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]

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Risk Factors

Alsike clover toxicosis appears to be more common during spring and fall when there is a prevalence of wet, humid weather. [2]

Several factors can increase a horse’s risk of developing Alsike clover toxicosis:

  • Limited access to high-quality pastures: Horses with inadequate grazing options may resort to consuming Alsike clover due to lack of palatable alternatives.
  • Seasonality: Spring and fall are associated with higher humidity levels, potentially amplifying the toxic effects of Alsike clover.
  • Geographical location: Horses living in regions that are native or endemic to Alsike clover are more likely to have access to the clover in pastures.

As with all forms of poisoning, the risk of toxicity is directly related to body weight and dosage, putting foals and miniature horses at increased risk.

Diagnosis of Alsike Clover Toxicosis

There is no specific diagnostic test for Alsike clover poisoning in horses. Diagnosis relies on a combination of clinical evaluation, imaging, blood tests, urinalysis, and assessment of exposure history and feed samples. [2]

Horses presenting symptoms associated with Alsike clover toxicosis generally undergo the following diagnostics: [2][4]

  • Blood tests: to review liver function
  • Physical examination: to assess general condition and potentially detect skin irriation and liver enlargement
  • Liver biopsy: to rule out other potential conditions causing symptoms
  • Diagnostic imaging: to visualize potential liver enlargement and other abnormalities
  • Skin biopsy: useful in cases of photosensitization to rule out other causes of symptoms
  • Feed analysis: analyzing hay and other feed sources can confirm the presence of Alsike clover and other potentially harmful plants

If you are unsure about potential mycotoxins in your horse’s forage, consider getting a forage analysis to confirm they are not at risk.

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Differential Diagnosis

A number of other forms of poisoning cause symptoms that can be confused with Alsike clover toxicosis, including: [4]

  • Pyrrolizidine alkaloids
  • Locoweed
  • Horsetail
  • Pteridium aquilinum (bracken fern)
  • Fumonisin mycotoxins

In the absence of a specific test, the diagnostic process aims to rule these other conditions out before confirming a diagnosis.

In addition to other forms of poisoning, diagnostic evaluation rules out other neurological, dermatological, and liver diseases with similar symptoms, including: [4]

Treatment of Alsike Clover Toxicosis

Currently, there is no antidote for Alsike clover toxicosis, and treatment is supportive and symptomatic in nature.

Once Alsike poisoning is confirmed, the first step in treatment is removing all horses from pastures containing the clover to prevent further ingestion.

Treatment options vary according to which syndrome is affecting the horse.

Treatment for Dew Poisoning

Treatment options for horses that develop photosensitization include: [4][7]

Proper wound management is crucial as necrotic skin attracts a number of insects that may lay eggs inside the wounds, which can lead to complications. [8]

While horses are recovering from dew poisoning, they may benefit from light blanketing and wearing UV-protective face and eye coverings while outdoors during daylight hours. [9]. Minimizing exposure to ultraviolet light during the recovery period helps prevent further development of symptoms while the body is clearing the toxin. Always choose an appropriate blanket weight for the weather to ensure your horse doesn’t overheat.

Treatment for Big Liver Disease

Management and treatment options for big liver disease are limited and mainly focus on supporting the liver’s natural detoxification processes and minimizing further damage. [4][8]

Nutritional support plays a vital role in recovery from big liver disease. Depending on the severity of the case, veterinarians might recommend switching to more frequent, smaller meals. [4][8]

The recommended diet for horses with hepatic diseases generally includes digestible carbohydrates and low levels of protein, but individual protocols vary. [4][8]

Dietary management is particularly important in cases where big liver disease has progressed to hepatic encephalopathy. Affected horses may have difficulty swallowing, putting them at risk of nutrient deficiencies. [4][8]

In some cases, horses with big liver disease may be unable or unwilling to eat. These horses may need to feed via nasogastric intubation during the recovery period.

Nasogastric intubation should only be performed by a veterinarian. There is no safe way to intubate a horse at home.

Prognosis

The prognosis of Alsike clover toxicosis varies and depends on which related syndrome the horse is suffering from.

Horses with photosensitization are generally expected to recover with appropriate medical intervention. Recovering horses may require long-term treatment, and even in cases of severe skin necrosis, the majority of lesions heal with supportive care. [8]

Unfortunately, full recovery from the hepatic form of toxicosis is rare. [4]

The prognosis for big liver disease varies from guarded to poor. The extent of liver damage significantly influences the outcome. Early intervention and aggressive supportive care can improve the chances of survival, but some cases might prove fatal. [2][4]

Prevention of Alsike Clover Toxicosis

Due to the lack of specific treatment options and poor understanding of how this toxin acts on the body, preventive measures are essential in protecting horses from Alsike clover toxicity. [4]

Pasture Inspection

Regular inspection of pastures to identify and remove Alsike clover plants can significantly reduce the risk of ingestion. Maintaining healthy pastures with diverse vegetation deters horses from grazing excessively on Alsike clover, as the plant is unpalatable, and animals prefer other forage options if available.

In North America, Alsike clover is most commonly found in the prairies. The plant has a distinct appearance, making it easy for horse owners and caretakers to identify in pastures.

When in bloom, the clover has white and pink bulbous blossoms made up of many small, curled petals. The shade of the blossoms varies from completely white to completely purple and all shades in between.

The plant is distinct from red clover and other clover species with smooth, unwatermarked leaves. When Alsike clover is uprooted, it shows a well developed crown and taproots that are branched and shallow.  [10]

Aslike Clover Horse Pasture Identification

 

Forage Analysis

Horse owners and caretakers should also ensure any forages coming from external suppliers are free from poisonous plants, including Alsike clover. This is especially important during the winter when northern farms may not have viable pastures and horses primarily consume cut hay.

Regular forage analysis can confirm that hay does not contain excessive amounts of mycotoxins, which may play a role in Alsike clover poisoning. Working with agronomists and veterinary toxicologists can identify potential toxic elements in forage.

Balanced Diet

Providing horses with a balanced, forage-based diet is essential for their overall health and can help mitigate the risk of Alsike clover toxicosis. Providing free access to appropriate pastures helps discourage excessive consumption of Alsike clover and other potentially harmful vegetation.

Grazing Management

Controlled grazing practices protect horses by limiting access to Alsike clover-infested areas. Rotational grazing, weed control, and temporary fencing can help prevent overexposure to Alsike clover.

Rotating pastures and providing adequate rest periods also promotes biodiversity of forage species.

Environmental Monitoring

Humid weather conditions are associated with increased toxicity levels in Alsike clover, possibly due to increased mycotoxins during these conditions. [2] Monitoring environmental conditions can help anticipate and mitigate potential outbreaks of toxicosis. [2]

Regular Veterinary Care

Keeping up to date with vaccinations and undergoing routine veterinary examinations is essential for early detection of many conditions, including chronic, low-levels of toxicity.

In addition, veterinarians and equine nutritionists can provide guidance on preventive measures tailored to specific farm environments and individual horse populations.

Summary

Alsike clover toxicosis, a potentially fatal condition, affects horses that ingest large quantities of the Trifolium hybridum plant (Alsike clover).

  • Symptoms vary depending on the syndrome: big liver disease causes abdominal pain, weight loss, and jaundice, while photosensitivity presents with skin redness, itchiness, and eye irritation after sun exposure
  • No specific antidote exists, so treatment focuses on removing horses from Alsike clover and providing supportive care including pain management, IV fluids, and dietary adjustments
  • The prognosis for photosensitivity is good with prompt removal and supportive care, while big liver disease has a guarded to poor outlook depending on severity
  • Prevention is critical to avoid equine Alsike toxicity. Regularly inspect pastures to remove Alsike clover plants and maintain diverse vegetation for horses to graze on

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References

  1. Awasthi, L.P. Viral Diseases of Field and Horticultural Crops. Elsevier Science, 2023.
  2. Wilson, D.A., Ed. Clinical Veterinary Advisor: The Horse. St. Louis, Mo: Elsevier Saunders, 2012.
  3. Plumlee, K.H., Ed. Clinical Veterinary Toxicology. St. Louis, Mo: Mosby, 2004.
  4. Lavoie, J.-P. and Hinchcliff, K.W. Blackwell’s Five-Minute Veterinary Consult Equine, Second Edition. Wiley-Blackwell, 2009.
  5. Foreman, J.H. Hepatotoxins in Large Animals. MSD Veterinary Manual, 2023
  6. Müller, J.-M.V. et al. Ataxia and Weakness as Uncommon Primary Manifestations of Hepatic Encephalopathy in a 15-Year-Old Trotter Gelding. Equine Veterinary Education. 2011. doi: 10.1111/j.2042-3292.2010.00132.x.
  7. Nation, P.N. Alsike Clover Poisoning: A Review. Can Vet J. 1989. View Summary
  8. Foreman, J.H. Disorders of the Liver in Horses. Merck Veterinary Manual, 2019.
  9. Alsike clover poisoning, photosensitization or photodermatitis in horses. Ontario Ministry of Agriculture, Food and Rural Affairs, 2023.
  10. Harder, C. et al. M&Ms of Controlling Alsike Clover. Peace River Forage Association of British Columbia, 2019.