blindness in chickens

Western Meeting of Poultry

Clinicians and Pathologists

Blindness in Chickens

May VanderKop

Alberta Agriculture

Eight roasters, 5 weeks of age were submitted to Lethbridge Animal Health Laboratory with a history that 80% of the birds on one side of the barn were poorly grown, still had down present and were acting as if they were partially to completely blind.

The birds were in poor condition and small. They moved normally, but four of the birds showed minimal papillary light response to bright light. The eyes appeared somewhat enlarged, as if the lids were spread apart and would have difficulty in closing completely. There were no further abnormalities noted on gross examination. Eyes were collected in Bouin’s fixative and remaining body tissues including brain collected in 10% neutral buffered formalin. Eyes of the birds with some pupil light response were collected as controls.

Histological lesions were confined to the eyes. The lens, cornea, sclera and iris were normal. There was separation of the retina due to fixation artifact. The retina showed severe vacuolation and degeneration. This was most severe in the inner nuclear layer and pyknosis was prominent. Differential diagnosis included light toxicity or some other type of toxin. Dr. Bruce Grahn, WCVM, was consulted and after examining the slides, he detected thinning in the photoreceptor layer with loss of rods and cones. There was also thinning the outer plexiform and outer nuclear layers, with variable degrees of pyknosis. He detected asymmetry in the degeneration with one side of the eye more severely affected, but since we had not oriented the eyes consistently on the slides, we could not determine if this was significant.

Light toxicity is described in association with continuous exposure to artificial light, which results in enlargement of the eyeball, retinal thinning, detachment and secondary degenerative changes including cataracts, fibroplasias and metaplasia. Susceptibility is reported to increase with temperature, and high environmental temperatures may have been a factor with these birds. The most notable lesion is reported to be degeneration in the rods and cones and their nuclei, but lesions are not specific and cannot be differentiated from nutritional or toxic retinopathy. In this case we were unable to detect anything other than 24-hour light, which might have been the cause.

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Last modified: 5/28/2009