Pituitary pars intermedia dysfunction (PPID) affects approximately 21% of the aged equine
population and is characterised by excessive hair growth, delayed shedding, and metabolic disturbances. While pergolide is the standard therapy, some horses continue to exhibit coat abnormalities and lethargy. Seasonal regulation of hair growth is photoperiod-dependent, and previous research has demonstrated that exposure to blue light can influence coat shedding.
A study by Sinead Parmantier and colleagues in Ireland and Kentucky examined whether extending day length using blue light masks could enhance coat condition and reduce PPID-associated signs when used alongside medical management. A full report of the study is published in Animals.
Fifty-two horses (>15 years) with a veterinary diagnosis of PPID and visible hypertrichosis were recruited via an online owner survey.
The horses were assigned to treatment and control groups. The treatment group (n = 29) wore Equilume® blue light masks from mid-December, extending their daily photoperiod to 15 hours. The control group (n = 23) experienced the natural seasonal light–dark cycle. Because most of these horses (44 out of 52) were receiving daily pergolide medication for PPID, a second unmedicated control group (n = 17) comprising research horses in the USA was included. These horses were maintained under natural photoperiod conditions for the 13-month study period.
Hair length, shedding, and body condition were measured monthly. Owners of groups T and C1 completed bimonthly questionnaires on coat condition, fat coverage, energy, and other PPID symptoms. Data were analysed using linear mixed models and non-parametric tests to assess effects of time, group, and group × time interactions.
There was no difference in hair lengths between the blue-light-treated and the first control group, but the second control group had shorter hairs. Shedding started a month earlier in the spring for the blue light group. Owners’ questionnaires revealed better coat condition in April, reduced fat coverage in April and June, and more energy in February for the blue light group.
In a final assessment, treatment-group horses showed significant improvements in coat condition, PPID clinical signs, and perceived quality of life.
Although blue light treatment did not significantly alter overall hair length compared to medicated controls, it advanced shedding and improved clinical signs of PPID.
These results align with previous findings that artificial photoperiod extension can influence hair follicle activity. Owner-reported outcomes indicate improved welfare, though the potential for observer bias warrants caution. The consistent positive trend suggests that blue light may act synergistically with pergolide, helping to mitigate coat-related symptoms and enhance comfort in PPID horses.
Extended photoperiod achieved via blue light masks advanced spring shedding and improved coat condition, energy, and perceived wellbeing in PPID horses.
Blue light therapy shows promise as a complementary, non-invasive management strategy alongside conventional medication. Further controlled research is warranted to clarify the physiological mechanisms and optimise treatment protocols.
For more details, see:
Parmantier, Sinead, Panoraia Kyriazopoulou, Margaret McClendon, Amanda Adams, and Barbara A. Murphy.
Influence of Extended Photoperiod Using Blue Light Masks on Hypertrichosis, Coat Condition and General Health Parameters in Horses with Pituitary Pars Intermedia Dysfunction"
Animals (2025) 15, no. 19: 2905.





