Hi ,I have 4 miniatures and the first thing I would suggest is to worm with an all round wormer (equest plus ,equimax ),secondly ,mine have suregrow (D&H) which is brill for the youngsters ,provides everything they need ,without giving too much which can so easily bring on laminitus in the little ones.
They are in all round good condition ,plenty of energy (too much in a couple ) ,and have qualified for Miniature Horse of the Year show .
I would'nt discount the suggestion it could be a virus ,but it sounds very much like a little one that was on my yard ,and that became as bright as a button after a good worming-worth a go