OP have a look at this site - the section about head shakers.http://www.calmhealthyhorses.com It refers to deficiencies in grazing and the importance of adding salt to the feed.
I have completely cured by TB mare who previously headshook for 3 or 4 summers by feeding salt in the spring. If I stop the salt she is likely to start again, but last year no flicking at all, she was transformed.
I have to ride mine in a fly mask during the summer as the second the midges are out he starts flicking his head.
The fly mask stops its straight away . its anything near his eyes that he hates.
