. Also available from Amazon: A guide to the poisonous plants and weed seeds of Canada and the northern United States.
The Bracken Fern grows commonly in pastures and old hayfields, especially in shaded parts, and also in open woods. It has long been suspected of containing a poison, but definite proof was wanting until the publication in 1917 of the experiments of Drs. Hadwen and Bruce of the Health of Animals Branch of the Canadian Department of Agriculture. Their investigations have proved that it causes the disease called "staggers" which has given trouble to horse owners in British Columbia and some of the western United States. The following quotation from the work of these men will illustrate the extent of the harm which may be done: "During the hard winter of 1915-16' the mortality amongst horses in the Fraser Valley and on Vancouver Island was very heavy. As an extreme case we cite the following: In the little village of St. Elmo, B.C., out of twenty-four horses owned by eleven farmers, sixteen died of Bracken poisoning, four recovered, and the balance (four) did not take the disease."
The symptoms of the poisoning were determined by feeding experiments with hay to which a definite amount of Bracken was added. No irregularity was noticed until after several weeks of feeding, the first indication being generally an unsteady gait, accompanied by nervousness and constipation. Then the eye became congested, and later there was constitutional or systemic disturbance with manifest symptoms of intoxication. Increasing weakness finally rendered the animal unable to stand, or even to lie in a natural position. In this condition he showed great excitement and usually battered himself up badly before death ensued. The appetite was good throughout.
Fig. 4. - Bracken Fern - Pteris aquilma. Rootstock and frond.
In rare cases horses have been known to show similar symptoms from eating Bracken in pastures. Except in the case of very greedy animals, they will not eat the fern either in hay or pasture if a proper amount of ordinary feed is provided.
Drs. Hadwen and Bruce refer to the treatment recommended by Dr. S. F. Tolmie of Victoria, late Dominion Minister of Agriculture, who has had a great deal of experience with such cases: "Remove all ferny hay and bedding. Administer a good brisk purgative, such as Barbados aloes seven drachms, calomel one drachm and ginger one drachm. Half-ounce doses of potassium bromide twice a day in the feed or drinking water. One or two drachms of potassium iodide three times a day is recommended. Give medicine in the feed or drinking water or with a syringe. Feed laxative food such as bran mash and carrots. Give enemas if necessary. When animal is very groggy place in slings with extreme care, avoiding excitement as much as possible. ... In some cases cold packs to the head are recommended." In certain remarks for the farmer who is unable to secure professional aid, the importance of a warm, quiet stable and of guarding against excitement is emphasized, and as an alternative purgative a quart of raw linseed oil recommended.
The Bracken is of wide distribution, growing under a variety of soil conditions, in both hemispheres. Its slender, glossy, dark-coloured, underground stem forms a tough mat exposed on ploughing but below the level of the grass roots. It gives off at intervals rigid erect stalks, two to four feet high, bearing at their summits broad branching fronds. The firm, upright, rigid character of the leaf stalk and the restriction of the foliage to its summit make a Bracken plot look solid from above but like a miniature open forest from within. In this respect the Bracken is distinct from our other native ferns, and so its habit forms an easy means of identification. At fruiting time the sporangia are also of value for identification. They are borne in a continuous line under the infolded margin of the leaflets.