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Prepared by Crystal Smith, Extension Agent, Animal Science; and Allison Steele, VCE Summer Intern

Common Pasture Weeds Causing Toxicity in Horses

FIgure1

Brackenfern (Pteridium aquilinum) 

  • Toxin(s) involved: Thiaminase.
  • Potential for Toxicity: Moderate.
  • Toxic when dry? Yes.
  • Clinical Signs: Thiamin deficiency resulting in neurologic symptoms including incoordination and severe tremors. Requires significant intake over 1-2 months.
  • Treatment: Daily thiamin injections for up to two weeks. If not treated, death may occur within 2-10 days.
Figure 2

Buckwheat (Fagopyrum esculentum)

  • Toxin(s) involved: Fagopyrin.
  • Potential for Toxicity: Low.
  • Toxic when dry? Yes.
  • Clinical signs: Photosensitization after a moderate-large intake. Most common when horses fed hay contaminated with the weed.
  • Treatment: Remove horse form the source. Protect from sunlight. Recovery is generally quick.
figure 3

Buttercup (Ranunculus spp)

  • Toxin(s) involved: Protoanemonin.
  • Potential for Toxicity: Low.
  • Toxic when dry? No.
  • Clinical Signs: Oral and gastrointestinal irritation and blistering.
  • Treatment: Recovery is uneventful when animals removed from source.
figure 4

Curly Dock (Rumex crispus)

  • Toxin(s) involved: Soluble Oxalates.  
  • Potential for Toxicity: Moderate.
  • Toxic when dry? N/A
  • Clinical Signs: Oxalates bind to calcium and magnesium in the blood leading to muscle tremors, weakness, depression, and recumbency.
  • Treatment: Intravenous Ca, Mg, glucose,electrolytes. Oral lime water to decrease further oxalation.
figure 5

Groundsel/ ragwort (Senecio vulgaris

  • Toxin(s) involved: Pyrrolizidine alkaloids.
  • Potential for Toxicity: Extremely high.
  • Toxic when dry? Yes.
  • Clinical Signs: 15 mg/kg BW over 2 weeks induces irreversible liver disease. May also cause photo-sensitization, weight loss, and jaundice.
  • Treatment: Once liver damage is done, treatment is unsuccessful. Humane euthanasia recommended.
Figure 6

Hemp Dogbane (Apocynum cannabinum)

  • Toxin(s) involved: Cynarin and apocynein.
  • Potential for Toxicity: Low-moderate.
  • Toxic when dry? Yes.
  • Clinical Signs: Diarrhea, colic, hemorrhagic,gastroenteritis, abnormal heartbeat. 15-30 grams of leaves can be lethal.
  • Treatment: Symptomatic treatment.
Figure 7

Horse Nettle (Solanum carolinense

  • Toxin(s) involved: Solanine.
  • Potential for Toxicity: Moderate.
  • Toxic when dry?Yes.
  • Clinical Signs: Toxic effects more common when plant is in processed feed. Symptoms include Salivation, colic,diarrhea, muscle tremors, and weakness.
  • Treatment: Fluid therapy, activated charcoal, via stomach tube. Physostigmine may be used cautiously in severely poisoned animals.
FIgure 8

Jimsonweed (Datura stramonium

  • Toxin(s) involved: Hyoscyamine, Hyoscine, and Atropine.
  • Potential for Toxicity: High.
  • Toxic when dry? N/a.
  • Clinical Signs: Within minutes to hours of ingestion, symptoms such as behavioral changes, colic or diarrhea appear.
  • Treatment: Symptomatic therapy and activated charcoal to prevent further absorption. Severely affected animals may benefit from treatment with Physostigmine.
Figure 9

Milkweed (Asclepias species)

  • Toxin(s) involved: Cardenolides.
  • Potential for Toxicity: Moderate.
  • Toxic when dry? Yes.
  • Clinical Signs: Colic, incoordination, tremors, heart problems, respiratory difficulty.
  • Treatment: Supportive therapy.
FIgure 10

Onions and Garlic (Allium spp)

  • Toxin(s) involved: N-propyl disulphide.
  • Potential for Toxicity: Low.
  • Toxic when dry? Yes.
  • Clinical Signs: More than 25% of the diet as onions will result in fast, weak pulse; staggering and collapse as a result of anemia.
  • Treatment: Reduce stress, whole blood transfusions in severely anemic animals.
Figure 11

Poison Hemlock (Conium maculatum)

  • Toxin(s) involved: Coniine, gamma-coniceine.
  • Potential for Toxicity: High.
  • Toxic when dry? Less toxic when dry.
  • Clinical Signs: Toxins block spinal cord reflexes leading to muscle tremors, incoordination, paralysis, frequent urination, and sudden death. 4-5 pounds of leaves are lethal to a horse.
  • Treatment: Supportive Therapy.
Figure 12

Pokeweed (Phytolacca Americana)

  • Toxin(s) involved: Phytolaccatoxin and Phytolaccigenin.
  • Potential for Toxicity: Low.
  • Toxic when dry? Yes.
  • Clinical Signs: Colic and diarrhea.
  • Treatment: Supportive Therapy.
Figure 13

Water Hemlock (Cicuta maculate)

  • Toxin(s) involved: Cicutoxin, and cicutol.
  • Potential for Toxicity: Extremely High.
  • Toxic when dry? Yes.
  • Clinical Signs: The most toxic poisonous plant known. 0.05%BW intake is lethal. Signs include convulsions and death due to respiratory failure.
  • Treatment: Due to rapid 15 minute - 8 hours following ingestion, veterinary intervention is unlikely. In some cases sodium Phenobarbital may help.
FIgure 14

Yellow and White Sweet Clover (Melilotus spp

  • Toxin(s) involved: Coumarin.
  • Potential for Toxicity: Moderate.
  • Toxic when dry? Yes *fresh undamaged sweet clover is safe for consumption.
  • Clinical Signs: Toxin, coumarin,can be converted to dicoumarol in moldy hay containing sweet clover. Signs include weakness, visible bleeding, and pale mucous membranes.
  • Treatment: Vitamin K administration.

 

Common Forage Plants Causing Toxicity in Horses

FIgure 16

Alsike Clover (Trifolium hybridum)

  • Toxin(s) involved: Unknown.
  • Potential for Toxicity: Low-moderate.
  • Toxic when dry? Yes.
  • Clinical Signs: Primary sign is photosensitization especially in non-pigmented areas. May advance to chronic liver damage with prolonged intake.
  • Treatment: Remove horse from the source. Prognosis is good if photosensitivity is only sign,poor when liver damage is involved.
Figure 17

Tall fescue (festuca arundinacea)

  • Toxin(s) involved:  Acremonium coenophialum.
  • Potential for Toxicity: Moderate.
  • Toxic when dry? Yes.
  • Clinical Signs: Toxic effects in brood mares only including prolonged gestation, retained placenta and agalactia.
  • Treatment: Remove the mare from fescue for the last 30-90 days prior to expected foaling date. Treatment with oral Domperidone at least 15 days prior to expected foaling date.

 

Common Trees Causing Toxicity in Horses

figure 18

Black Locust (Robinia pesudoacacia, and neomexicana)

  • Toxic(s) involved: Robin.
  • Potential for Toxicity: Moderate.
  • Toxic when dry? Unknown.
  • Clinical Signs: Colic, constipation, diarrhea, muscle weakness, laminitis and irregular heartbeat may occur within one hour of eating; Fatalities are rare.
  • Treatment: Prevent further ingestion and treat clinic signs.
Figure 19

Black Walnut (Juglans nigra)

  • Toxin(s) involved: Unknown.
  • Potential for Toxicity: Moderate.
  • Toxic when dry? Yes.
  • Clinical Signs: Horses bedded on shavings containing 20% more black walnut develop severe laminitis, limb edema and colic within 12/18hours.
  • Treatment: Remove the bedding, treat the clinical signs.
Figure 20

Buckeye/ Horse Chestnut (Aesculus spp.)

  • Toxin(s) involved: Aesculin, fraxin and possibly narcotic alkaloid.
  • Potential for Toxicity: Moderate.
  • Toxic when dry? Unknown.
  • Clinical Signs: Toxin is found in leaves and young sprouts. Clinical signs include colic and neurologic signs such as trembling, staggering, and difficulty in breathing.
  • Treatment: Supportive Therapy.
Figure 21

Cherry (Prunus spp.)

  • Toxin(s) involved:Cyanide
  • Potential for Toxicity: High
  • Toxic when dry? Probably not
  • Clinical Signs: Breathing difficulties, anxiety, staggering,convulsions, collapse, and death, within minutes of ingestion
  • Treatment: If horse is alive after 2-3 hours, chances are good it will recover. Veterinary treatment includes intravenous administration of sodium thiosulfate and sodium uirite.
Figure 22

Oak (Quercus spp.)

  • Toxin(s) involved: Gallotoxins.
  • Potential for Toxicity: Moderate.
  • Toxic when dry? Unknown.
  • Clinical Signs: New young leaves and green acorns most toxic leading to poor appetite,weight loss, diarrhea or constipation, increased drinking, increased urination, edema, death is possible.
  • Treatment: Aggressive fluid therapy and low stress environment.
FIgure 23

Red Maple and hybrids of red maple (Acer rubrum)

  • Toxin(s) involved: Unknown.
  • Potential for Toxicity: Extremely High.
  • Toxic when dry? Yes.
  • Clinical signs: Massive destruction of red blood cells leading to breathing difficulties, jaundice, dark brown urine, and death.
  • Treatment: Supportive therapy, Ingestion of 1 1/2 kg is toxic, 3 kg is lethal to horses (50-75% death/euthanasia rate).

 

Common Ornamentals Causing Toxicity in Horses

Figure 24

Rhododendron, Mountain Laurel, Azalea (Rhododendron spp.)

  • Toxin(s) involved: Grayanotoxins (glycosides)
  • Potential for Toxicity: Moderate.
  • Toxic when dry? No. 
  • Clinical Signs: 0.2% BW green leaves will cause colic, abnormal heart rate and rhythm, convulsions, coma, and death.
  • Treatment: Supportive Therapy.
Figure 25

Spurge (Euphorbia spp.)

  • Toxin(s) involved: Diterpene esters.
  • Potential for Toxicity: Moderate.
  • Toxic when dry? Yes.
  • Clinical Signs: Blistering upon contact, colic and gastrointestinal irritation.
  • Treatment: Remove plants from animal’s diet and they will recover uneventfully.
FIgure 26
Figure 27

 Yew, English or Japanese (Taxus spp.)

  • Toxin(s) involved: Taxine (alkaloid).
  • Potential for Toxicity: Extremely High.
  • Toxic when dry? Unknown.
  • Clinical Signs: Within one hour of ingestion:paresis, ataxia, trembling and death within 15minutes of appearance of clinical signs.
  • Treatment: Supportive therapy including activated charcoal and saline cathartic. Atropine to counter depression.

 

Resources

  • Equine Nutrition Problems: Toxic Plants in the Mid-Atlantic
      Erin D. Pittman, Institute of Applied Agriculture, University of Maryland, College Park, MD 20742.
  • Photos courtesy of Virginia Tech Weed ID Guide.

Virginia Cooperative Extension materials are available for public use, reprint, or citation without further permission, provided the use includes credit to the author and to Virginia Cooperative Extension, Virginia Tech, and Virginia State University.

Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; M. Ray McKinnie, Administrator, 1890 Extension Program, Virginia State University, Petersburg.

Publication Date

August 3, 2009