Sunday, July 16, 2017
Baxter Black: Photosensitization
The animal is sensitized to the sun's rays so that even a lot of the weaker rays can cause damage. An animal cannot be sunburned through a window glass but can be photosensitized through it.
It occurs in sheep, grazing cattle and I've seen it show up in the feedyard. Most commonly is related to the ingestion of specific plants the animal has eaten. St John's wort (Klamath weed) in the Northwest, agave and sachuiste in the Southwest, plus others including cultivated rape, horse brush and buckwheat. Sudan grass, clover or alfalfa have even been incriminated.
Either through liver damage or directly, breakdown products of the plant enter the blood stream and circulate to the skin. There they become exposed to the sun's penetrating rays. A chemical reaction occurs which damages the surrounding capillaries and tissue. Thick hair or darkly pigmented skin usually blocks out the sun's rays so the reaction only occurs in the lighter areas.
Following ingestion of enough plant toxins the affected critter's skin begins to redden, itch and swell. By the second day the muzzle and eyelids have a burned appearance. With continued exposure to sunlight the skin dies, becomes hard and leathery and starts to peel at the edges of the eyes or muzzle.