Photosensitivity in the American Paint Horse

Along with “lethal white syndrome,” the Paint Horse, like any horse breed, is susceptible to a handful of genetic disorders and congenital conditions. But there’s one health condition in particular that Paints are known for being susceptible to, and that’s photosensitization. Some aspects of this condition and the horses’ vulnerability to it are easy to understand. Others are more subtle and counterintuitive. So, let’s discuss what equine photosensitization is, what it looks like, and why the Paint Horse is vulnerable to its effects.


Primary Photosensitivity

Photosensitization doesn’t just affect pigmented or non-pigmented areas of the skin. Any part of the body that is exposed to direct sunlight may be affected. Skin lesions around the eyes and nostrils is often the first sign of trouble. That said, it is believed that the Paint Horses white hair and unique color profile contributes to its vulnerability. Likewise, giving the horses a break from direct sunlight is a common and smart precaution, especially during the late spring and early summer when UV radiation is worst. In terms of primary photosensitivity, the Tobiano Paint Horse with its typical white legs, white back, and hindquarters is particularly vulnerable. While the condition can become quite severe if left untreated, recovery from primary photosensitivity is often easier and more reliable than secondary photosensitivity. As you’ll see, the real culprit often comes from within.


Secondary Photosensitization

It’s secondary photosensitization that can be the real danger and killer. Most grasses are not easy to digest, even for the animals that have evolved to use grazing pastures for sustenance. If the horse’s liver becomes damaged or diseased, or if toxins are introduced into the system, the body becomes unable to excrete phylloerythrin, a substance created when chlorophyll is broken down in the gut. The phylloerythrin builds up in the blood and eventually the skin. When it becomes exposed to sunlight, it oxidizes with harmful effects to the skin and blood vessels, eventually becoming fatal. Pyrrolizidine alkaloids are the biggest risk factor for this illness. Tansy ragwort, groundsel, fiddleneck, common heliotrope, vipers bugloss, and rattlebox are in this group. Other photosensitizing agents include St. John’s wart, buckwheat, perennial ryegrass, sulfa antibiotics, and tetracycline.


Other Causes and Presentations of Photosensitization

Most cases of photosensitization require at least some component of both primary and secondary photosensitization. It’s difficult to completely rid a horse’s diet of phylloerythrin, nor is it realistic or even a good idea to try to keep the horse out of the sun all the time. The worst cases often involve heavy elements of both causes of photosensitization. There is also a genetic disorder, porphyria, that creates photosensitivity through abnormal growth of pigmentation, but this is extremely rare even for Paint Horses.

Now, that we’ve covered everything, it’s worth providing some perspective in summation. For with a little planning and the right type of grazing pastures, Paint Horses typically live long, healthy, and productive lives.