If you google thrush, you’ll find that most people say it’s a bacterial infection of the frog. Some will say it can also be fungal. A few might say it’s mainly fungal. So it’s hard to sort out the truth of what causes thrush and what cures it. But whatever bug it is that causes thrush, we can all agree that thrush is an infection of the frog, usually the central sulcus, collateral grooves (also sometimes called the collateral sulci), and the frog itself. White Line disease is also an infection that compromises the health and stability of the hoof.
My preferred prevention of and treatment for thrush are the clay based products like Pure Sole Hoof Mud and Artimud Hoof Clay which use antimicrobial ingredients that work on both bacteria and fungus.
For really deep cracks of the central sulcus, sometimes going all the way up the heel bulbs to the hair line, I like to use a syringe to get the treatment material all the way to the bottom of the crack. You can also use your hoof pick to wedge the stuff way down in there, but the syringe is more effective. Problem is, with a syringe, you can’t use the mud based products (although Pure Sole Hoof Mud is soft enough is kept warm and the syringe tip isn’t too narrow).
Whatever product you use to treat thrush, be sure that it isn’t caustic and won’t harm living tissue.
In any case, as the saying goes, one ounce of prevention is worth a pound of cure. Things that you can do to prevent serious thrush are cleanliness (keep the footing clean and as dry as possible and pick out feet regularly), give them plenty of movement, which stimulates the hoof, balance minerals and provide an appropriate diet, keep a regular trim cycle and trims that allow for frog pressure and release, and treat with thrush products once a week or so (more often in wet conditions).
Lastly, don’t panic if your horse has thrush: at some point thrush is just a matter of fact for horses, no matter what the environment. It’s when it gets out of control that it becomes a big problem. If left unchecked, thrush can eat its way up through the frog into the sensitive tissues of the foot, and, not only is that painful for your horse, it can lead to all sorts of problems for your horse.
Please have a look at the following articles (and video) for a more in-depth look at what thrush is and how to prevent and treat it.
Equine Thrush: What It Is and How to Deal with It, Walt Friedrich
Equine Thrush, The Study of the Equine Hoof by Lindsey Field (video)
Hoof Infections and Their Management, Red Horse
Identifying and Treating Thrush, The Horse.com
Mistaken Identity, Steve Hebrock (Enlightened Equine)
Preventing and Treating Thrush in Horses, Michigan State University
THRUSH!!!, Healthy Hoof.com
Thrush Treatment, Pete Ramey