Influence of Extended Photoperiod Using Blue Light Masks on Hypertrichosis, Coat Condition and General Health Parameters in Horses with Pituitary Pars Intermedia Dysfunction †

Fifty-two horses aged >15 years, diagnosed with pituitary pars intermedia dysfunction (PPID), and displaying hypertrichosis were recruited via an online survey of PPID horse owners. From mid-December, group T (n = 29) wore Equilume^® light masks extending photoperiod to 15 h daily, while group C1 (n = 23) remained under natural photoperiod. As 85% (44/52) of recruited study horses received pergolide medication, a second unmedicated PPID research herd (C2; n = 17) was recruited and remained under natural photoperiod. Hair coat samples, shedding and body condition scores were collected monthly by owners for 13 months and analysed by the research team. Data related to management, coat condition and PPID clinical signs were collected using bimonthly questionnaires (BMQ). Time (p < 0.001), group (p = 0.025) and time-by-group interaction (p = 0.005) affected hair length. Group differences were attributable to shorter hair lengths in C2, and no differences in hair length occurred between T and C1 (p > 0.05). Time affected shedding scores (p < 0.001) which was advanced by one month in T (p < 0.05). In group T, BMQ responses showed improved coat condition in April (p = 0.035), decreased fat coverage in April and June (p < 0.05), and increased energy/alertness in February (p = 0.022). Improvements in coat condition (p = 0.043), clinical signs of PPID (p = 0.018), and general quality of life (p = 0.035) were reported in T only in a final study questionnaire. Blue light treatment merits further investigation as a complementary treatment for PPID.