Alsike clover (Trifolium hybridum) and red clover (Trifolium pratense) are valuable forage crops in North America and are often included in pasture seed mixes. However, these plants can be associated with disease conditions in all animals including horses. In addition to the risk of slaframine production, these clovers also can be associated with less common conditions in horses such as photosensitivity and liver disease. The specific toxins that cause these conditions have not yet been identified and are hypothesized to be either mycotoxins or secondary plant metabolites that are produced only under specific circumstances.

Horses with photosensitivity due to clover ingestion develop sunburn-type lesions on nonpigmented areas of skin and mucous membranes. Affected areas are red, edematous (fluid-filled), ulcerated, and sloughing. Treatment involves removing horses from sunlight and clover exposure. Horses with clover-associated liver damage often show neurologic signs such as depression, listlessness, head pressing, circling, ataxia (incoordination), poor appetite, and other unusual behaviors. Other signs include weight loss, poor body condition, and poor hair coat. Blood tests show abnormalities indicative of liver failure. Treatment consists of supportive care and ceasing exposure to clover. Horses that show only photosensitization have a good prognosis, while others that develop liver damage or liver damage combined with photosensitivity have a poor prognosis.

Occurrence of clover-related photosensitization and liver disease varies greatly from year to year and from region to region, which suggests the toxin involved might be influenced by environmental conditions. Signs can be associated with ingestion of clover in pasture or hay and can occur any time of year, although cases are less common in winter months. The exact amount of clover that must be ingested before signs develop is unclear. However, most cases have been associated with diets consisting of at least 20% clover, and consumption of the flower appears to increase the risk for disease. Sporadic cases in a herd might be related to individual predisposition or feeding preferences (e.g., some horses will seek out clover while others might avoid it). Clinical signs can develop within weeks after the start of clover ingestion, or they can take months to develop. No specific diagnostic tests are available, as the toxin(s) involved have yet to be identified. Presumptive diagnosis is based on clinical signs and blood test results indicative of liver damage, along with a history of exposure to clover in the diet.

Prevention is key and consists of minimizing or avoiding clover in equine diets. Much work still needs to be done to better understand the conditions and identify the toxin(s) associated with these diseases