Q. I'd like to see more information on alsike clover sensitivity, especially in horses with large areas of white skin such as Paints, pintos, and Appaloosas. All the information I can find suggests that the only problem you will encounter is a severe sunburn on the white areas with sloughing of the damaged skin.

My Paint horse's first signs were severe itching all over his body and colic! His urine output was severely diminished, and he had severe edema (swelling) in his legs, loss of appetite, and extreme distress. My veterinarian was at first puzzled by all the signs and told me he would be able to go out and graze at night and I could avoid the skin damage. I've read that alsike clover poisoning causes liver malfunction and that is what caused all the other signs.


A. Alsike clover (Trifolium hybridum) poisoning is a relatively common cause of photosensitivity in horses, especially in the northwest United States, or where alsike clover and red clover (Trifolium praetense) are common in pastures. As little as 20% alsike clover in the diet may result in poisoning, with clinical signs appearing two to four weeks later. The toxin responsible for the liver disease associated with alsike clover poisoning is unknown, but the presence of "sooty blotch" disease, a fungus disease of clover and alfalfa, appears to be associated with the liver disease. Also referred to as dew poisoning or trifoliosis, horses with alsike poisoning develop severe photosensitivity secondary to biliary (pertaining to the bile and/or bile ducts) disease.

Sunburn of non-pigmented skin, and also of the mucous membranes of the mouth, eyes, and vulva, are often the first noticeable signs. In more severe cases, signs of liver failure include progressive weight loss, loss of appetite, depression, jaundice, circling, yawning, colic, recumbency, and eventually coma and death. Diagnosis of alsike poisoning is based upon presence of clover in the diet, detection of the fungus Cymodothea trifolii on the clover, elevated liver enzymes, and the presence of photosensitization.

If the poisoning is recognized early, horses will recover if they are removed from the alsike or red clover and kept out of the sun until the liver enzyme profile returns to normal. Your veterinarian can evaluate liver function and provide comprehensive treatment in more severe cases. The best method of preventing the disease is to limit or prevent horses having access to alsike clover, especially if "sooty blotch" fungus is present.

About the Author

Anthony Knight, BVSc, MS, Dipl. ACVIM

Anthony P. Knight, BVSc, MS, Dipl. ACVIM, is a professor of large animal medicine in the College of Veterinary Medicine and Biomedical Sciences at Colorado State University. He received his veterinary degree from the University of Nairobi, Kenya, in 1968. After completing a master’s degree at Colorado State University, he joined the faculty in 1974. His current professional interests include livestock heath, foreign animal diseases, emergency management, and plant toxicology. He has written two books on poisonous plants of animals in North America, and maintains a poisonous plants website for use by anyone wanting poisonous plant information.

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