The Horse:Your Guide To Equine Health Care provides monthly equine health care information to horse owners, breeders, veterinarians, barn/farm managers, trainer/riding instructors, and others involved in the hands-on care of the horse.
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27 October 2015 THE HORSE TheHorse.com influencing vitamin D synthesis remain poorly understood," said Azarpeykan. She explained that in humans, factors such as hair and skin pigmentation, sea- son, latitude and altitude, type of clothing, and sunlight exposure can influence the skin's ability to synthesize vitamin D3. Because many horses in New Zealand spend much of their time turned out blan- keted, the researchers looked at whether these horses had lower blood serum concentrations (which can indicate a lack of vitamin D) than unblanketed horses in similar living situations. The team studied 21 mature horses— five of which wore blankets with neck covers—living on pasture and consum- ing grass and hay. They collected and evaluated blood and pasture samples monthly for 13 months. Specifically, they looked for levels of 25- hydroxyvitamin D2 and D3 (25OHD2 and 25OHD3, respectively) and 1,25- dihydroxyvitamin D (1,25[OH]2D)—all forms of vitamin D—in the serum. They also tested pasture samples. The team determined that: ■ 25OHD2 was most abundant; ■ No 25OHD3 was detected; ■ There was no difference in serum 25OHD2 and 1,25(OH)D2 between blanketed and unblanketed horses; ■ The pasture had high vitamin D concen- trations that exceeded horses' minimum daily recommended intake; and ■ 25OHD2 and pasture vitamin D concen- trations were "directly correlated" with the amount of sunshine each month. These results suggest equine skin doesn't actually appear to synthesize vitamin D3 (and, thus, blankets have no effect on it), so Azarpeykan stressed the importance of horses' diets containing adequate vitamin D. Hair Cortisol Levels in PPID Horses Researchers and veterinarians are still on a noble quest for the Holy Grail of equine Cushing's disease (PPID or pituitary pars intermedia dysfunction) management: a reliable diagnostic test that can detect early stage disease. Canadian researchers recently examined an innovative method for this by testing hair cortisol levels. Heidi Banse, DVM, PhD, Dipl. ACVIM, an assistant professor at the University of Calgary Faculty of Veterinary Medicine, in Alberta, shared her team's findings. "Pituitary pars intermedia dysfunction is the most common endocrine disorder of aged horses, affecting 15 to 30% of horses 15 to 20 years of age," Banse said. Caused by an enlargement of the pitu- itary gland's pars intermedia (the central part of the gland at the base of the brain; essentially, as it enlarges, clinical signs be- come more severe), PPID affects all horse breeds and types. Clinical signs include hypertrichosis (abnormal hair growth), abnormal sweating patterns, polyuria and polydypsia (excessive urination and drinking, respectively), and muscle wast- ing with or without weight loss. Veterinarians currently have several op- tions for diagnosing PPID. One common test measures resting levels of adrenocor- ticotropin hormone (ACTH) produced and secreted by the pituitary gland; these levels tend to be higher in PPID horses than in healthy ones. Another frequently used option is the thyrotropin-releasing hormone (TRH) stimulation test. Admin- istering TRH causes the gland to release more hormones. In PPID horses this results in increased ACTH concentra- tions. Both tests are useful for detecting disease—especially later-stage Cushing's— but early stage is still difficult to accurate- ly diagnose due to seasonal variability in circulating hormone concentrations. Cortisol is commonly referred to as the "stress hormone," which researchers and veterinarians measure to determine physiological stress. "Hair cortisol has been shown to be higher in people with Cushing's than in healthy control patients and higher in dogs with Cushing's compared to healthy or chronically ill dogs," Banse said. So she and university colleagues, along with Moore Equine and TD Equine Veterinary COURTESY DR. SARA AZARPEYKAN Researchers took a closer look at equine vitamin D metabolism and factors infuencing vitamin D synthesis.