Poisonous Plants

By Marcia King

May 1, 2001
At a training barn in Washington, two prized half-Arabian horses munched on wood chip bedding that had been delivered to the facility. Moments later, both were dead. Among the chips lay a branch of yew, a type of evergreen that can be fatal to horses. Every year, untold numbers of horses eat toxic plants, shrubs, and trees, sometimes with dire consequences.

Toxic plants can arrive on the premises in bedding, as leaves blown in from a neighbor's property, or baled up in hay. Poisonous plants and trees can be found growing wild and uninvited in pastures, along fence-rows, adjacent to trails, or beneath larger trees. Harmful plants can also be unwittingly introduced by owners in the form of ornamental shrubs, plants, and trees.

The old adage that animals won't eat toxic substances is nonsense. Says Audrey Pavia, author of Horses For Dummies, "Many horse owners are under the impression that horses have an instinctive knowledge of what is okay to eat and what is not. Unfortunately, this is far from the truth. Horses go more by what they think will taste good, rather than what they think might be bad for them. That's why it's vital for horse owners to know which plants in their area are harmful to horses."

Risk Management

The first defense against plant poisoning is to know the enemy (see below). County extension agents can provide a list of plants common to your area, and various web sites and reference books have descriptions and photos of toxic plants.

You also need to be aware of when plants are most dangerous. Michael Knight, DVM, Diplomate American Board of Veterinary Toxicology, and Medical Director of the ASPCA Animal Poison Control Center, notes that the leaves of red maples become more toxic after they turn and drop in the fall than when they're fresh. He also says that snakeroot, which grows under trees, is usually more tempting to horses in the autumn after the normal pasture grasses die back.

Don't assume that plants lose their potency or their appeal when they die. "Nightshade becomes more succulent and tasty as the plant is dying, for example after it's been sprayed with an herbicide," Knight says. "The poison lives on if nightshade is cut and put in with the hay. With cherry trees, wilted leaves release a hydrocyanic gas that can kill a horse within minutes."

Don't create inviting conditions for toxic invaders. "Make sure you're practicing good pasture management so the plants you want are growing there," explains Bob Coleman, PhD (animal science), Extension Horse Specialist at the University of Kentucky. "Typically, some of these poisonous plants start to invade because the desirable grasses are no longer competitive. By and large, poisonous plants are coarser, larger, and not terribly palatable. They often wouldn't be selected unless there's nothing else for the horse to eat."

Inspect the grounds where your horses graze and congregate, and your hayfields if you grow your own hay. Toxic weeds that grow in hayfields can get bound up with the hay, and many varieties retain potency when dried. "If you are unsure of what you're looking for, get an expert to walk your pastures and hayfields," Coleman advises. You can also mail in a questionable plant to local cooperative extension services, veterinary toxicologists at veterinary schools, or veterinary diagnostic laboratories. Call first to find out shipping procedures and to whom to direct the plant.

The experts advise that you also be wary of plants when traveling with your horse.

"Japanese yew (a popular decorative, easy-keeping plant) is a very common problem," Knight warns. "You see it where horses are tethered around arenas, grazing on the hedge." Pavia cautions vigilance when trail riding: "Keep strict control of your horse so he doesn't reach out and grab mouthfuls of harmful plants."

Weed Whacking

Eradication methods for unwanted vegetation generally consist of digging out the plant or spraying it with an herbicide. Consult references or experts (perhaps even your local plant nursery) to ascertain the safest and most efficient methods for dealing with specific plants, as you can create problems by using the wrong technique. Ragwort, for example, is one of those plants that retains potency and becomes more succulent when sprayed and dying.

Be careful when using an herbicide. Notes William M. Fountain, PhD (horticulture), Extension Specialist and Professor of Horticulture at the University of Ken-tucky Department of Horticulture, "Safety of herbicides and other chemicals depends on the material being applied, rate, time of year, weather conditions, and so on, as well as the condition and type of the animal being exposed."

Carefully follow labeled directions when using an herbicide. The time frame in which a horse can return to a treated area will vary with the plant's potency/succulency and the chemical used. Again, a county extension agent can lend advice on what, when, and how to use an appropriate herbicide.

When weeding out a toxic plant, be sure to get the entire root. If the root is left behind, the plant might return in an even more vigorous state.

When handling toxic plants, wear waterproof gloves, as some toxins can be absorbed through the skin. Dispose of plant materials; don't leave them lying around.

Possibly Poisoned

If you suspect that your horse might have ingested a toxic plant, remove the suspected material from the pasture or place the horse in a deeply bedded stall without food, and talk to your vet as quickly as possible. While waiting for the vet, contact an animal poison control center. Knight says it's helpful if the caller knows the scientific name of the plant.

"One plant may have as many as a dozen common or trivial names; what is vernacular for one part of the country may be completely different for another part of the country," he says. (See "Information Please" on page 73 for some poisonous plant referral services.)

Veterinary experts at the Poison Control Center, who work with the owner as well as the vet, will try to obtain a detailed history on each case. "There are certain questions we want to answer for ourselves so we can zero in on the problem," Knight says. "When we get that information, we can make an assessment on what can be done, and help develop a management approach.

"Sometimes the caller needs to get a veterinarian out there for immediate treatment, as some plants cause problems very quickly. With other plants, problems develop subtly and over time."

Diagnosing plant poisoning is difficult if based only on clinical signs. Says Robert H. Poppenga, DVM, PhD (veterinary toxicology), University of Pennsylvania School of Veterinary Medicine, New Bolton Center, "Ingestion of many plants produces non-specific clinical signs that must be differentiated from other disease conditions. Relatively few tests are available to detect plant toxins in either antemortem or postmortem samples. In many cases, the best way to support a diagnosis of plant poisoning is to confirm the presence of a toxic plant in the animal's environment, to confirm that the plant has been ingested (noting that the candidate plants have been chewed and/or finding plant fragments in vomitus or gastrointestinal tract samples), and to correlate clinical findings, where possible, with those known to be associated with the suspect plant."

Adds Dennis J. Blodgett, DVM, PhD (veterinary toxicology), Associate Professor at the Virginia-Maryland Regional College of Veterinary Medicine, "Different toxins affect different 'target organs.' Organ effects dictate clinical signs. Some poisonous plants cause only minor clinical signs, like loose stools. Others can be life-threatening with heart or central nervous system effects. Some toxic plants cause problems after a single ingestion (i.e., yew). Other toxic plants have to be eaten for a long period of time (chronically for weeks to months) to cause a problem (i.e., Senecio) or have to be eaten during a specific time to cause a problem (i.e., tall fescue in late gestation of mares). Similar or same toxins in some different plants may produce an identical syndrome of clinical signs (i.e., liver problems with Senecio and Crotalaria plants; central nervous system and heart problems with white snakeroot and rayless goldenrod)."

Treatment varies. Says Wilson Rumbeiha, DVM, PhD (toxicology), Diplomate American Board of Toxicology, Diplomate American Board of Veterinary Toxicology, Clinical Veterinary Toxicologist at Michigan State University, "What the veterinarian does depends on the clinical signs. Unfortunately, we do not have an antidote for most toxic plants, so treatment is symptomatic."

The best approach to equine poisoning usually consists of routine decontamination procedures, such as the administration of activated charcoal and a cathartic (a purging medication) to hasten elimination of the plant from the gastrointestinal tract, Poppenga says. "In addition, symptomatic and supportive care needs to be provided."

For example, in the case of red maple poisoning, Blodgett says the veterinarian might administer "intravenous fluid therapy to keep hemoglobin casts (masses of plastic matter formed in cavities of diseased organs) out of kidneys, parenteral vitamin C to reduce methemoglobin back to regular hemoglobin (that carry oxygen to cells and release it), blood transfusion if hematocrit (volume of red blood cells) is very low from red blood cell hemolysis (disintegration), and activated charcoal with a saline cathartic if you suspect multiple ingestions with the latest being within the last six hours. For yew poisoning, the veterinarian may administer atropine sulfate to relieve bradycardia (slow heartbeat) and activated charcoal with a saline cathartic after the heart rate has stabilized."

Prognosis depends on the identity and the amount of toxin ingested, time between ingestion and when treatment is started, and the degree of clinical signs present. In some cases, full recovery is possible with minimal support; in other cases, ingestion is fatal.

Britain's War Of The Ragwort

The British Horse Society (BHS) has been waging a war against ragwort (Senecio spp.), a deadly poisonous plant that has become as invasive and widespread as a contagious disease. Some reports indicate that ragwort poisoning in Britain causes a higher economic loss in cattle than all other plants combined. Yet despite a widespread educational publicity campaign and stepped-up measures to eradicate the weed, the prevalence of ragwort seems to be increasing.

"Five or six years ago, it became apparent that ragwort was becoming much more prevalent, mainly on roadside verges, railway lines, and motor-way embankments," says Nichola Gregory, BA, Head of Public Relations of the British Horse Society. "This meant that horse owners were finding it much more difficult to keep pastures clear." Although ragwort is not especially palatable, the danger lies in the plant getting into the hay. "When dried, ragwort becomes more palatable, but no less toxic, and mixed in with other fodder, it is usually eaten. The spread of ragwort in the UK has meant that very few hay fields are free of ragwort, and many irresponsible landowners fail to remove ragwort from hay fields before they are cut."

Authorities are uncertain why the weed proliferates despite stepped-up eradication efforts. Gregory cites several possibilities: "Each plant carries 150,000 to 250,000 seeds, which can be carried several miles by the wind, and seeds can lie dormant in the soil for up to 20 years. Local authorities stopped cutting the roadside verges on a regular basis some years ago (because of a combination of lack of money and environmental concerns to conserve wildflowers); these verges are prime sites for ragwort, and they have been allowed to flower and seed." Additionally, warmer winters with subsequent favorable growing conditions, disturbed soil at road building projects, and the reduction of natural predators might be contributors.

Ragwort is highly toxic to equines. "The poison damages the liver and is cumulative, which means that a little will damage the liver a little," says Gregory, "and each subsequent ingestion will damage the liver a little more. Horses can live for years with ragwort poisoning, and then suddenly start to show symptoms and die."

To eradicate the plant from pastures, remove the "ro-settes" (new plants) as they appear. "Dig them out by hand, being sure to remove all roots," Gregory advises. "Putting rock salt in the hole will ensure any roots remaining are killed." If ragwort is found blooming in the field, pull it out before it starts to seed.

Gregory recommends spot spraying with Barrier H from Barrier Animal Healthcare (www.ragwort.com). "This will kill the plants fast (in a few days), but you must remove horses and not put them back until all dead plants have been collected (they lift off easily once dead); this could take up to six weeks. It is a new environmentally friendly herbicide. Spray the field with 2,4-D (a common, active ingredient used in many commercial herbicides or weed killers) in the autumn and spring each year."

(Editor's note: Barrier H might not be available in the United States.)

Don't miss page 2 of this article, including a list of plants that are toxic to horses, clinical signs of poisoning, and treatments!

Information Please

Who To Call

For crisis/emergency information, contact your local or state animal poison control hotline or the ASPCA Animal Poison Control Center (formerly the National Animal Poison Control Center). Consultation is $45 per case including all follow-up calls. Telephone: 888/4ANI-HELP (888/426-4435), billed to your credit card, or 900/680-0000, billed to your phone bill.

‚ÄčThis article appeared in TheHorse Magazine  May 1, 2001

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