Alsike clover toxicosis is a potentially life-threatening condition in horses caused by ingesting large quantities of the Trifolium hybridum plant, commonly referred to as Alsike clover.

Horses consuming large amounts of Alsike clover — over 20% of their diet — can lead to two different health conditions: equine hepatic failure (big liver disease) and photosensitivity (dew poisoning). To date, neither the precise mechanism of action nor the specific toxin from Alsike clover have been identified.

Symptoms vary widely and depend on which syndrome develops in the affected horse. Alsike clover-induced photosensitivity results in skin inflammation and itchiness, sensitivity to light, and eye irritation.

Big liver disease causes colic, weight loss, weakness, dry coat, jaundice, and excitement preceding sudden death. In severe cases, horses have acute neurological syndrome presenting with head pressing, incoordination, coma, teeth grinding, and death.

Since the toxicology of alsike clover poisoning is not well understood, treatment options are mainly symptomatic and supportive. The first step is removing the horse from clover-infested fields.

Pasture management plays a central role in preventing Alsike clover toxicosis. The plant is not palatable, and horses only ingest it when they do not have adequate access to other forages.

What is Alsike Clover?

There are over 300 species of clover worldwide, belonging to the Trifolium family. Most types of clover are a safe and healthy source of protein for horses. [1]

Alsike clover (Trifolium hybridum), named after the Swedish town of the same name, is primarily found in temperate climates and is highly tolerant of wet, acidic, alkaline, and saline soils. [1]

Alsike clover is unpalatable, so horses do not generally eat it unless they have inadequate access to better sources of feed and forage.

Alsike Clover Poisoning

Alsike clover can cause two separate syndromes in horses: photosensitivity and big liver disease. Horses are only at risk of Alsike toxicity when they ingest large quantities (~20% of total diet) of the clover. [2]

The precise chemicals in Alsike clover that result in toxicosis are currently unknown. However, poisoning cases tend to be linked to grazing in wet and humid wheat fields, indicating a possible connection to fungal contamination.

Fungi growing on plants can produce harmful toxins known as mycotoxins, which could contribute to poisoning symptoms in horses. [3]

Symptoms of Alsike Clover Toxicosis

Symptoms of Alsike clover toxicosis vary greatly depending on the specific syndrome developed by the affected horse. Since the physiological mechanism of this form of poisoning is not well understood, it is unclear why some horses develop dew poisoning and others develop big liver disease.

There is some evidence that dew poisoning is associated with short-term exposure, while big liver disease is a result of chronic toxicity, but more data on prevalence and rates of exposure is needed. [9]

Symptoms of Big Liver Disease

Horses that develop the hepatic (liver) form of Alsike clover poisoning can present the following symptoms: [4][5]

  • Abdominal pain (colic)
  • Lethargy
  • Diarrhea
  • Head pressing
  • Recumbency (inability to stand)
  • Jaundice (yellowing of the skin and mucous membranes)
  • Weight loss

In severe cases, affected horses can develop hepatic encephalopathy, a syndrome related advanced liver dysfunction. Hepatic encephalopathy results in an excessive accumulation of neurotoxins in the blood.

Neurological symptoms of Alsike clover toxicosis include: [6]

  • Ataxia (incoordination)
  • Aimless wandering
  • Alternating states of depression and excitement
  • Head pressing
  • Excessive yawning
  • Bruxism (teeth grinding)
  • Recumbency
  • Coma
  • Sudden death

Symptoms of Dew Poisoning

Horses with Alsike photosensitivity develop the following skin symptoms two to four weeks after ingestion: [2][3][4]

  • Photosensitization (skin reddening upon exposure to sunlight)
  • Edema (swelling)
  • Lesions
  • Tissue necrosis (death) and sloughing
  • Open, draining wounds
  • Pruritus (itching)
  • Corneal irritation (eye discomfort)
  • Hair loss

In severe cases of Alsike clover photosensitivity, horses may suffer from lack of appetite and subsequent weight loss. [4]

Dew poisoning primarily affects the muzzle, feet, and tongue. [4]

Mad About Horses
Join Dr. Chris Mortensen, PhD on an exciting adventure into the story of the horse and learn how we can make the world a better place for all equines.
Apple Podcasts Spotify Youtube
Mad Barn - Equine Nutrition Consultants | Mad Barn Canada

Risk Factors

Alsike clover toxicosis appear