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Plant Poisoning in Horses & Cattle
Plant Poisoning in Horses & Cattle

Larkspur.

by Heather Smith Thomas of Salmon, Idaho

PLANT POISONING IN HORSES

CASE #1: Your child just brought his horse to the house through the front yard, even though you don’t want horses leaving manure piles on the lawn or trampling the flower beds. But little Billy is in a hurry because he forgot something and didn’t want to ride around through the barnyard to the back door. He ties old Trusty to a shrub by the sidewalk and runs inside. When he returns, he finds Trusty collapsed on the lawn, jerking and trembling.

CASE #2: It’s a crisp Fall morning and you’re going for a ride. But when you go to the pasture to get your horse, he’s by the bushes along the creek and doesn’t come. As you approach him you see he is agitated and breathing heavily. At first you think he’s been running and exerting, then you see his neck muscles trembling, his eyes watering, and he staggers when he walks toward you.

CASE #3: Your horses have been grazing a pasture where a number of weeds have been encroaching from the dry roadway next to it. Today you notice that one of the horses seems to have trouble eating and acts like there is something stuck in his mouth or throat. He is holding his mouth open, flicking his tongue in and out of the mouth and making chewing movements. He also seems a little dull and drowsy.

All three of these horses were poisoned. In Case #1, old Trusty was gasping for breath and died before you could arrive on the scene, alerted by your son’s screams. Your veterinarian’s diagnosis is yew poisoning; the shrub in your yard contains a highly poisonous alkaloid called taxine, which has a strong depressing affect on the heart. Just a couple bites of this evergreen will kill a horse very quickly.

Case #2 was chokecherry poisoning. Wilted leaves after fall frosts contain a chemical that produces cyanide under the right conditions — a very fast-acting poison. Most poisoned animals are dead by the time you find them, for they rarely survive more than one or two hours after eating the wilted leaves. But prompt treatment with intravenous injection of sodium nitrite and sodium thiosulfate can counteract the poison (the first ingredient combines with it to produce a non-toxic compound which is taken up and absorbed by the other ingredient so it can be harmlessly excreted from the body). If your vet got there in time and had the proper chemicals with him, he could save your horse.

Plant Poisoning in Horses & Cattle

Chokecherries.

Case #3 was poisoning from eating yellow star thistle — a noxious weed that is spreading and becoming more common in many states. Some horses, after sampling it, develop a taste for it and may eat enough to permanently damage their brain. Some of the signs of poisoning include twitching lips, open mouth and involuntary chewing movements; it’s often been called “chewing disease”. Affected animals have trouble eating and drinking but are still able to swallow. Muscles of the jaw and lips eventually become rigid and the horse can’t grasp or chew food.

Poisoning sometimes occurs in horses; various plants are toxic. The horse owner must try to make sure there are no poisonous plants, trees or shrubs in the pasture or hanging over the fence. Some of the toxic substances or plants that a horse might taste or chew on, even in small quantities, can cause death.

SYMPTOMS AND TREATMENT

You don’t always know what you are dealing with when you discover a sick horse. Poisoning can have a wide variety of symptoms, since some poisons affect the digestive tract and cause colic as a first sign while others affect the nervous system and cause the horse to become uncoordinated and staggering or mentally deranged. Other toxins may cause slobbering.

It can be difficult to determine whether you are dealing with poisoning or some type of infectious disease. And unless you know what the animal ate, you won’t know what to do for emergency treatment, since treatment will differ for different poisons. If you suspect poisoning, get immediate veterinary help, and try to figure out what the animal might have eaten. If treated in time, some kinds of poisoning can be resolved and the horse will fully recover. But other poisons may leave him permanently disabled, and still others are almost always fatal.

If your vet can determine from symptoms and any other evidence what the horse ate, he can more readily give proper treatment — if there is an antidote that will work for that particular poison. Depending on the toxin involved, he may try to remove any residual poison from the gut with purgatives and an adsorbent like activated charcoal, which collects some types of poison. A purgative (such as mineral oil) hurries the offending material on out.

POISON PLANTS

There are many plants that can cause illness and death in horses. Some can vary in toxicity while others are always extremely poisonous. Some are harmless under normal conditions (in small amounts or at certain seasons) but prove poisonous if eaten in large amounts or if chemically changed by freezing or enzyme activity (such as glycosides in chokecherry leaves which can turn into deadly cyanide).

Poisoning often depends on palatability (some deadly plants are readily eaten by horses and others are not, unless a pasture is overgrazed and feed is short, or the horse is bored and nibbling), stage of development, conditions under which it grew (drought or freezing may increase toxicity levels), portion eaten (some have highest concentration of poison in roots, seeds, or new sprouts).

Harmful plants may be inadvertently baled in hay, or given to horses in lawn or hedge trimmings. Some plants and shrubs used in landscaping (yard or garden, barnyard or fairgrounds) or growing along a fencerow, road, trail or ditch bank may be poisonous. A horse may nibble the shrubs that hang over or through his paddock fence, or nip off plants along the trail or at a rest stop when you are riding, if you aren’t paying attention to what he’s doing.

Plant poisoning should also be suspected if horses experience sudden illness or death after being moved to a new pasture or fed different hay. Poisonous weeds in hay can be a danger, since the hay-fed horse has no other food options (as does a horse at pasture). Some horses will sift and sort out the strange weeds, but greedy eaters may eat them. Even a not-so-hungry horse may eat harmful hay because the strange or obnoxious taste of the live poisonous plant may have decreased during the drying process.

The best way to avoid plant poisoning is to make sure your hay is good quality, containing no weeds, and familiarize yourself with the types of poisonous plants that grow in your geographic region or climate. Make sure none are growing in your pasture. Look at all the plants in a pasture or paddock before putting horses in, including the trees and shrubs, even if they are on the outside of the fenceline but within reach of horses.

If unsure about identity of plants or trees, take leaf samples to your county extension agent. Some plants are easily eradicated (taking out a tree or hedge, or several individual plants growing along a fence or ditch) but large patches may require herbicides. Your extension agent can advise you on how best to get rid of them.

Make sure no trimmings, pruned branches or discarded garden plants or weeds get put in or near your horse pasture or pens. Bored or curious horses may nibble strange plants, even reaching under or through a fence to taste them. Never let a horse have access to ornamental shrubbery or flowers, and remove any fallen tree limbs from pen or pasture immediately after a storm. If using straw for bedding, inspect it carefully as you put it into stalls, to make sure there are no weeds in it. When feeding hay, inspect it for strange plants and discard anything you aren’t sure about.

Horses usually avoid poisonous plants, but hunger, curiosity or boredom may lead them to try things they shouldn’t. If you ever suspect a horse may have eaten something toxic (showing behavioral changes, digestive upset or any unusual signs), get veterinary help immediately. Some types of poisonings are irreversible and always fatal, but others can be resolved with fast emergency treatment. Do all you can to prevent such problems, but also be prepared to act fast if the suspicion of poison should ever arise.

A SAMPLING OF PLANTS THAT ARE POISONOUS TO HORSES

Garden flowers and plants that are poisonous include buttercup, narcissus, daffodil, lily of the valley and delphinium. Wild delphinium is called larkspur. Rhubarb leaves contain oxylates that may crystallize in the kidneys, causing kidney failure and death.

Other plants that may poison horses include brackenfern, cocklebur, castor beans (grown as a crop in some areas), crotalaria (sometimes called rattleweed or showy crotalaria), death camus, equisetum (horsetail), fiddleneck, fireweed, tar weed, buckthorn, yellow burr weed, lantana (an ornamental flower), locoweed, nightshade, jimsonweed, ground cherry, poison hemlock, water hemlock, Russian knapweed, yellow star thistle, the senecios (groundsel, stinking willie, ragwort, tansy ragwort), Saint-John’s-wort (also called Klamath weed or goat weed) and white snakeroot. Potato skins can also poison to horses. There are hundreds of toxic plants, but some only grow in certain geographic areas — so you need to find out which ones might be a problem where you live.

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