Moldy Sweet Clover Poisoning (Dicoumarol Toxicosis) in Horses

Key takeaways

Moldy sweet clover toxicosis is a potentially life-threatening condition caused by the ingestion of moldy sweet clover plants. 

  • Moldy sweet clover plants produce the natural anticoagulant dicoumarol.
  • Ingestion of dicoumarol can cause fatal hemorrhages in horses
  • Symptoms of sweet clover toxicosis include bruising, red spotted gums, nose bleeds, stiffness and lameness
  • There is no specific diagnostic test for sweet clover toxicosis, so diagnosis is usually based on clinical findings and history of exposure
  • There is no antidote for dicoumarol poisoning; treatment options include blood transfusion and synthetic vitamin K1 administration
  • In rare cases, sudden death caused by internal bleeding related to concurrent surgery, injury, or giving birth can occur without showing prior symptoms
  • The only way to completely prevent toxicosis is to remove sweet clover from the diet
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A closer look: Moldy Sweet Clover Poisoning (Dicoumarol Toxicosis) in Horses

Sweet clover is native to the European and Asian continents, but can be found worldwide. Sweet clover contains coumarin, an organic chemical compound. When the plant molds, coumarin is transformed into the natural anticoagulant dicoumarol.

If horses consume a toxic dose of dicoumarol when eating moldy sweet clover, the normal blood clotting process is interrupted. Abnormal blood clotting can result in internal bleeding, which is life threatening.

If exposure is witnessed or if symptoms develop, immediate medical attention is warranted.

In most cases, sweet clover toxicosis is a herd condition; as such, all animals living on a shared pasture should receive medical attention. The herd nature of the condition is also helpful in ruling out other potential conditions.

Risk factors

Moldy sweet clover toxicosis is rare in horses. Moldy sweet clover toxicosis is a potentially life-threatening condition and must be treated as an emergency.

In some cases of dicoumarol poisoning, death can occur without the appearance of prior symptoms. Prevention and removal of sweet clover from the diet are of the utmost importance.

Symptoms vary depending on the amount of moldy sweet clover ingested. Horses that consume large amounts of dicoumarol are more likely to experience sudden death, usually attributed to massive hemorrhage into the chest, abdomen, or around the brain.

Horses at higher risk of bleeding such as those pending surgery and pregnant mares are most at risk of severe complications from dicoumarol poisoning.

Possible causes

Sweet clover toxicosis is caused by the ingestion of Melilotus officinalis, commonly known as sweet clover, which has gone moldy. In sufficient quantities, the natural anticoagulant dicoumarol present in the moldy clover interrupts the natural blood-clotting process, leading to internal bleeding.

Main symptoms

Testing and diagnosis

Diagnosis of dicoumarol poisoning is typically based on symptoms and history of exposure. Other diagnostic tests include:

  • Blood clotting time
  • Blood work
  • Diagnostic imaging to assess for internal bleeding
  • Chemical analysis of feed for dicoumarol levels

Steps to Recovery

Once diagnosed, treatment options include:

  • Blood transfusion
  • Synthetic vitamin K1 administration to improve blood clotting
  • Removing the affected feed from the diet

Left untreated, the prognosis for moldy sweet clover toxicosis is extremely poor. Death can occur suddenly without showing any symptoms.

With prompt and proper treatment, animals suffering from mild cases of dicoumarol poisoning are able to recover.


Moldy sweet clover toxicosis is not contagious, but it usually affects all animals pasturing in contaminated fields together.

The only way to completely prevent dicoumarol poisoning is to eliminate sweet clover from the diet. The risk of toxicosis can be reduced by:

  • Making sure that feed is mold free
  • Choosing low-coumarin sweet clover varieties
  • Testing hay for the presence of dicoumarol if contamination is suspected

Is Moldy Sweet Clover Poisoning (Dicoumarol Toxicosis) in Horses common?

Moldy sweet clover toxicosis is a rare condition in horses. The majority of cases affect cattle.

Typical Treatment

  • Blood transfusion
  • Vitamin K1 administration

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