An inherited retinopathy in commercial breeding chickens.

Impaired vision was observed in individual birds from several related flocks of layer breeding chickens. The condition was usually apparent by eight weeks of age. Affected birds were unable to perceive food particles on a flat surface or to negotiate a 50 mm bridge, although apparently able to discern large objects and to respond to bright light. On ophthalmoscopic examination of the fundus the choroidal vasculature was usually more readily visualised than in normal chickens and in addition discrete white streaks were often seen to extend peripherally from the base of the pecten. Cataract formation was frequently present by several months of age. The electroretinograms of two affected birds were of abnormally low amplitude. Light microscopy of the retinas of most birds exhibiting blindness showed a generalised reduction in the number of photoreceptors and in the thickness of the outer nuclear layer, although in others there were no obvious abnormalities. The linear fundus lesions represented abrupt thinnings of the retina in which, to a varying degree, the retinal pigment epithelium and photoreceptor inner and outer segment, outer nuclear and outer plexiform layers were disorganised or absent. A mating of affected individuals produced progeny of which 13/14 were obviously ' blind, indicating an hereditary basis for the condition.