Time-of-year effects on clinical signs of Pituitary Pars Intermedia Dysfunction and evaluation of long day photoperiod delivered via blue light to one eye (Equilume) on hypertrichosis and general parameters in horses/ponies

Pituitary pars intermedia dysfunction (PPID) is an incurable neurodegenerative condition that affects roughly 21% of the aged (>15 years) horse population. Treatment options are currently limited to medication with dopamine agonists, which directly address causative dopamine loss. PPID causes a spectrum of clinical signs which affect quality of life. Hypertrichosis, the persistent growth of a long hair coat, and disrupted seasonal shedding, are frequently observed. Changes in equine hair coat growth are controlled by changes in daylength, communicated via light input through the retina to the suprachiasmatic nucleus, the central pacemaker for biological clocks. The circadian (~24-hour) clock is important for regulating an array of physiological processes that contribute to health. Artificially extended photoperiod using blue light exposure was previously shown to modulate seasonal hair growth and promoted shedding in horses. Blue spectrum light is also a potent entrainment factor for the circadian clock, synchronising internal rhythms to the external environment and co-ordinating rhythmicity of internal clocks. This study evaluated the effects of extended daily photoperiod via blue light administration on hypertrichosis and general health parameters in horses and ponies diagnosed with PPID. Owners of horses and ponies with PPID and hypertrichosis were recruited to a year-long study via social media channels. A university herd of unmedicated PPID horses was also recruited. Beginning in mid-December 2021, blue light masks extended photoperiod continuously to 15 hours per day in the treatment group, while the control group and university herd was managed under natural photoperiod conditions. Participating owners provided monthly hair samples, shedding scores and body condition scores for their horse, and completed bimonthly questionnaires (BMQ) for a period of 13 months and an end of study questionnaire captured feedback related to changes in health parameters relevant to PPID. To analyse the effects of time and group, a linear mixed model was applied to hair lengths, and non-parametric tests (Friedman and Mann-Whitney tests) were applied to the ranked shedding and body condition scores. Fisher’s exact tests were used to analyse individual BMQs and the final questionnaire to compare responses between groups. Results revealed an effect of time (p<0.001) on hair length but no effect of group (p=0.74) or time by group (p=0.24). Shedding was advanced by one month in the treatment group, and scores were higher in the treatment group than in the control group in January, February, November and December (p=0.01, p=0.02, p=0.002, p=0.05 respectively). Body condition scores did not vary with time or group (p>0.05). Analysis of each BMQ showed an effect of treatment on improving coat condition in April (p=0.03), decreasing fat coverage in April (p=0.007) and June (p=0.04), and increasing energy/alertness in February (p=0.02). The final study questionnaire distributed after the end of the study revealed overall improvements in the coat condition (p=0.059), clinical signs of PPID (p=0.02), and general quality of life (p=0.047) in the treatment group. The results demonstrate that a circannual rhythm in hair coat growth is maintained in horses with PPID and that provision of extended photoperiod via blue light masks does not affect seasonal hair length patterns. However, shedding scores and questionnaire responses clearly indicated improvements in coat condition suggesting that aspects of hypertrichosis are remediated by treatment. The reports of reductions in clinical signs of PPID and increases in quality of life by over half of the treatment group participants are promising and provide justification to further evaluate extended photoperiod using blue light as a complementary treatment for PPID.