notice symptoms later. Fortunately, contact dermatitis is generally not serious and will typically resolve without treatment if the horse is prevented from having further contact with the irritating substance. More worrisome is photosensitivity, a condition in which exposure
to sunlight causes the skin to become itchy, red, swollen, crusty and blistered, aſt er which it will eventually slough off . While that sounds suspiciously like severe sunburn, there is more to it than that. T ree factors are usually at play in the development of photosensitization: the presence of a photodynamic substance in the skin, a lack of skin pigment and exposure to ultraviolet (UV) light. It can develop in completely dark-skinned horses, but that is less common. It is a serious condition that can lead to secondary infections and take a long time to heal. It may also be a sign of liver impairment. T ere are two common types of photosensitization:
Primary Photosensitization T is occurs when a horse eats something containing a photody-
namic pigment (pigment activated by light), or in response to certain drugs. When the photodynamic substance travels through the blood- stream and reaches unpigmented areas of skin, it comes into contact with UV rays, triggering a reaction. Plants that cause primary photosensitization include St. John’s
Wort, perennial ryegrass and buckwheat, with pyrrolizidine alkaloid producing plants (e.g. fi ddleneck, ragwort, hound’s tongue) being especially problematic. Drugs that cause photosensitization include sulfonamides, phenothiazine, thiazides and tetracycline.
Hepatogenous (Secondary) Photosensitization T is occurs when the liver is damaged and is unable to excrete phyl-
loerythrin, a photodynamic end product of chlorophyll metabolism normally excreted in the bile. Most oſt en, the liver damage is caused
To reduce mosquito exposure, stable horses at dusk and dawn; use fl y sheets, masks and leg wraps; turn barn lights off ; use fans to move air; and use mosquito repel- lents. Photo courtesy of
www.CashelCompany.com
by ingestion of one of the many hepatotoxic plants that include ragwort, fi ddleneck, fi reweed, rapeseed, comfrey and but ercup. Various members of the clover family are also known to cause
photosensitization, though the mechanism by which they do so is not entirely understood. Explains Dr. Wright, “Alsike clover is well recognized for causing photosensitization as well as oral ulcers and hepatitis. It is unclear whether the photosensitization is primarily a photodynamic agent problem or a secondary phototoxic reaction due to liver damage, or whether alsike clover and its metabolites is truly the culprit. T e photosensitization reaction could also be associated with mycotoxins (toxins produced by fungi) produced on the plant. A similar syndrome has also been observed with horses consuming lush white clover in the fall.” Because other symptoms of liver damage are oſt en vague and fre-
quently go undetected, photosensitization can be the fi rst sign that the horse’s liver is impaired. Any case of photosensitization should there- fore be followed up with blood work to assess liver function. Because photosensitization is much less common in dark-skinned horses, liver damage in these animals can be more diffi cult to detect. Treatment for photosensitization requires
removing the horse from direct sun exposure, supportive care for lesions and appropriate evaluation and treatment for any liver damage. Pastures should be carefully inspected for any plants known to cause primary or secondary photosensitization.
Susan Kauff mann is an equestrian journalist and photographer who has been working with, training and writing about horses for nearly 30 years. Her
articles on horse health, training and welfare appear regularly in publications in both the US and Canada, and she also writes content for horse-related courses for Michigan State University. Some of her articles can be viewed on her blog, www.T
eEquinist.blogspot.com, which is popular with horse owners seeking in-depth information. Susan rides bitless, treeless and shoeless in the rocky terrain of Northern California, where she is proudly owned by a Morgan, a Tennessee Walker, a Welsh pony, a Paint and a miniature donkey.
60 | June 2013 •
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