Analysis of the variations in clinical signs shown by 254 cases of equine headshaking.

A national survey of headshaking in 254 horses was undertaken to describe the clinical signs of the condition as observed by horse owners. Principal component analysis was used to determine the underlying structure of 11 signs and the criteria by which the affected horses could be most effectively differentiated; the analysis suggested five components with a variance greater than one which together explained over 60 per cent of the total variance. Other analyses of the data indicated that headshaking could develop at any age and that twice as many males were affected as females; 64 per cent of the horses shook their heads seasonally and geldings were more likely than mares to be seasonally affected. Seasonal headshaking tended to be significantly worse on sunny days but improved on rainy days, windy days, at night and indoors.