It's a glorious summer day as you pull up to the barn. You've got a perfect plan for your workout: a schooling session in the ring, followed by a relaxing trail ride to cool your horse down while you soak up some sun. You put on your sunglasses and maybe a cap to shade your brow and reach for that bottle of sunscreen you keep in the glove compartment. You know that you need to shield yourself from the summer sun's rays but what about your horse? Can too much fun in the sun hurt him?
Horses are not as susceptible to that all-over sunburn that people can get with over-exposure to strong sunlight, but a few individuals may be at greater risk for sun-related problems. Fortunately, the majority of these problems in horses are easy to identify and simple to cure.
True allergy, or hypersensitivity, to the sun is rare in horses. More common is "photosensitivity," sun-related skin injury that's triggered by chemical imbalances in the body. Forage plants are one possible culprit in the process but certain drugs or underlying liver disease may also be factors. Scientists distinguish two kinds of photosensitivity, primary and secondary, and plants can be implicated in both.
Primary photosensitivity results when a horse eats significant quantities of a plant that contains a photodynamic, or light-sensitive, chemical compound. Examples include buckwheat, Saint-John's-wort, bishop's weed and spring parsley. After it is absorbed from the gut, the photodynamic compound circulates in the bloodstream until it reaches the capillaries of the skin. In a horse with significant areas of pink or white skin, which lack the normal protective level of melanin, exposure to UV-A (ultraviolet containing the highest energy) radiation causes a fluorescent reaction, releasing free radicals, which damage the surrounding skin tissue.
To understand secondary photosensitivity, we first have to look at how horses normally digest the food they eat. Every green thing a horse eats contains chlorophyll. In the large intestine, bacteria break the chlorophyll down into a derivative--and photodynamic--compound known as phylloerythrin. Normally, phylloerythrin is absorbed from the horse's gut and transported in the bloodstream to the liver for extraction and excretion into the bile. But if, for some reason, the liver is unable to perform this function, the phylloerythrin is carried to capillaries in the skin and the result is secondary photosensitivity.
There are two ways that plants can contribute to this condition. One is by overwhelming the liver's extracting capacity. Some plants--particularly in the clover and related alfalfa families--may deliver unusually large amounts of chlorophyll to the large intestine, producing a higher "dose" of phylloerythrin than the liver can process. The excess, circulating through the bloodstream, triggers the photosensitive reaction. Other factors, such as altitude, can also affect the process: For example, horses grazing on pure alfalfa in the high, clear air of the interior West have exhibited increased photosensitivity because of more incoming UV-A.
Too much chlorophyll is not the only plant-related cause of photosensitivity. Some plants contain alkaloids and other toxic compounds that can directly damage a horse's liver, rendering it unable to process phylloerythrin. Common culprits here include hound's tongue, creeping indigo and members of the Senecio family.
The signs of photosensitivity vary. In mild cases, the most common sign is a reddened area under (or following) dry, crusty scabs on the white or light-colored parts of the body (especially the face, fetlocks and coronets or, on a pinto or Paint horse, the back and shoulders). Affected parts are obviously sensitive to touch. Though the skin is damaged, the hair growing in it remains normal; you will see it sticking up through the scabs.
With severe secondary photosensitivity, the symptoms are much more alarming. The first obvious signs are dark, purplish blisters that form and then weep or slough and scab over on the areas where UV rays are most perpendicular to the horse's body: back, fetlocks, coronary bands and the edges of the nostrils. A horse in this condition is in severe discomfort and deserves immediate medical attention. In addition, if you heed this "early warning" system, you might be able to nip a potentially life-threatening condition, such as liver failure, in the bud.
Fortunately, the majority of photosensitivity cases are mild, and the outlook for a full recovery and future prevention is very good.
This excerpt is from the article "Sun Smart" which originally appeared in the June 1997 issue of EQUUS magazine.