Poisonous Plants

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.

The North Shore Animal League Poison Hotline also provides a 24-hour consulting service: 888/232-8870, $35 fee per case (includes follow-up) is billed to your credit card.

Books

  • Horse Owner's Field Guide to Toxic Plants by Sandra M. Burger, published by Breakthrough Publications, Inc.
  • Toxic Plants of North America by George E. Burrows and Ronald J. Tyrl, published by the Iowa State University Press.
  • Natural Poisons in Horses by Jeffery O. Hall, published by the National Animal Poison Control Center.
  • A Guide to Plant Poisoning of Animals in North America by Anthony P. Knight, BVSc, MS, and Richard G. Walter, MA Botany, published by Teton NewMedia.

Web Sites

Gardens

You can see poisonous plants in "sinister" or "poison" gardens planted throughout the United States, such as the University of Illinois Poisonous Plant Garden, the Cornell University garden, and Greenspace at the University of Wisconsin.


Editor's Note: Decontamination (with activated charcoal, cathartics, and mineral oil) will be considered as treatment for nearly all poisoning cases. If ingestion of a poisonous plant is suspected before clinical signs appear, decontamination as discussed above might be advised to prevent and/or minimize symptoms. Symptoms of poisoning vary from mild to lethal with the toxin ingested, the amount ingested, and the horse's medical history.

BLACK WALNUT (Juglans nigra)

  • Description
  • Trees with dark, deeply furrowed bark, compound leaves with approximately 20 leaflets and a spherical fruit (thick, green husk and a hard, brown, furrowed nut)
  • Geographic Location
  • Native to northeastern America and to the central plains
  • Source Of Toxicity
  • Shavings and sawdust are toxic when used as bedding
  • Clinical Signs
  • Signs appear within 24 hours of exposure--laminitis, bounding digital pulse, lower leg edema, fever, increased respiratory rate
  • Treatment
  • Remove source of black walnut, wash legs and feet, symptomatic and supportive care 

BOXWOOD (Buxus sempervirens)

  • Description
  • Evergreen with simple leathery leaves, usually grown as hedge
  • Geographic Location
  • Used throughout North America
  • Source Of Toxicity
  • Leaves, twigs, clippings
  • Clinical Signs
  • Nausea, vomiting, diarrhea, dizziness, convulsions, respiratory failure
  • Treatment
  • Activated charcoal, symptomatic care. Boxwood ingestion can be fatal

 BRACKEN (Buxus sempervirens)

  • Also Known As
  • Bracken fern, brake fern, hog brake
  • Description
  • Fern-like leaves up to six feet long
  • Geographic Location
  • Throughout North America in forests, open woods, fields
  • Source Of Toxicity
  • Toxic whether green or dry. Shows up in hay
  • Clinical Signs
  • Clinical signs appear one to two months after ingestion--weight loss, weakness, gait abnormalities, stance with an arched back and legs spread far apart, staggering, slow heart rate, abnormal cardiac rhythm
  • Treatment
  • Death usually within 10 days of the onset of symptoms. Can be treated with thiamine injections if diagnosed quickly

BUTTERCUP (Ranunculus spp.)

  • Also Known As
  • Tall field buttercup, creeping buttercup
  • Description
  • Leaves grow low on the stem, flowering stalk contains five-petaled flowers in spring
  • Geographic Location
  • Throughout North America in fields, gardens, woods
  • Source Of Toxicity
  • Toxins contained in bulbous root
  • Clinical Signs
  • Pain and inflammation of mouth, salivation, diarrhea, dizziness, skin irritation, and depression
  • Treatment
  • Decontamination (with activated charcoal and purgatives), demulcents (oily substances for soothing and coating irritated membranes), corticosteroids, antibiotics

CASTOR BEAN (Ricinus communis)

Activated charcoal, supportive fluid and electrolyte balance, anticonvulsants. This plant is fatal in very small amounts.

  • Also Known As
  • Castor oil plant, palma Christi
  • Description
  • Annual up to 12 feet, large palmately lobed leaves, showy red three-parted fruity pods
  • Geographic Location
  • Southeast, southwest U.S. naturally, but cultivated everywhere
  • Source Of Toxicity
  • Seeds are very poisonous
  • Clinical Signs
  • Rapid signs. Dopiness, incoordination, sweating, shock, neck and shoulder spasms, weak and rapid pulse, fever, diarrhea, colic-like pain
  • Treatment

CHERRIES, WILD AND CULTIVATED VARIETIES (Prunus spp.)

  • Description
  • Trees or shrubs, white or pinkish spring flowers, simple leaves
  • Geographic Location
  • Throughout North America
  • Source Of Toxicity
  • Cyanide contained in seeds, leaves, bark, and fruit (highest to lowest concentrations). The biggest factor in toxicity is the condition of the plant--wilted plants (such as on a branch blown down from a storm) are the most toxic, while dried and fresh cherry leaves are not as bad
  • Clinical Signs
  • Nausea, vomiting, diarrhea, dizziness, convulsions, respiratory failure
  • Treatment
  • There is an antidote, but it is often not available quickly enough

FOXGLOVE (Digitalis purpurea)

  • Description
  • Large bell-shaped multi-colored flowers
  • Geographic Location
  • Ornamental and wild plants found everywhere
  • Source Of Toxicity
  • Normally only eaten if found in hay. Whole plant is toxic
  • Clinical Signs
  • Diarrhea, abdominal pain, tremors, and convulsions
  • Treatment
  • Activated charcoal to prevent further toxin absorption, anti-arrhythmic drug

GROUND IVY (Glecoma hederacea)

  • Also Known As
  • Creeping Charlie
  • Description
  • Low, prostrate perennial herb with slender four-sided stems that hug the ground, root at their joints, and often cover areas of many square feet. Roundish, scalloped leaves
  • Geographic Location
  • Present throughout much of North America
  • Source Of Toxicity
  • Often baled in hay, whole plant is toxic
  • Clinical Signs
  • Severe sweating accompanied by frothing at the mouth and/or difficulty breathing, pupils dilated, panting
  • Treatment
  • Rarely fatal except in large amounts. Supportive care

HOARY ALYSSUM (Berteroa incana)

  • Description
  • Member of the mustard family. Hairy stems and leaves, white, four-petaled flowers, up to four feet tall
  • Geographic Location
  • Throughout the northern U.S. and southern Canada
  • Source Of Toxicity
  • Usually requires significant ingestion (up to 25% of the diet). Toxicity retained on drying. Whole plant is toxic
  • Clinical Signs
  • Clinical signs appear 12-48 hours after ingestion--laminitis, lower leg edema, fever, colic, anorexia, dehydration
  • Treatment
  • This plant can be lethal-- symptomatic and supportive treatment is recommended

HORSE CHESTNUT (Aesculus hippocastanum)

  • Also Known As
  • Buckeye. Other varieties: California buckeye, Ohio buckeye, sweet buckeye, red buckeye
  • Description
  • Trees or shrubs with opposite, palmately compound leaves. The glossy brown seeds are one inch in diameter
  • Geographic Location
  • Several species grow in the Pacific coast region, Midwest, South, Southeast U.S. Grown as ornamentals
  • Source Of Toxicity
  • Young sprouts, leaves, seeds
  • Clinical Signs
  • Vomiting, abdominal pain, muscle twitching, weakness, odd gait
  • Treatment
  • Supportive care

HORSETAIL (Equisetum spp.)

  • Also Known As
  • Mare's tail, scouring rush
  • Description
  • Rush-like ridged stalks, papery leaves
  • Geographic Location
  • Widespread in North America, often found near bogs and streams
  • Source Of Toxicity
  • Leaves contain toxin that destroys vitamin B1 (thiamine) in the body
  • Clinical Signs
  • Weakness, inappetence, incoordination, quietness, unresponsiveness, coma
  • Treatment
  • Horsetail ingestion can be fatal if not treated with thiamine injections

LARKSPUR (Delphinium spp.)

  • Description
  • Deeply lobed palmate leaves, blue spurred elongated flowers
  • Geographic Location
  • Native species throughout North America, grown in gardens. Especially a problem in the Rocky Mountains
  • Source Of Toxicity
  • Entire plant highly toxic
  • Clinical Signs
  • Nervousness, incoordination, staggering, salivating, bloating, abnormal heartbeat, breathing difficulty, paralysis, convulsions, death
  • Treatment
  • Larkspur ingestion can be fatal. Supportive care is recommended 

LOCOWEED (Astragalus and Oxytropis spp.)

  • Also Known As
  • Many varieties, i.e. purple loco, wooly loco, stemless loco, etc.
  • Description
  • Low-growing ground covers to two-foot-tall clumps of flowers
  • Geographic Location
  • Throughout the western U.S. in semi-arid foothills and plains
  • Source Of Toxicity
  • Horses usually avoid locoweed, but once sampled it might be addictive
  • Clinical Signs
  • Signs appear after prolonged exposure--locoweed affects the brain. Altered gaits, aimless wanderings, impaired vision, erratic behavior, listlessness, overreactive, darkened feces, fever, abortion, birth defects, convulsions
  • Treatment
  • Quality diet, slow recovery. Horses with severe clinical signs may not recover completely. Supportive care

LOCUST TREES (Robinia pseudoacacia)

  • Also Known As
  • Black locust, yellow locust, false acacia, clammy locust
  • Description
  • Sized from shrub to large tree. Compound leaves, pea-like flowers, long pod-like fruit
  • Geographic Location
  • Eastern and central U.S., southern Canada
  • Source Of Toxicity
  • Bark, young sprouts, branches, posts from locust trees somewhat toxic
  • Clinical Signs
  • Onset within hours. Loss of appetite, general weakness depression, mild colic
  • Treatment
  • Activated charcoal, supportive fluid and electrolyte balance. Occasionally lethal

LUPINE (Lupinus spp.)

  • Description
  • White, blue, purple or pinkish pea-shaped flowers on upright stalks
  • Geographic Location
  • Found in a wide variety of landscapes from ocean beaches to high mountain pastures
  • Source Of Toxicity
  • Young lupines and those going to seed are the most toxic
  • Clinical Signs
  • Gastrointestinal irritation, diarrhea, altered gait, nervousness, leg twitching, loss of muscle control, prostrations, convulsions
  • Treatment
  • Symptomatic treatment

MILK VETCH (Astragalus spp.)

Ingestion is usually fatal due to sudden asphyxiation
Various species cause different problems requiring different treatment approaches

  • Description
  • Small compound leaves, pea-like flowers in elongated clusters, various-shaped pods
  • Geographic Location
  • Wide range of habitats throughout North America
  • Source Of Toxicity
  • Leaves, pods, seeds
  • Clinical Signs
  • A roaring sound when exhaling, salivating, staggering
  • Treatment

NIGHTSHADES (Solanum spp.)

Central nervous system signs: depression, incoordination, tremors, posterior weakness, prostration
Gastrointestinal signs: inappetence, salivation, vomiting, diarrhea

  • Also Known As
  • Plant species include black night-shade, bittersweet nightshade, horse nettle, silverleaf nightshade, ground cherry, Jerusalem cherry
  • Description
  • Up to four feet tall, white potato-like flowers, clustered berries. Some varieties are evergreens with yellow or red fruit
  • Geographic Location
  • Fields, open woodlands, pastures, and waste places throughout North America
  • Source Of Toxicity
  • Leaves and green berries (toxicity retained on drying)
  • Clinical Signs
  • Treatment
  • Symptomatic and supportive care

OAKS (Quercus spp.)

Decontamination with active charcoal and purgatives, symptomatic and supportive care

  • Description
  • Range in size from small shrubs to tall trees. Fruit is a smooth nut with a cap (acorn)
  • Geographic Location
  • All parts of North America
  • Source Of Toxicity
  • Oak buds, leaves, acorns contain gallotannin. Spring and fall most common periods for poisoning
  • Clinical Signs
  • Loss of appetite, depression, constipation, black tarry feces, weakness, prostration. Animals may be jaundiced, have blood in the urine, and be dehydrated
  • Treatment

OLEANDER (Nerium oleander)

  • Also Known As
  • Rose laurel, adelfa, roseniorbeer
  • Description
  • Ornamental evergreen shrub with long linear leaves, white or pink flowers
  • Geographic Location
  • Southern U.S., California
  • Source Of Toxicity
  • All varieties and plant parts extremely toxic. Leaves get mixed in with clippings and hay
  • Clinical Signs
  • Profuse diarrhea which might be bloody, abnormal heartbeat, chilled extremities
  • Treatment
  • Immediate veterinary attention to prevent death, which often occurs within eight to 24 hours after ingestion. Evacuation of digestive tract, administration of activated charcoal/cathartic

POISON HEMLOCK (Conium maculatum)

  • Also Known As
  • European hemlock, spotted hemlock, wild carrot
  • Description            
  • Plants up to ten feet tall. Smooth, hollow stems covered with purple spots. Leaves are finely divided (similar to carrots or parsley). Large white to pale yellow taproot
  • Geographic Location
  • Throughout North America along fencerows, meadows, streams
  • Source Of Toxicity
  • Roots, young plants, seeds
  • Clinical Signs
  • Signs appear within minutes--muscle tremors, ataxia, frequent defecation, excessive urination, increased respiratory rate, abdominal pain
  • Treatment
  • Decontamination with activated charcoal and purgatives. Horse might die or make a full recovery

RED CLOVER (Trifolium pratense)

  • Description
  • Reddish three-leaf clover
  • Geographic Location
  • Cultivated extensively in North America
  • Source Of Toxicity
  • Toxicity is caused by a fungus of red clover. The fungus can also appear in white clover, though this is less common
  • Clinical Signs
  • Signs appear after 30-60 minutes--excessive salivation, diarrhea, frequent urination
  • Treatment
  • Horses usually recover in one to four days with supportive care. Remove clover from pasture 

RED MAPLE (Acer rubrum)

  • Description
  • Tree trunk and leaves are broad; leaves have three to five lobes with palmately arranged veins. The fruit is a two-winged, two-seeded structure
  • Geographic Location
  • Native to Eastern U.S., Canada, westward to Missouri and eastern Texas. May be planted elsewhere
  • Source Of Toxicity
  • Fresh, wilted, and dried leaves
  • Clinical Signs
  • Depression, jaundice, anemia, discolored urine, increased respiratory and heart rates
  • Treatment
  • Symptomatic and supportive care. Red maple ingestion can be fatal

RHODODENDRON (Rhododendron spp.)

  • Also Known As
  • Rhodora, azalea, white laurel, great laurel, rose bay, California rose bay, Lapland rose bay
  • Description
  • Large shrubs or small trees with evergreen leaves and terminal clusters of large, attractive pink to purple flowers
  • Geographic Location
  • Native and ornamental species found throughout North America
  • Source Of Toxicity
  • Leaves and nectar
  • Clinical Signs
  • Salivation, emesis, diarrhea, muscular weakness, slow heart rate, difficulty breathing, depression, prostration
  • Treatment
  • Activated charcoal, cathartic (purgative), symptomatic and supportive care

ST. JOHN'S WORT (Hypericum perforatum)

  • Also Known As
  • Klamath weed
  • Description
  • Erect herb up to three feet tall, dark dots on leaves when held up to light, yellow flowers in terminal clusters
  • Geographic Location
  • Aggressive weeds of roadsides, pastures, ranges throughout most of U.S. and southern Canada
  • Source Of Toxicity
  • Less toxic when dried, but still poisonous when found in hay
  • Clinical Signs
  • Photosensitization in unpigmented skin, which can become red and irritated when exposed to sunlight
  • Treatment
  • Supportive care

SUDAN GRASS (Sorghum vulgare var. sudanense)

  • Also Known As
  • Hybrid Sudan, sorghum, sorghum grass, milo, Johnson grass
  • Description
  • Tall plant with coarse, dull green leaves, terminal flowers, seeds are dark brown to purple-black
  • Geographic Location
  • Central and great plains, can be grown throughout North America
  • Source Of Toxicity
  • Foliage, not seeds. More deadly after frost (wilted)
  • Clinical Signs
  • Might graze for one week to six months prior to clinical signs. Urinary incontinence and dribbling of urine, bladder inflammation, posterior ataxia, incoordination
  • Treatment
  • Supportive care is recommended. Sudangrass poisoning is survivable, but nerve regeneration or recovery is not expected once nerve signs develop

TALL FESCUE (Festuca arundinacea)

  • Description
  • Grass
  • Geographic Location
  • Widely grown forage grass in the U.S.
  • Source Of Toxicity
  • Grass invaded by fungus
  • Clinical Signs
  • Late-term abortions, prolonged gestation, difficult births, thickened or retained placentas, foal deaths
  • Treatment
  • Supportive care. Domperidone to reverse the effects in late-gestation mares as prescribed by a veterinarian

WATER HEMLOCK (Cicuta spp.)

  • Also Known As
  • Spotted hemlock, cowbane, spotted cowbane, poison parsnip, false parsley, spotted parsley, muskrat weed, fever root, mock-eel root
  • Description
  • Plants two to 10 feet tall. Smooth, hollow stems covered with purple spots, compound leaves, small white compound flowers
  • Geographic Location
  • All areas of North America along ditches, flood plains, low mead-ows, fertile uncultivated areas
  • Source Of Toxicity
  • Roots, leaves
  • Clinical Signs
  • Rapid onset of signs including muscle tremors, teeth grinding, abdominal pain, convulsions, death
  • Treatment
  • Death can occur within 45 minutes, but water hemlock ingestion is survivable. Supportive care 

WHITE SNAKEROOT (Eupatorium rugosum)

  • Also Known As
  • Snakeroot, burrow weed, white sanicle
  • Description
  • Erect plant two to three feet tall, heart-shaped, serrated leaves, white flowers in top clusters in the fall
  • Geographic Location
  • Varieties in eastern, midwestern, southern U.S., westward to Minnesota. Similar toxin exists in jimmy weed, rayless goldenrod (Haplopappus)
  • Source Of Toxicity
  • Vegetative plant parts
  • Clinical Signs
  • Signs appear in two to three weeks--depression, weakness, heart failure, tremors, posterior weakness
  • Treatment
  • Symptomatic and supportive treatment. Poisoning is survivable, but with potential long-term cardiac muscle scarring

YELLOW STAR THISTLE/RUSSIAN KNAPWEED (Centauria solstitialis/Centauria repens)

  • Yellow Star Thistle Also Known As
  • Barnaby's thistle
  • Description
  • Yellow flowers with starburst thorns. Russian knapweed has the same toxin
  • Geographic Location
  • Western U.S.
  • Source Of Toxicity
  • Toxic when dried
  • Clinical Signs
  • Must be consumed for 30 days or more before clinical signs appear--inability to swallow, eat, or drink; chewing; unusual tongue and lip movements; frequent yawning; unusual postures
  • Treatment
  • Causes brain damage. Once symptoms appear, nerve symptoms are irreversible

YEW (Taxus spp.)

  • Also Known As
  • English Yew, Japanese Yew, American Yew, Western Yew
  • Description
  • Landscape shrubs with small, narrow, strap-like evergreen leaves. Fruit is red and fleshy
  • Geographic Location
  • Varieties found throughout North America
  • Source Of Toxicity
  • Whole plant is extremely toxic except red fleshy part of fruit (as little as six to eight ounces of fresh yew can kill a horse)
  • Clinical Signs
  • Rapid onset of signs including trembling, muscle weakness, difficulty breathing, collapse, death
  • Treatment
  • Symptomatic and supportive care. Treatment is often unrewarding

About the Author

Marcia King

Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.

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