Page 106 - July 2019

bloodstream from the gut. When it reaches the skin, it sensitizes unpigmented areas to sunlight. Light-skinned horses or those with white mark- ings develop red, swollen, sore and itching skin that peels or comes off in sheets. Affected animals must be moved out of the sun and treated.
• WhitesnakerootgrowsinsandyareasoftheMid- west. Because it often stays green in early fall after other plants become dry, horses may be tempted to eat it. Symptoms of poisoning are weakness, lethargy, stiffness and incoordination. The plant contains the alcohol tremetol. A lethal dose is two to 10 pounds of plant material. Enzymes in the plant damage the heart muscle, after which stress can cause heart failure. Trematol also damages
the liver. It can be excreted in a mare’s milk and
poison a foal if a lactating mare eats the plants.
• YellowstarthistleiscommonalongthePacific Coast as well as in western, plains, and eastern
states — and it is spreading. Some horses devel- op a taste for it and eat enough to permanently damage the brain. Signs of poisoning include twitching lips, flicking tongue, involuntary chewing movements (ranchers call it “chew- ing disease”) and drowsiness. Affected animals have trouble eating and drinking, but are able to swallow. The horse may hold his mouth open as if something is stuck in his throat. The muscles of jaw and lips become rigid, and the horse has trouble grasping or chewing food.
TOXIC GRASSES
Tall fescue causes several problems in mares, including abortion, retained placenta and failure to produce milk. Although fescue grass itself is not toxic, the fungus Acremonium coenophialum that infests many stands of it is. This fungus can impair circulation and make it harder for horses to stay warm in cold weather, causing ear tips to freeze, and harder to dissipate heat in hot weather.
Some grasses, such as perennial rye grass and dried buckwheat, can cause photosensitization.
Sorghum and Sudan grasses, common in pastures in the Southwest and Southeast, can also be harmful. Their toxic effects are intensi- fied during a drought or after a frost because the glycosides in the plants are changed to cyanide, a potent poison. The affected horse is starved for oxygen, breathes heavily, grows weak, falls into a coma, and dies within a few hours.
Torpedo grass is a lush pasture grass in Florida and the South that is toxic to horses. Animals who eat this grass often develop severe anemia and die.
POISONOUS TREES AND SHRUBS
Many trees and shrubs cause problems for horses — even apple trees. If a horse eats too many apples, he may develop colic. When horses have access to fallen apples in a pasture or paddock, they may overeat, especially if the fruit is ripe. Some crab apples are especially troublesome. The horse may be fine until he exerts himself, then the com-
“Most poisonous plants that might be eaten by horses are broad-leafed flowering plants.”
Some grasses, such as dried buckwheat, can cause photosensitization and can affect light-skinned horses or those with white markings similarly to Saint John’s wort.
Horses bedded on wood shavings from Black walnut trees and horses at pasture who have access to them can suffer from colic and laminitis. The pollen also causes allergic respiratory reactions.
Oleander contains the toxins oleandrin and nerioside that affect the heart much like digitalis, producing erratic heartbeat and diarrhea. Just a few leaves of Oleander can be deadly, and death can occur within a few minutes or hours.
Castor bean seeds are the most toxic part of the plant, causing mild colic, diarrhea, sweating and a pounding heartbeat. Just a few ounces of castor beans can kill a horse.
Cocklebur poisoning can occur when horses eat the leaves in early spring because seedlings are more toxic than mature plants, causing rapid and weak pulse, labored breathing and leg and neck muscle spasms.
104 SPEEDHORSE, July 2019
Yew, a small ornamental evergreen with red berries and needlelike leaves, contains the highly poisonous alkaloid taxine which has a depressing effect on the heart. About 4 ounces, or one mouthful, of yew needles can kill a horse, often within minutes.
EQUINE HEALTH