This section is from the book "The Farmers Ready Reference Or Hand Book Of Diseases Of Horses And Cattle", by S. C. Orr. . Also available from Amazon: The Farmer's Ready Reference;.
There are two plants, commonly known as "loco" or "crazy weed," which, when eaten for some length of time, produce the peculiar condition called "locoism" or more commonly," locoed." These terms, like many other oddly-sounding words and phrases of the West are of Spanish origin. The word "loco" in the Spanish tongue signifies fool; hence, the literal translation of the phrase "loco weed" is fool weed. Botanically, one of the plants is called Astragalus Mollissimus, the other Oxytropis Lamberti, both belonging to the natural order Leguminosoe. It is not during the summer months when the range is covered with green grass that animals first learn the habit of eating loco; but after the pasturage has become closely eaten and dry and the ground is covered with snow, then the feed being scarce, the half-starved animals wandering about in search of something with which to satisfy the pangs of hunger, are tempted to eat the bunches "of loco weed." The center of the bunch, being well protected by the outer leaves, is tender and juicy, even in midwinter, and also has a peculiar sweetish taste, which, once learned, seems to be relished by the animal and is never forgotten. However, an animal when first beginning to eat the weed will not, as a rule, leave other food to search for it, but will only eat it as it accidentally finds it in the way.
I have seen horses that had been eating it in this casual way for a month or more, and were then taken up and fed upon hay and grain, which never exhibited any of the symptoms of the disease whatever. And I have also seen horses that had become affected to quite an extent - even so much that they would not lead, but would run backward instead - taken up and fed upon hay and grain, but given no medicines, and in time recovered sufficiently to be broken to harness and become good workers in the team, but would never learn to lead by the halter and were never safe under the saddle.
Then I have seen others in which the habit of eating "loco" had become so confirmed that the appetite seemed to crave it in preference to any other food. Such an animal will, at first, generally feed apart from, but within sight of, its companions; it will follow a short distance behind them as they travel to water: wait till they drink and start back, then approach the water cautiously, drink and follow back to the grazing grounds. But after a while it will abandon the herd entirely, search out the patches of "loco," where it will remain, only leaving when forced by thirst to go to water; then it will start off in a dazed sort of a way, walking with feeble and uncertain step, returning to the same place again as soon as it has quenched its thirst.
The poor victim continues in this routine, gradually growing thinner in flesh until it finally dies, apparently more because of mal-assimilation than starvation, unless, as frequently happens, it falls into a pit or runs over a bluff and breaks its neck.
The active principle, whatever it is, of the plant seems to affect chiefly the brain, but to some extent the entire nervous system. What that active principle is or under what conditions it must be eaten has never yet been ascertained. One of the peculiar effects of the plant upon the animal eating it is the dilatation of the pupil of the eye, as after administering belladonna or its alkaloid atropine.
Treatment. - In extremely bad cases treatment is useless; but in only moderately-affected cases the symptoms can be so palliated as to render the horse capable of doing good service in the team for several years. Place the animal in a stall where all is quiet; give a sufficient dose of Barbadoes aloes to operate moderately; give laudanum in drench, or morphia hypodermic-ally, often enough to correct the dilatation of the pupil of the eye; give bromide of potassium in doses of two or three drachms each often enough to quiet the symptoms of nervousness, adding an equal quantity of chloral hydrate to each dose if the bromide does not have the desired effect. As soon as the foolish actions of the animal begin to subside it should be given, twice a day as a tonic, a dose composed of two drachms each of gentian and foenugreek, one drachm of sulphate of iron and one drachm of nux vomica. Feed liberally and give pure water to drink. Turn the animal out in a yard every day for exercise, and after it has become tractable it may be put to light work. The degree of success in managing such a horse will depend much upon the coolness and patience of the driver. A "locoed" horse and a "locoed" driver should never be allowed in the same team.