Visual-evoked potentials and electroretinograms in ruminants with thiamine-responsive polioencephalomalacia or suspected listeriosis

SUMMARY Electrodiagnostic visual testing (electroretinogram [ERG] and visual-evoked potential [YEP]) was performed on 5 ruminants (3 lambs, 1 kid, and 1 steer) with thiamine­ responsive polioencephalomalacia.(PEM) and. on 2 ?h~ep with listeriosis. The lambs and kId had tYPIcal clImcal signs of PEM, especially blindness. In these animals, the ERG was normal but the YEP was abnormal. Follow-up recordings in the kid and 1 lamb indicated an improve­ ment in VEP recordings accompanying a gradual return of vision after thiamine treatment. Possible subtle changes in YEP peak latencies could not be assessed because ~f lack of normative YEP data for sheep and goats. All am­ mals had complete return of vision (owner-assessed). The steer did not have signs of blindness, and the ERG and YEP were normal. Changes in YEP accompanying per­ manent PEM blindness are not known. One sheep with suspected listeriosis had lack of menace response and palpebral and corneal reflexes, but had in­ tact vision. The ERG and YEP were normal. The second sheep with suspected listeriosis had in.tact men.ace re­ sponse and vision, but became acutely blmd and dIed; the ERG was normal, but YEP amplitudes were depressed.