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SUMMARYImpaired 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.

Thirty-four rat carcases from 11 poultry farms were examined for the presence of Pasteurella multocida; 14 out of 34 (41 per cent) proved positive after mouse inoculation, compared with five out of 34 (14 per cent) using media alone. No salmonellae were recovered from 27 rat carcases using enrichment media. Poultry pasteurellosis was present on two farms with infected rats and the same serotype was present in rats and poultry in those cases.

To investigate to what extent healthy chickens and turkeys may be carriers of Pasteurella multocida, oral swabs were examined from birds in flocks with or without a history of avian pasteurellosis. In all 841 specimens were examined by cultural methods but 142 were also tested by mouse inoculation. P multocida was not found in specimens from normal healthy flocks but was recovered from some live chickens in known infected flocks and from dead turkeys on an infected farm. The implications of these findings are briefly discussed.

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