older horse with long hair
Exposing horses with pars pituitary intermedia dysfunction (PPID, formerly known as equine Cushing’s disease) to a few hours of blue light every day for part of the year could make them grow lighter, shorter winter coats, reported researchers on a new study.

While blue light therapy does not affect adrenocorticotropic hormone (ACTH) levels, it might lessen hair growth—probably by mimicking the longer days of summer, said Barbara Murphy, PhD, assistant professor in University College Dublin’s School of Agriculture and Food Science, in Ireland.

The findings do not offer a cure for the disease itself but offer a way to improve welfare by making PPID-positive horses more comfortable, said Murphy. “If there’s any way we can mitigate one of the major symptoms [of PPID], let’s do it!” she said.

Horses with PPID have greater-than-normal increases of ACTH in autumn, and their winter coats often grow in heavier and thicker than other horses’, Murphy explained. This excessive hair growth, known as hypertrichosis, can make horses uncomfortably warm and put them at risk for skin infections, especially as the days begin to get warmer the following spring and the hair does not shed normally. While the drug pergolide and body shearing can help with hypertrichosis, many horses continue to grow thick coats that might affect their ability to thermoregulate.

Researchers have suggested that blue light—one of the wavelengths found in natural sunlight—influences the horse’s natural circadian rhythms. This affects seasonal hormone regulations that influence hair growth, Murphy said. Aiming to help owners better control the amount of “daylight” horses get, Murphy developed a light mask for horses—which projects a small ray of blue wavelength light onto one eye. Aside from using the masks to manipulate mares’ estrous cycles, people have been using the masks on horses starting in mid-spring to help sport horses maintain a summer coat year-round, she said.

“What blue light does is regulate melatonin,” Murphy said. “And melatonin is a hormone which basically governs the seasonal activity and a bunch of other hormones for horses—one of them being prolactin—which is responsible for hair shedding. And my work has shown previously that we can shed out horses at different times of the year.”

Blue-Light-Masking Senior PPID Study Horses

Amanda Adams, PhD, an associate professor and specialist in equine immunology at the University of Kentucky, in Lexington, read about Murphy’s sport-horse-based work and wondered if blue light might also help PPID-affected horses. Along with her student, Ashton Miller, PhD, Adams teamed up with Murphy to study 18 PPID-positive senior horses of various breeds, all living at the same research center near Lexington. None were being treated with pergolide at the time of the study.

The team equipped eight of the horses with blue light masks from mid-July to late-October, and the other horses had no exposure to artificial lighting. The masks provided horses with a total of 14.5 hours per day of blue light, including the natural sunlight hours. Those without masks had only about 10.5 hours of blue light via sunlight per day, Murphy said.

Average ACTH levels did not vary between the two groups of horses at the start of the study or at several time points in mid-October, Murphy said. There were also no differences in average body weight, body temperature, heart rate, or respiratory rate between groups.

However, masked horses’ individual hairs—sampled from behind the hip on seven separate days throughout the experiment—weighed less than those of control horses, Murphy said. And although their coats were lighter, the horses still developed warm enough winter coats to not require blanketing the following winter, she added.

“I was really excited by those results,” Murphy said. “We did see a lessening of the thickness of and the length of the hairs that grew. The hairs were finer, and they didn’t grow as heavy a coat by the end of the study. We care so much for these animals, and we really want to see their welfare improved with this condition.”

Bigger, Longer, International Study Now Underway

The findings confirm some of the stories Murphy heard from owners who had already tried the masks on their PPID horses. Those anecdotal tales of success combined with the encouraging results in this preliminary study motivated Murphy to take a deeper look on a larger scale.

In December 2021 Murphy and her research colleagues initiated a yearlong study of 60 client-owned PPID horses in Europe and North America wearing the masks for a full year. “We’re getting into the exciting time now when horses who are wearing the blue lights—if it’s working for hair shedding—should start having a lighter coat than they normally have [due to PPID].”

The study, Impact of blue light therapy on plasma adrenocorticotropic hormone (ACTH) and hypertrichosis in horses with pituitary pars intermedia dysfunction, Domestic Animal Endocrinology, appeared in Domestic Animal Endocrinology in January 2022.