Page 105 - July 2019

• Thenightshadefamilyincludesblacknightshade, aka horse nettle, jimsonweed or thornapple, ground cherry, and cultivated varieties — potato, tomato, and eggplant. Fruits of the cultivated plants are not poisonous, but potato skins are toxic to horses. The leaves and stems of potato plants and other members of the nightshade family con- tain the alkaloid solanine, which causes nervous system impairment (weakness and incoordina- tion), digestive problems and abdominal pain. Green parts of the plants are toxic, and all parts
of the wild nightshades are poisonous. One to
10 pounds of black nightshade is fatal to a horse. Deadly nightshade is a vine-like weed that looks somewhat like a tomato or potato plant with tiny purple blossoms and small red berries. Horses usu- ally don’t eat the plant, but might nibble on it in a pen where there is nothing else to eat.
• Poisonhemlockispartofthecarrotfamilyand should not be confused with water hemlock, which is actually more poisonous but less often eaten. Water hemlock grows in shady moist areas and it only takes a very small amount to kill a horse. Poison hemlock grows two to 10 feet tall, with hollow, purple-spotted stems and a solid taproot. The leaves are finely divided, like carrot leaves. They smell like parsnip when bruised and have a nauseating taste. Poisoning is most com- mon in spring when the tender, succulent, highly toxic new leaves appear. Horses generally don’t eat hemlock unless they are short on other feed. Poison hemlock is found most often in pastures and along roadsides, and less often near creeks and irrigation ditches. Once eaten, it causes inco- ordination, staggering, drooling, and slow heart rate. Death is usually from respiratory failure, and treatment is generally of no help. Whether the animal lives or dies depends on how much he has eaten. It takes less than a pound to kill a horse.
• Russian knapweed is a member of the same family as yellow star thistle and causes similar symptoms if eaten over several weeks. Brain damage from this type of poisoning is irreversible and horses starve from the inability to eat. Horses affected are typically euthanized to spare them a slow death.
• Senecioandotherspeciesofsenecio,suchas groundsel, stinking willie, ragwort and tansy rag- wort, contain pyrrolizidine alkaloids that destroy the liver. A horse who eats these plants or con- taminated hay over a period of several weeks will eventually show signs of liver failure. Because the plants grow in early spring, they are often found in first-cutting hay. A horse is more apt to eat them in hay than at pasture unless good forage is lacking.
• SaintJohn’swort,alsocalledKlamathweedor goat weed, causes reaction to sunlight (photosen- sitization) and severe lesions in unpigmented ar- eas of skin with white markings. Saint John’s wort is toxic in all stages of growth, but is most likely to be eaten in spring when young and tender. Hay containing this plant can also cause prob- lems. The toxic compound is absorbed into the
“Horses may eat harmful plants inadvertently baled in hay or raked into lawn or hedge clippings.”
Torpedo is toxic to horses, who will often develop severe anemia and die.
Saint John’s wort causes reaction to sunlight and lesions in unpigmented areas of skin, and is toxic
in all stages of growth. On light-skinned horses
or those with white markings, the toxic compound sensitizes unpigmented areas to sunlight and causes red, swollen, sore and itching skin that peels or comes off in sheets. Animals must be moved out of the sun and treated.
Wilted Red maple leaves carry a toxin that breaks down red blood cells, causing rapid breathing, weakness, depression and brown urine. They can kill a horse within a day or two after they’ve eaten small amounts.
Crotalaria, a member of the pea family, contains pyrrolizidine alkaloids that destroy the liver.
Chokecherry leaves produces cyanide and hydrocyanic acid poisoning when damaged, interfering with oxygen transfer to the cells.
The horse will breathe heavily, grow excited and agitated, then become weak and suffer from convulsions before going into a coma. Symptoms appear soon after the leaves are consumed and, unless treatment is immediate, the horse may die.
Tall fescue itself isn’t toxic, but the fungus Acremonium coenophialum can infest the grass and cause problems in mares, including abortion, retained placenta and failure to produce milk, as well as impair circulation.
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EQUINE HEALTH