Laminitis occurs when the  laminae or tiny interlocking areas that glue together your horse's hoof wall to their hoof become inflammed or separate.  

When the laminae separate, your horse's coffin or pedal bone can rotate or sink your horse's bones and the sensitive foot structures inside their hoof capsule. Laminitis can be mild, moderate or severe. It is very painful for your horse.

(This information was adapted from an article by UQ lecturer and Equine veterinary specialist Dr. Allison.J.Stewart BVSc(hons), MS, DACVIM, DACVECC for Hoofbeats magazine. Read the article)