UNDERSTANDING PHOTO SENSITIVITY

As the days get long in spring, there’s one more condition for horse owners to watch out for: photo sensitization .“It’ s like sunburn, except it affects the deeper layers of skin,” says Rosanna Marsella, DVM, DACVD, of the University of Florida in Gainesville. “Many horses with white legs develop ulcerated lesions triggered by ultraviolet exposure. Thus we tend to see this at the same time we see allergies, with longer days and warm weather—when the horses are outside a lot. Many of them get pastern dermatitis, which is not a disease in itself, but a syndrome. In some horses photosensitivity is the underlying cause.”

Photo sensitization occurs when concentrations of photodynamic (light sensitive) compounds circulate in the bloodstream. When the sun’s ultraviolet rays penetrate the upper layers of a horse’s skin, they interact with the chemicals and cause localized burning that begins inside the capillaries. Photo sensitization usually occurs on white markings with pink skin, because dark skin contains the pigment melanin, which blocks the sun’s rays. “It’s not limited to white areas in some horses, however,” says Stephen D. White, DVM, DACVD, of the University of California–Davis.

Primary photo sensitization occurs when a horse ingests plants, medications or other substances that contain photodynamic compounds. These plants can include common weeds, such as alsike clover, buckwheat and St. John’s wort; some horses develop photo sensitization after eating alfalfa hay.

Secondary photo sensitization is a result of liver damage. A healthy liver filters wastes from the blood; when a liver is not functioning well, wastes accumulate in the bloodstream, including one photodynamic byproduct of the metabolism of chlorophyll, which is found in green plants. Even a horse with a healthy liver may overload his system with more chlorophyll than his liver can handle when he suddenly consumes large quantities of spring grass after months of dry hay. In addition, says White ,“photo sensitization can also be due to horses eating plants that are toxic to the liver.”

Signs of photo sensitization include dark, purplish blistering and crusting and scabbing of (mainly) pink skin under white markings. The lesions will be very painful.

The first step when dealing with photo sensitization is to keep the horse out of the sunlight and remove access to plants that could be causing the reaction. Covering the affected areas with ultra-violet-resistant masks, boots, sheets or other clothing may also help.

If the healing goes slowly, or the horse seems otherwise ill, call your veterinarian. “We need to find out what the animal is eating, and what the liver values are in case there’s been liver damage,” says White. “We can also take a biopsy of the affected area to see if there is inflammation of the blood vessels. There are a number of diagnostic techniques that your veterinarian might use.”

Treatments depend on the specific source of the problem. At the very least, your veterinarian may prescribe medications to reduce pain and inflammation.

Also ask your veterinarian or local extension agent for a list of plants in your region that can trigger photosensitivity and remove any that grow in your pastures. Horses who have plenty of grass or hay are less likely to nibble on strange weeds. If your horse tears through his hay, a slow feeder may make his ration last longer. Finally, make sure all your pastured horses have access to a deep run-in shed or another source of shade.