Loco Plants. Crazy Weeds.—These are plants growing in the far Western States, the eating of which by horses and cattle is believed to produce loss of flesh, disordered vision, delirium, convulsive movements, or stupor and death. The chief local plants are Aragallus Lamberti, commonly known as rattle weed or white loco, extending from Alaska southward. A plant of more limited range is the purple loco, woolly loco or Texas loco (Astragalus mollissimus Torr... There are quite a number of other plants causing heavy losses to stock men of the West.
Isaac Ott found a plant, which he believed to be A. mollissimus, to act as a violent spinal poison and mydriatic (N. R., Aug.,J882), and, according to Carl Ruedi, the decoction produces in the rabbit great hilarity, excitement, and even ferocity, and contains an alkaloid, locoine, and an acid. (Tr. Col. State Med. Soc., 1895.) On the other hand, O'Brien, of the State Agricultural College of Colorado, was not able to obtain any active chemical substance from six U. S. species of the genus, including the two previously mentioned, while in a very careful study of a plant which was neither in flower nor in seed, but whose identification as A. mollissimus was clear, H. C. Wood found that it was not poisonous to rabbits or dogs, a result which has been confirmed by Ingersoll (Proc. A. Ph. A., 1890) and O'Brien (Bulletin 25, Agricultural Experiment Station of Colorado) and others. It would appear that A. mollissimus is not a poisonous plant. Other species of the genus, especially A. Hornii A. Gray, A. Bigelovii A. Gray, and A. caryocarpus Ker. Gawl, have had poisonous properties attributed to them, and in the fruit of the last-mentioned species G. B. Frankforter believes he has demonstrated the presence of an alkaloid. In 1908 Crawford brought forward the interesting theory that the poisonous effects of the loco weed were due to barium, but Alsberg and Black (Bull. 246, U. S. Bureau of Plant Industry, 1912) have shown that many other plants in the same localities contain equal quantities of barium, but do not produce the characteristic symptoms, and that the barium is in such an insoluble form as to render improbable that it would be absorbed in the intestinal tract. There can be no doubt that domestic animals are destroyed in the West in very large numbers from an affection which is known as "loco," and hundreds of thousands of dollars have been spent in bounties by the State of Colorado for the extirpation of the supposed poisonous astragalus. It is probable that many of the deaths attributed to loco have been from other causes. Thus, Ingersoll of the Colorado State Agricultural College, found in a large number of locoed sheep masses of Taenia expansa, which he believed to be the cause of death. It is a probable explanation that the phenomena of loco disease are due to the fermentation of an astragalus or other plant in the intestines of the animals, and the production of one or more poisons which are absorbed and produce narcotic symptoms. If this be correct the loco disease is parallel in its etiology and nature to the lathyrismus of Europe. See Lathyrus sativus;also N. Y. M. J., 1889, xlix. Similar effects are produced in horses by the pods of the mesquite (Prosopis juliflora DC.., when eaten too freely.
According to Haeckel (Die naturlichen Pflanzen Familien), the Russian grass Stipa capillata L. frequently kills sheep, not, however, by a direct poisonous action, but by its glumes working through the skin into the vital organs; and Stipa Vaseyi, which is said in some parts of New Mexico and Texas to be known by the name of "sleepy grass," is believed by some ranchers to be the cause of loco disease. Lescohier (M. R., 1911, xx, p. 273) finds that the sleepy grass is an active respiratory poison, and also has some depressant action upon the heart muscle. He suggests that the apparent narcosis produced by the injection of it into the lower animals, is due to partial asphyxiation. This explanation, however, seems improbable in light of the symptoms which are reported in animals which have eaten the grass. Gillespie (B. M. J., 1908, ii, p. 1059) reports that the drug is not only a narcotic, but also a diuretic and sudorific.
The Dispensatory of the United States of America, 1918, was edited by Joseph P. Remington, Horatio C. Wood and others.