PHOTIC HEADSHAKING IN THE HORSE - 7 CASES

Seven horses with headshaking are described, No physical abnormalities were detected in any of the cases, Six of these horses had onset of c linical signs in the spring. The role of light was assessed by applica tion of a blindfold or dark grey lens to the eyes, covering the eyes w ith a face mask and observing the horse in total darkness outdoors, Ce ssation of headshaking was observed with blindfolding (5/5 horses), ni ght darkness outdoors (4/4 horses) and use of grey lenses (2/3 horses) . Outdoor behaviour suggested efforts to avoid light in 4/4 cases, The photic sneeze in man is suggested as a putative mechanism for equine headshaking. Five of 7 horses had improvement with cyproheptadine trea tment (0.3 mg/kg bwt b.i.d.). Headshaking developed within 2 calendar weeks of the same date for 3 consecutive years in one horse, Neurophar macological alterations associated with photoperiod mechanisms leading to optic trigeminal summation are suggested as possible reasons for s pring onset of headshaking.