The Horse:Your Guide To Equine Health Care provides monthly equine health care information to horse owners, breeders, veterinarians, barn/farm managers, trainer/riding instructors, and others involved in the hands-on care of the horse.
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51 August 2015 THE HORSE TheHorse.com investigate the undesirable plants. Other horses will simply nibble on anything. Some of the Nastier Ones So, what trees and shrubs are toxic, and what do they do to a horse? Let's look at 10, starting with the most poisonous. Japanese yew "It is an ornamental shrub that can kill a full-grown horse with just a mouthful," says Williams. "Only (consuming) 0.1% of their body weight is necessary (for poisoning). This will cause respiratory and cardiac col- lapse typically too quickly for the owner to even call a veterinarian." Red maple "Any of the species of maple trees is toxic (box elders are in the maple family, for instance), but the red maple is the most toxic," says Williams. "When those three-lobed leaves wilt, they become very sweet but also very toxic, so horses will crave them even if they have other grass available. "The most important time to watch for problems is after a storm that might have caused a limb of the tree … to fall into the pasture," she continues. "Those wilting leaves are most toxic then. Only 1.5 pounds of leaves are necessary to cause symptoms of depression, lethargy, increased respiratory rate and heart rate, which will progress into coma and death." Leaves still are slightly toxic in the fall, she says, but most of them are already wilted and dead when they fall off the trees. Once the leaves die, they are neither as toxic nor as palatable to horses. Wild cherry This tree's leaves, seeds, and bark are all toxic. Williams says only about 2.5 pounds of leaves are necessary to cause toxicity in horses. Cherry leaves contain a precursor to cyanide that binds and interferes with the cellular use of oxygen. If an animal consumes a large enough amount of this toxin, he will basi- cally suffocate. Poisoning is difficult to catch in time to treat because signs come on so rapidly. Black locust Michigan State University Extension reports that this tree grows throughout the United States and Canada, and consumption of certain tree parts can be deadly to horses. Black locust seeds, leaves, bark, and twigs—whether fresh or dried—contain toxic proteins that affect the gastrointestinal tract as well as the nervous system. Just eight ounces of the leaves or bark can kill a 1,000-pound horse. Clinical signs begin as soon as one hour after consumption and include paralysis, abdominal pain, diarrhea, and abnormal heart rate and/or rhythm. Death can occur within a few days. Black walnut Horses suffer this tree's toxic effects not by consuming parts of the tree itself, but when bedded or stand- ing on black walnut shavings or sawdust. That dangerous contact with the tree's toxin can cause depression, lethargy, and laminitis. Clinical signs usually appear in a matter of hours, and it only takes a few shavings and a short exposure time to cause the toxicity. Oak These trees are moderately toxic. New leaves and young acorns, especially when green, are the most dangerous parts. The acorns' tannic acid can cause portions of the intestinal lining to slough, says Williams. Clinical signs of oak toxic- ity include poor appetite, weight loss, and impaction, followed by diarrhea, kidney failure, and edema. In severe cases, oak poisoning can be fatal. Boxwood shrubs This dense evergreen plant comes in several varieties, but all types and parts are toxic. Horses that consume boxwood can suffer neurologic signs, colic, and potentially even respira- tory failure, says Williams. Black Walnut Boxwood Shrubs Black Locust WIKIMEDIA COMMONS Oak Horse Chestnut Holly Tree Box Elders: A Relatively New Concern The rather recent discovery of box elder trees across the United States and Canada as the cause of a deadly muscle disease in horses coincides with the discovery of European sycamore trees in Southern and Central Europe as the cause of a similar disease there. Stephanie Valberg, PhD, DVM, director of the University of Minnesota Equine Center, who led a team of collaborators to discover seasonal pasture myopathy, proposes the name be changed to hypoglycin A myopathy (H-AM). She explains that the toxin hypoglycin A is pres- ent in seeds obtained in the autumn from box elder (Acer negundo) and sycamore maples (Acer pseudoplatanus) and is the likely cause of the problem, she says. Hypoglycin A breaks down muscle fibers, which then release their contents into the bloodstream and damage the kidneys. The disruption in muscle cells causes the horse's postural and respiratory muscles to deteriorate. There is a clear seasonality that coincides with ripe seeds, Valberg notes. Seed burdens vary annually, potentially explaining discrepancies in outbreaks from year to year. It's a bit of a mystery, but these trees might have been growing in some horse farms' pastures for years without causing problems. Whether the horses avoided eating the seeds, have some form of immunity, or just had plenty of other forage to eat is unclear. Valberg has discovered that horses particularly at risk for H-AM are those that are young or new to overgrazed pastures with Acer trees, are turned out more than 12 hours a day, and receive little supplemental feed. However, any adventurous eater can be affected, she notes. If possible, affected horses should be hospitalized and treated for muscle damage right away, says Valberg. With rapid and labored breathing, fast heart rate, and rapidly progressive muscle weakness and stiffness, death occurs in most cases within 72 hours of the onset of signs. About 50% of horses that die have cardiac damage, says Valberg. Even if the horse will be euthanized, the veterinarian might want to save blood and urine for testing at a specialized metabolic laboratory to determine the cause, as a standard post-mortem exam might not provide a specific diagnosis, says Valberg. For those North American horses that have fallen victim to the disease, a reporting site at the University of Liege, in Belgium, is interested in cases: http://labos.ulg.ac.be/ myopathie-atypique/declare-case-owners.—Maureen Blaney Flietner