Experimentally Produced Progressive Retinal Degeneration (Bright Blindness) in Sheep

SUMMARY Twenty cases of progressive retinal degeneration (P.R.D.), including 15 definite clinical cases of bright blindness, were produced in a group of 22 Dorset Horn ewes fed a concentrate ration containing 50 per cent dried bracken at approximately 1 kg/day for up to 63 weeks. No sheep were affected in a group of 22 control ewes fed a similar ration with dried grass replacing the bracken. Retinal lactate dehydrogenase isoenzyme patterns for the bracken-fed ewes differed markedly from the control ewes. No significantly greater changes were seen in severely affected compared with less severely affected ewes, suggesting that the enzyme changes preceded the morphological ones. Blood platelet and leucocyte counts in the bracken-fed group were significantly lower than those in the controls, particularly towards the end of the experiment. The factor in or on bracken responsible for progressive retinal degeneration in sheep has not yet been identified.