Clover Photosensitivity

A rainy, cool summer in Manitoba, Canada, was turning to fall when Thunder, a 2-year-old Paint/Arab cross gelding with lots of white across his body, began to show signs of colic. He was treated for colic four times in two days. Annette Fleming, DVM, a  veterinarian at Oakbank Birds Hill Animal Hospital in Manitoba, diagnosed a high impaction and made note of excessive gastric reflux. A seemingly incidental finding at the time was a small patch of skin on the inside of his thigh that was hard and crinkled, but warm to the touch. Thunder improved and was eating and drinking normally after the second day.

Two days later, he deteriorated with severe swelling and pain in all four legs, difficulty walking, and was lethargic. He had developed another abnormal skin patch on his neck, and the area on his thigh was expanding. Fleming's colleague Neil Charnock, DVM, was called in to help diagnose Thunder. Charnock queried Thunder's owner, Pamela Austman, about travel off the farm for Thunder or any of the other horses, any other sick horses, eating habits, and pasture turnout.

The description of the pasture clued Charnock in to Thunder's problem. He recalled a lecture from veterinary school about alsike clover (Trifolium hybridum) causing liver damage and photosensitivity in horses. Aust-man's pasture was timothy, brome, and orchard grasses, alfalfa, and "a bit" of clover. However, the wet, rainy summer had allowed the clover to take over the other grasses.

Signs of alsike clover toxicity typically manifest two to four weeks after ingestion. The main insult to the liver, through an unknown toxin, causes damage to the biliary tree that is responsible for excreting phylloerythrin (a compound formed by the breakdown of chlorophyll in the intestine). The phylloerythrin circulates in the blood stream to the skin, where exposure to ultraviolet light causes oxidative injury to the blood vessels. The resulting vasculitis (inflammation of the vessels) constricts the blood supply to the skin, causing tissue necrosis (death). There is more UV exposure in non-pigmented skin, and therefore Thunder's risk was increased due to the large proportion of his white skin (about 60% of his coat).

Charnock confirmed his diagnosis with a blood chemistry panel that showed mild liver damage and skin biopsies that showed severe vasculitis with some of the vessels completely occluded with blood clots.

Thunder's treatment plan was to remove him from the clover exposure, to support his liver function during healing with good nutrition, and to minimize further skin damage by housing him indoors. The vasculitis was managed with corticosteroids. The secondary skin infection related to the dying tissue was managed with antibiotics.

Wound care was the biggest obstacle. His owner took two weeks off from work to nurse Thunder during his crisis. Austman described her horse thusly: "His poor nose turned black. That lovely pink diamond was a crust of dead skin. All four legs were heavily damaged, with the back of both knees and the left thigh being the worst. The coronary bands on all four feet had gaping sores, and we were very concerned that he could slough his hooves." He did develop hoof damage that required acrylic filling and shoeing of all four feet.

Six months later, his hooves are almost completely grown out. The scarred and hairless areas are mostly limited to his legs and thighs. Austman said, "Perhaps with a few shed-and-grow seasons, his coat will surprise me and fill in a lot of the naked areas. One can only hope. But despite his patchiness, he is still a loving and friendly young horse who holds a very special place in my heart."

There are three species of clover with flow-ering heads typically found in pastures in the northern United States and Canada. Two generally non-toxic species are the common white Dutch clover (Trifolium repens) and red clover (Trifolium praetense, a more common pasture legume; for more information on red clover, see page 24). Whole alsike clover is the toxic one; alsike has pink to purple flowers on stalks, does not produce runners, has multiple branches of leaves and flowers, and grows 12-36 inches tall. Clover likes poorly drained topography and thrives in moisture and soil that ranges from neutral to acidic. Control is through whole-pasture spraying with a broadleaf herbicide.

If you have alsike clover in your horse pastures, discuss the possibility of health-related problems with your veterinarian.

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