I've come across a pair of young mares with enlarged head, at first I thought these horses had a misdirected jaw tooth. Both mares seem to have good weight on, but appears to be stunted in growth. It was the first time I have seen something like this and finally went on facebook last night and described the condition of the mares and asked if any of the members if they had any knowledge what it was. Someone said it was Nutritional Secondary Hyperparathyroidism It happens when a horse does not get enough dietary calcium. When there is a dietary calcium deficiency calcium is mobilized from the bone to keep blood calcium levels normal. Has anyone had any experience with this type of conditions? And what was the out come? From what I have read about it, mares are mostly affected, if developed at an older age it's reversable with treatment, if it developed while the mares were juvinile than it maybe 50/50 chance their bones will recover. Attatched picture is of a horse I found on the internet with the same features of both mares. But their faces were morphed. Owner has said they both have had it for a year.
This is a condition referred to as "equine big head" (unfortunate huh?). Your description is accurate and the only details that I would add are that the bone is replaced by fibrous tissue and, as you might imagine, it is irreversible, and it can also be due to too much phosphorus in the diet as opposed to too little calcium. Big head can also be caused by chronic renal (kidney) failure but since both mares in the same pasture are showing signs it is more likely a nutritional deficiency unless the CRF is due to a toxin of some sort (much less likely). Secondary hyperparathyroidism is likely to cause other lesions in bones, big heads are just the most visually obvious one.