Thrush in horses is a fairly common problem especially at the time of year when horses stand around in wet muddy areas.  Although standing in water or mud does not cause the disease, I think it decreases the integrity of the sole and frog to fight off infection and enable infection to enter the tissues easier than it would if the horse was on a hot dry summer pasture.  Dr. Steve O’Grady says thrush is a degenerative condition of the frog caused by a bacterial infection, which leads to deterioration of the frog and a black necrotic type of discharge with a foul odor.  If the infection is severe, it can infect the sensitive tissues of the foot causing pain and lameness. 

Almost all cases of thrush are caused by incorrect trimming of the frog and sole that allow deep sulci (the clefts on either side of and in the middle of the frog) to develop. Some will have one deep central sulcus of the frog extending to the hairline at the heel.  Although there are many products available to treat thrush, none of these products will be effective until the foot is trimmed correctly.  If the frog is recessed and the hoof wall is too long, allowing a deep sulcus to develop, no treatment is going to be effective without appropriate foot trimming.  The frog should be in contact with the ground when the horse bears weight on the foot.  Horses that are not trimmed often enough in whom the hoof wall has grown too long, or those with shoes that raise the frog above the ground, can develop contracted heels; this allows thrush to develop. 

So, if your horse develops thrush, make sure the horse is trimmed correctly and do not expect any treatment to be effective until the foot is correctly trimmed.  In general, the hoof wall at the heel should be on the same plane as the frog for the foot to function properly.