Hoary alyssum toxicity

Veterinary advice should be sought before applying any treatment or vaccine.

Hoary Alyssum Toxicity

Hoary alyssum (Berteroa incana) is an erect, densely haired, annual to short-lived perennial forb which is toxic to horses if ingested. Hoary alyssum can cause lameness resulting from limb edema and in severe cases, laminitis if the plant is consumed in the pasture or contaminated hay.

Hoary alyssum has gray-green, hairy stems which branches out near the top and stands about 1 to 3 feet in height. It has gray-green, hairy, oblong, narrow, alternate leaves with smooth edges that are roughly 0.5 to 3 inches in length. It's flowers appear as small white notched petals, blooming from early spring to late fall. Hoary alyssum has hairy, oblong seedpods with short beaks on the end. Its seeds can germinate while the plant is in bloom. Seedlings which are late to establish themselves will remain as rosettes and produce flowers and seeds the following year, reproducing as winter annuals or biennials. Sometimes it can form a seed bank which causes seeds to remain dormant for up to nine years. It's seeds are a frequent contaminate of forage and lawn seed, as well as contaminated hay.

Contamination of more than 30% or more of hay with hoary alyssum is toxic to horses, causing clinical signs of lameness, limb edema, haemolysis, hypovolemic shock, and laminitis. The severity depends on the amount of the plant consumed and on the horse, as just under 50% of horses ingesting hoary alyssum show clinical signs. It usually takes 12 to 24 hours following ingestion of the plant for the horse to begin to show signs of toxicity.


Limb edema (lower leg swelling)
Fever of 103°F (39.4°C)
Increased digital pulse
Joint stiffness
Abortion in mares
Red-colored urine
Hemorrhagic diarrhea


  • History
  • Clinical signs
  • Physical exam
  • Complete blood count
  • Serum biochemical analysis
  • Urine analysis
  • Radiography



Supportive therapyIV fluids and electrolytes if severe enough
Antiinflammatory and analgesic therapyflunixin meglumine or phenylbutazone
Cold water hydrotherapyfor treatment of limb edema
Removal of plant from diet
Treatment for laminitis, if it develops


  • Hay containing greater than 30% hoary alyssum should not be fed to horses.
  • Make yourself aware of the weeds and plant species that can be invasive in pastures and/or poisonous to horses.
  • Take periodic walks around pastures to check for the presence of potentially poisonous plants
  • Check that hay does not contain dried up poisonous plants
  • If you borrow or hire farm machinery ensure it is clean prior to arriving on your property, the same goes for lending of your own equipment.
  • Quarantine new animals in a separate paddock the first 10 days to 2 weeks after arrival. Weed seeds can be passed through an animal's digestive tract.


1 to 2% of cases are fatal

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Risk Factors

  • Letting horses graze on pastures containing hoary alyssum