Merck Manual

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Poisonous Range Plants of Temperate North America

Poisonous Range Plants of Temperate North America

Common and Scientific Names

Habitat and Distribution

Affected Animal(s)

Toxic Parts and Effects

Comments and Treatment

Dangerous Season: Spring and Fall

Water hemlock; Cicuta species

Open, moist to wet environments; throughout North America

All

Roots, stem base, young leaves. Toxicity retained when dry, except in hay. Rapid onset of signs, with death in 15 to 30 minutes. Drooling, muscular twitching, dilated pupils. Violent convulsions, coma, death. Poisoning in humans common.

Sedatives to control spasm and heart action. Outlook good if alive 2 hours after ingestion.

Dangerous Season: Spring

African rue; Peganum harmala

Arid to semiarid ranges; southwest

Horses likely

Seeds, leaves, stems; seeds more toxic. Loss of appetite, hindleg weakness, knuckling of fetlock, listlessness, excess drooling, subnormal temperature, frequent urination. Tissue changes include inflammation of the stomach and intestine, with bleeding on heart and under liver capsule.

Unpalatable. Eaten only under drought conditions.

Cocklebur; Xanthium species

Fields, waste places, exposed shores of ponds or rivers; throughout North America. Dangerous season spring and occasionally fall.

All animals

Seeds and young seedlings. Loss of appetite, depression, nausea, vomiting, weakness, rapid weak pulse, difficulty breathing, muscle spasms, convulsions. Tissue changes include inflammation of the stomach, intestines, liver, and kidney.

Seedlings or grain contaminated with seeds. Oils and fats given by mouth may be beneficial; warmth, stimulants given in the muscle.

Death camas; Zygadenus species

Foothill grazing lands, occasionally boggy grasslands, low open woods; throughout North America

Horses

Entire plant. Drooling, vomiting, muscle weakness, incoordination or laying down, fast weak pulse, coma, death. No distinctive tissue changes.

Seeds most toxic. Leaves and stems lose toxicity as plant matures. Atropine and picrotoxin may be effective.

Oaks; Quercus species

Most deciduous woods; throughout North America

Horses

Young leaves and swollen or sprouting acorns. Loss of appetite, constipation, followed by dark tarry diarrhea, dry muzzle, frequent urination, rapid weak pulse, death. Tissue changes include swelling around the kidneys with inflammation, inflammation of the stomach and intestine.

Diet must consist of more than 50% oak buds and young leaves for a period of time. Kidney failure with diet history diagnostic. Treatment symptomatic. Oral ruminatorics helpful. (Also See also Quercus Poisoning (Oak Bud Poisoning, Acorn Poisoning).)

Pokeweed, Poke; Phytolacca americana

Disturbed rich soils such as recent clearings, pastures, waste areas; eastern North America

Horses, people

Entire plant; roots most toxic. Vomiting, abdominal pain, bloody diarrhea, low blood cell counts. Terminal convulsions, death from respiratory failure. Tissue changes include open inflammation of the stomach and intestine, bleeding of the mucosa, dark liver.

Oils and protectants (stomach and intestinal tract). Dilute acetic acid orally, stimulants. Blood transfusion (hemolytic anemia).

Dangerous Season: Spring and Summer

Buckeye; Aesculus species

Woods and thickets; eastern US and California

Horses

Entire plant, especially seeds and leaves. Depression, incoordination, twitching, paralysis, inflammation of mucous membranes.

Young shoots and seeds especially poisonous. Stimulants and purgatives.

Coffeepod, Sicklepod; Cassia obtusifolia

Found in cultivated (corn, soybean, or sorghum) and abandoned fields, along fences, roadsides; naturalized in eastern US

Horses

Toxic principles thought to be same as in Cassia occidentalis. Signs, although similar, less severe with Cassia obtusifolia.

Treatment ineffective in down animals.

Coffee senna, Coffee weed, Styptic weed, Wild coffee; Cassia occidentalis

Common along roadsides, waste areas and pastures; naturalized in eastern US

Horses, rabbits

Entire plant. Associated with stomach and intestinal dysfunction and degeneration of muscle. No fever, incoordination with diarrhea and coffee-colored urine. Affected animals are unable to stand but eat and are alert shortly before death. High blood pressure frequent. Tissue changes include heart and skeletal muscle degeneration. Congestion, fatty degeneration and tissue death in the liver and kidneys also reported. Death probably due to high blood pressure causing heart failure.

No specific treatment known. Symptomatic and supportive care essential. Although tissue changes are similar to those of vitamin E/selenium deficiency, this therapy is contraindicated. Remove animals from source.

Fly poison, Staggergrass, Crow poison; Amianthium muscaetoxicum

Open woods, fields, and acid bogs; eastern North America

Horses

Entire plant. Drooling, vomiting, rapid and irregular respiration, weakness, death from respiratory failure.

No practical treatment. Especially dangerous for animals new to pasture. Keep animals well fed.

Larkspurs; Delphinium species

Either cultivated or wild, usually in open foothills or meadows and among aspen; mostly western. Dangerous season spring and summer, also seeds in fall.

Horses

Entire plant, fresh or dry. Straddled stance, arched back, repeated falling, forelegs first. Constipation, bloat, drooling, vomiting. Death (respiratory and heart failure). Most often no tissue changes.

Young plants and seeds more toxic. Toxicity decreases with maturity.

Dangerous Season: Summer and Fall

Black locust, False acacia, Locust tree; Robinia pseudoacacia

Open woods, roadsides, pinelands, on clay soils preferably; eastern US

Horses

Entire plant, although flowers have been suggested as the toxic principles. Diarrhea, loss of appetite, weakness, hind end paralysis, depression, dilated pupils, cold extremities; frequently weak pulse. Death infrequent; recovery period extensive. Tissue changes after death restricted to stomach and intestinal tract.

Laxatives and stimulants suggested. Treatment supportive.

Dogbanes; Apocynum species

Open woods, roadsides, fields; throughout North America

All

Leaves and stems of green or dry plants. Increased temperature and pulse, dilated pupils, loss of appetite, discolored mucous membranes, cold extremities, death.

Intravenous fluids and stomach protectants suggested.

Flatweed, Cat’s-ear, Gosmore; Hypochaeris radicata

Native to the Mediterranean and South America; widely distributed in the US, including Pacific, eastern and southeastern states

Horses

Unknown; associated with but not proven cause of a neurologic condition called stringhalt, sudden onset of abnormal gait, knuckling of lower limb joints; paralysis of larynx; recovery possible, but condition could be permanent.

Tranquilizers, sedatives, mephenesin, and thiamine (questionable effectiveness); longterm phenytoin therapy seems helpful. Treatment with baclofen also reported helpful. Surgery reported helpful for gait problems.

Nightshades, Jerusalem cherry, Potato, Horse nettle, Buffalo bur; Solanum species

Fence rows, waste areas, grain and hay fields; throughout North America

All

Leaves, shoots, and unripe berries. Inflammation of the stomach and intestine with bleeding, weakness, excess drooling, difficulty breathing, trembling, progressive paralysis, laying down, death.

Pilocarpine, physostigmine, gastrointestinal protectants. Seeds may contaminate grain.

Perilla mint, Beefsteak plant; Perilla frutescens

Ornamental originally from India, escaped to moist pastures, fields, roadsides, and waste places; eastern North America

Horses

Green or dry plant. Signs 2 to 10 days after exposure include difficulty breathing (especially on exhaling), open-mouth breathing, lowered head, reluctance to move, death on exertion. Tissue changes include fluid and swelling in the lungs.

Treatment ineffective once signs are severe. Injectable steroids, antihistamines, and antibiotics may help. Handle gently (prevents exertion and death).

Red maple; Acer rubrum

Moist land and swamps; eastern North America

Horses

Wilted leaves. Anemia, and destruction of blood cells; weakness, rapid breathing, rapid heart beat, depression, jaundice, poor oxygenation of blood, brownish discoloration of blood and urine.

Not common. Fluids, oxygen, and blood transfusion can be helpful. Methylene blue therapy not rewarding.

Russian knapweed; Centaurea repens

Waste areas, roadsides, railroads, and overgrazed rangeland; not common in cultivated or irrigated pastures; mostly western and upper midwestern US

Horses

Fresh or dried plant. Chronic exposure, but sudden onset of signs. Inability to eat or drink, loss of facial tone, chewing, yawning, standing with head down, severe facial swelling, gait normal, head pressing, aimless walking or excitement most severe the first 2 days, become static thereafter. Death from starvation, dehydration, aspiration pneumonia.

More toxic than yellow star thistle (C. solstitialis;see below) but with similar pathology and prognosis. Some relief with massive doses of atropine but not an effective treatment. Euthanasia recommended.

White snakeroot; Eupatorium rugosum

Woods, cleared areas, waste places, usually the moister and richer soils; eastern North America

Sheep, cattle, horses

Complex benzyl alcohol (tremetol in leaves and stems). Excreted via milk; cumulative. Weight loss, weakness, trembling (muzzle and legs) prominent after exercise, constipation, acetone odor, fatty degeneration of liver, partial paralysis of throat, death in 1 to 3 days.

“Milk sickness” or “trembles.” Supportive treatment. Heart and respiratory stimulants and laxatives may be necessary. Remove animal from access to plant.

Yellow star thistle, Yellow knapweed; Centaurea solstitialis

Waste areas, roadsides, pastures; mostly western North America

Horses

Entire plant. Involuntary chewing movements, twitching of lips, flicking of tongue. Mouth commonly held open. Unable to eat; death from dehydration, starvation, aspiration pneumonia.

Horses graze because of lack of other forage. Extended period of consumption essential for toxicity. Death of certain brain areas is diagnostic. No treatment. Euthanasia recommended.

Dangerous Season: Fall and Winter

Black walnut; Juglans nigra

Native to eastern US; now from eastern seacoast, west to Michigan and most of the Midwest, south to Georgia and Texas

Horses

Shavings with as little as 20% black walnut are toxic within 24 hours of exposure. Reluctance to move; depression; increased temperature, pulse, respiration rate, abdominal sounds, digital pulse, hoof temperature; distal limb swelling; lameness. Severe laminitis with continued exposure.

Nonfatal; laminitis and edema of lower limbs. Remove shavings promptly. Treat for limb edema and laminitis. Improvement in 1 to 2 days with no complications.

Bladderpod, Rattlebox, Sesbane, Coffeebean; Sesbania vesicaria

Mostly open, low ground, abandoned cultivated fields; southeastern US coastal plain

All

Green plant and seeds.

Green seeds are more toxic. Remove animal from source immediately. General supportive treatment—saline purgatives, intravenous fluids.

Onions, (cultivated and wild); Allium cepa, A. canadense

Cultivated and grown on rich soils throughout US

Cattle, horses, dogs

Entire parts. Livestock readily consume onions; low blood cell counts develop within days of exposure. Signs are hemoglobin in the urine, diarrhea, loss of appetite, jaundice, incoordination, collapse, and possible death if untreated. Hemolytic anemia reported in livestock ingesting wild onions. Swollen, pale, dying liver.

Signs similar to toxicity induced by S-methylcysteine sulfoxide (a rare toxic amino acid in Brassica species) in livestock. Susceptibility to onion poisoning varies across animal species: cattle more susceptible than horses and dogs, which are more susceptible than sheep and goats. Remove animals from source and prevent future access to cull onions. Symptomatic and supportive care essential.

Rattlebox, Purple serbane; Daubentonia unica

Cultivated and escaped, in waste places; southeastern US coastal plain

All

Rapid pulse, weak respiration, diarrhea, death.

Seeds poisonous. Remove animal from source. Saline purgatives.

Rayless goldenrod, Burroweed; Haplopappus heterophyllus

Dry plains, grasslands, open woodlands, and along irrigation canals; southwest US

Horses

Primarily nursing young and nonlactating animals. Reluctance to move, trembling, weakness, vomiting, difficulty breathing, constipation, lying down, coma, death.

“Milk sickness.” Separate foals from mares.

Sweet clover, White sweet clover; Melilotus officinalis and M. alba

Commonly found on alkaline soils, fields, roadsides, and waste places; forage crop in southern and northern US

Horses

Dangerous Season: Fall, Winter, and Spring

Chinaberry; Melia azedarach

Fence rows, brush, waste places; southeastern US

Horses

Entire plant, fruit most toxic. Restlessness, vomiting, constipation, blue-tinged gums, rapid pulse, difficulty breathing, death within 24 hours.

Gastroenteritis usual. Recovery may be spontaneous. Laxatives and stomach and intestinal protectants suggested.

Dangerous Season: All Seasons

Astragalus species (certain species only—selenium accumulators)

Areas high in selenium, mostly western and midwestern

Horses

Selenium (chronic). Slow growth, reproductive failure, loss of hair, sore feet, acute death.

Avoid grazing seleniferous plants for extended periods. Also See Selenium Poisoning.

Bracken fern; Pteridium aquilinum

Dry poor soil, open woods, sandy ridges

Horses

Castor bean; Ricinus communis

Cultivated in southern regions

All

Entire plant, seeds especially toxic. Short to long course (death or recovery). Violent purgation, straining with bloody diarrhea, weakness, drooling, trembling, incoordination.

Diagnosis based on presence of seeds, red blood cell clumping, precipitin test. Specific antiserum, ideal antidote; sedatives, arecoline hydrobromide, followed by saline cathartics suggested.

Chokecherries, Wild cherries, Peaches; Prunus species

Waste areas, fence rows, woods, orchards, prairies, dry slopes

Horses

Excitement leading to depression, difficulty breathing, incoordination, convulsions, prostration. Death may occur in 15 minutes.

Mucous membranes, bright pink color; blood, bright red color (See Cyanide Poisoning).

Corn cockle; Agrostemma githago

Weed, grainfields, and waste areas; throughout North America

All

Seeds. Short course. Profuse watery diarrhea, vomiting, dullness, general weakness, rapid breathing, hemoglobin in the urine, death.

Oils and stomach and intestinal protectants. Neutralize toxin. Blood transfusions may be necessary.

Crotalaria, Rattlebox; Crotalaria species

Fields and roadsides; eastern and central US

All

Entire plant, especially seeds. Chronic course. Horses—unthriftiness, incoordination, walking in circles, jaundice; Death may occur from a few weeks to months after ingestion.

Cumulative, fresh or dry. No treatment.

Day-blooming jessamine and night-blooming jessamine, respectively; Cestrum diurnum, C. nocturnum

Open woods and fields; Gulf Coast states (Florida, Texas) and California

Horses, and dogs (ingesting cholecalciferol-based rodenticides)

Entire plant, including fruit and sap. Inflammation of the stomach and intestine develops on ingestion of fruits. Vomiting, depression, loss of appetite, chronic weight loss with normal appetite, choppy stiff gait, increased pulse, persistent increases in blood calcemia and phosphate, calcium deposits in arteries, tendons, ligaments, and kidneys, destruction of the parathyroid glands, overgrowth of the thyroid gland, and increased bone density reported with chronic ingestion of leaves.

Prevent further access of animals to plants. In early stages, treatment might be effective. Correct fluid and electrolyte (salt) imbalances in cases with persistent vomiting or diarrhea. Reduce or prevent changes in blood calcium. Maintenance therapy of diuretics and steroids may be necessary.

Fraser’s photinia, Chinese photinia, Red leaf photinia, Red tip photinia; Photinia fraseri, P. serrulata, P. glabra

Common ornamental (hedge or screen) in southern US

Horses

Same as for Nandina (see below).

Same as for Nandina (see below).

Groundsel, Senecio; Senecio species

Grassland areas; mostly western US

Horses, limited to US

Fresh or dry. Short-term poisoning not common. Dullness, aimless walking, increased pulse, rapid breathing, weakness, colic, delayed death (days to months). Nervous signs evident in later stages.

Liver biopsy diagnostic in early stages. No general treatment.

Hound’s tongue; Cynoglossum officinale

Common in waste places, roadsides, and pastured areas throughout US

Horses

Foliage. Unpleasant odor discourages consumption when fresh, becomes palatable in hay and is readily consumed. Signs are poor appetite, depression, rough hair coat, bleeding, bloody feces, incoordination, jaundice, death.

Know source and quality of hay. Treatment symptomatic and supportive at best. Affected animals seldom recover.

Jimson weed, Thorn apple; Datura stramonium

Fields, barn lots, trampled pastures, and waste places on rich bottom soils; throughout North America

All

Entire plant, seeds in particular. Short course. Weak rapid pulse and heartbeat, dilated pupils, dry mouth, incoordination, convulsions, coma.

All parts, mainly in hay or silage. Urine from animal dilates pupils of laboratory animals (diagnostic). Treatment nonspecific; cardiac and respiratory stimulants.

Johnson grass; Sorghum halepense

Weed of open fields and waste places; southern and scattered north to New York and Iowa

Horses

Same as for Sorghum vulgare (see below).

Same as for Sorghum vulgare (see below).

Laurel, Ivybush, Lambkill; Kalmia species

Rich moist woods, meadows, or acid bogs; eastern and northwestern North America. Dangerous all seasons, especially winter and spring.

Horses

Vegetative parts. Short course. Incoordination, excess drooling, vomiting, bloat, weakness, muscular spasms, coma, death.

Treatment includes laxatives, demulcents, nerve stimulants, atropine.

Laurel cherry, Cherry laurel; Prunus caroliniana

Woods, fence rows, and often escaped from cultivation; southern regions. Dangerous all seasons, especially winter and spring.

Horses

Wilted leaves, bark, and twigs. Short course. Difficult breathing, bloat, staggering, convulsions, followed by prostration and death. Mucous membranes and blood bright red.

Also See Cyanide Poisoning.

Locoweed; Astragalus species, Oxytropis species (certain species only)

Mostly western North America

All grazing animals

Depression, emaciation, incoordination, dry lusterless hair. Abortions.

Avoid grazing of source. Both green and dry plants toxic.

Lupines, Bluebonnet; Lupinus species

Dry to moist soils, roadsides, fields, and mountains; throughout, but poisoning mostly western North America

Horses

Seeds (fresh and dry). Short course. No appetite, difficulty breathing, struggle, convulsions, death from respiratory paralysis.

Do not disturb sick animals; remove from source as they begin to recover. No effective treatment, but survivors recover completely. Also See also Fungal Poisoning : Mycotoxic Lupinosis.

Milk vetch (and many other common names); Astragalus species (certain species only)

Nearly all

Horses

Hindlimb paralysis, goose-stepping, depression, rough coat, pulmonary emphysema, sudden death, spinal cord changes.

Avoid grazing of pre-flower stage.

Milkweeds; Asclepias species

Dry areas, usually waste places, roadsides, streambeds

All

Entire plant, green or dry. Staggering, tetanic convulsions, bloating, difficulty breathing, dilated pupils, rapid and weak pulse, coma, death.

Sedatives, laxatives, and intravenous fluids suggested.

Mustards, Crucifers, Cress; Brassica, Raphanus, Descurainia species

Fields, roadsides; throughout North America

Horses

Seeds and vegetative parts, fresh or dry. Sudden or long course. Loss of appetite, severe inflammation of the stomach and intestines, drooling, diarrhea, paralysis, sensitivity to light, hemoglobin in the urine.

Remove from source. Administer stomach and intestinal protectants (mineral oil).

Nandina, Heavenly bamboo, Chinese sacred bamboo; Nandina domestica

Common ornamental in southern US

Horses

Foliage and fruits. Hydrolyzed in stomach and intestinal tract to free cyanide, thereby affecting cellular respiration. Prognosis good if animal survives for 1 hour after signs begin. Also See Cyanide Poisoning.

Acute outcome precludes effective treatment for most; Intravenous sodium nitrite/sodium thiosulfate treatment of choice. Picrate test indicates toxic potential of the plant. Also See Cyanide Poisoning.

Oleander; Nerium oleander

Common ornamental in southern regions of the US

All

Entire plant, fresh or dry. Short course. Severe inflammation of the intestine and stomach, vomiting, diarrhea, increased pulse rate, weakness, death.

No specific treatment. Atropine in conjunction with propranolol reported helpful.

Poison hemlock; Conium maculatum

Roadside ditches, damp waste areas; throughout North America

All

Vegetative parts. Short course. Dilated pupils; weakness; staggering gait; slow pulse, progressing to rapid and thready. Slow, irregular breathing; death from respiratory failure.

Toxin excreted via lungs and kidneys, mousy odor of breath and urine diagnostic. Administer saline cathartics; neutralize alkaloids with tannic acid, together with stimulants.

Privet, Ligustrum, Hedge plant; Ligustrum species

An ornamental; common as hedge; found at abandoned farm home sites, along fences, and in bottomlands.

Horses

Leaves and fruit. Primarily intestine and stomach irritants. Diarrhea, abdominal pain, incoordination, muscle weakness, weak pulse, fever, convulsions, sometimes death.

Treatment symptomatic and supportive; correct dehydration.

Sorghum, Sudan grass, Kafir, Durra, Milo, Broom-corn, Schrock; Sorghum vulgare

Forage crops and escapes; throughout North America

All

Heavy in vegetative parts. Short course. Difficult breathing, bloat, staggering, convulsions, death. Blood bright red (cyanide) or chocolate brown (nitrate).

Hay safe for cyanide (volatile), not safe for nitrate (analyze). Also Veterinary.heading on page Cyanide Poisoning and Veterinary.heading on page Nitrate and Nitrite Poisoning.

St. John’s-wort, Goatweed, Klamath weed; Hypericum perforatum

Dry soil, roadsides, pastures, ranges; throughout North America

Horses

Photodynamic pigment (hypericin). Short course. Sensitivity to light, pruritus and erythema, blindness, convulsions, diarrhea, hypersensitivity to cold water contact, death.

Remove animals from source and sunlight. Corticosteroids given by injection, topical antibiotics.

Tall fescue; Festuca arundinacea

A coarse, hardy, drought-resistant grass; Pacific Northwest, Missouri, Oklahoma, and Kentucky; major pasture grass in southeastern US

Horses

Also see see Fescue Poisoning.

Also see see Fescue Poisoning.

Yellow jessamine, Evening trumpet flower, Carolina Jessamine; Gelsemium sempervirens

Open woods, thickets; southeast

All

Entire plant. Short course. Weakness, incoordination, dilated pupils, convulsions, coma, death within 48 hours.

No specific treatment. Relaxants and sedatives suggested.

Yew; Taxus species

Most of North America; Japanese and English yew common ornamentals

All

Bark, leaves, seeds. Gaseous distress, diarrhea, vomiting, tremors, difficulty breathing, dilated pupils, respiratory difficulty, weakness, fatigue, collapse, coma, convulsions, irregular heartbeat, circulatory failure, death. Death may be rapid.

Poisoning usually results when branches and trimmings fed to livestock.