Effect of prolonged photoperiod on ocular tissues of domestic turkeys

Objective: The objective of this study is to investigate the structural and functional ocular changes that develop in turkeys exposed to a photoperiod of 23 h of light (23L) compared with a photoperiod of 14 h of light (14L).

Procedures: Ten-day-old Nicholas heavy strain poults were exposed to either a 14L or 23L photoperiod. Between 16 and 18 weeks of age, equal numbers of turkeys per treatment group underwent ophthalmic examination (biomicroscopy, indirect ophthalmoscopy) (n = 14), refractometry (n = 20), keratometry (n = 20), tonometry (n = 20), and full-field electroretinography (ERG) (n = 14). Postmortem analyses included orbital magnetic resonance imaging (MRI) (n = 10) and light microscopy (n = 24) at 18 weeks of age.

Results: Autorefraction revealed a median of -0.13 for sphere in both groups (P = 0.69), which is approximately emmetropia. The radius of curvature of the cornea was significantly higher (P = 0.0001) and the refractive power of the cornea was significantly lower (P = 0.0001) in the 23L group. The astigmatic power was significantly greater in the 23L group (P = 0.0001). Mean intraocular pressure did not differ between groups (P = 0.085). Turkeys from the 23L group had significantly larger globes in nasotemporal (P = 0.0007), dorsoventral (P = 0.015), and anterioposterior (P = 0.021) directions, and anterior chambers were more shallow (P = 0.0002). ERGs revealed the 23L group to have lower a- and b-wave amplitudes and significantly lower cone flicker amplitudes (P = 0.0008). Light microscopic examination revealed 23L turkeys to have significantly decreased numbers of nuclei in the outer nuclear layer (P = 0.0001) and inner nuclear layer (P = 0.0186), and decreased choroidal thickness (P = 0.0008). The prevalence of cataract in the 23L group was significantly higher (P = 0.001).

Conclusions: Exposing turkeys to a prolonged photoperiod induces significant ocular disease.