The frog is, in my opinion, the unsung hero of the foot. The underdog. Often misunderstood, often mistreated and ignored. So what's the big deal?
The frog plays an incredible part in the support and function of a healthy hoof. It is the triangle-y, rubbery sometimes stinky apparatus at the back and middle of the bottom of the hoof. It was named such hundreds of years ago for bearing at least a tactile resemblance to the amphibian animal that lives in a pond and croaks. The amphibian also has a somewhat triangle shape when viewed from above.
The equine frog has some identifiable parts. The frog apex, is the very tip towards the front of the hoof, that points toward the toe. Along the sides of the frog, the grooves where dirt builds up, are the collateral sulci, or collateral groves. In the center, sometimes deep and sometimes shallow, is the central sulcus. The back of the frog, at it's widest point, is called the frog buttress.
The frog serves a critical purpose in the function of a healthy hoof. First, if viewed internally, the structure on the outside holds a mirror image on the inside of the hoof capsule. The underside (or top side, depending on your view) of the central sulcus forms the "frog spine" also called "frog stay" as seen in the following images:
(photos courtesy The Illustrated Horse's Foot by Christopher C. Pollitt, and Distal Limb Pocket Guide by Jenny Edwards & Paige Poss, respectfully)
Contact between the frog and the ground when the limb loads at foot fall, raises the frog spine and lifts the digital cushion at the base of the hoof, which in turn, both absorbs shock and transfers energy up into the bone column, expanding the collateral cartilages, and creating a vacuum effect, bringing the blood present in the foot back up into the limb and to the heart. Since there are no muscles in the hoof or lower limb, this is how circulation gets done. The greater part of the vascular system of the hoof is around the digital cushion.
Second, why do we care about a heel first landing? Well, the frog contains numerous proprioceptors, which are sensory receptors for the nerves, as our own heels do as well. When we walk we land heel first, our heel "seeks" the ground, and then immediately lets you know the footing you are walking on. If the footing is sharp and you are in bare feet, you immediately change your gait. Your next step will be more careful! It is the same with the horse. The frog lets the body and mind know what surfaces they are traveling on.
Finally, a healthy frog is part of the whole load sharing system of the hoof, participating in bearing the weight of the horse, in contact with the ground. The load is shared by the hoof wall, sole, bars and heels. Yes, application of a shoe can increase the load bearing on the hoof wall, but the frog will still contact the ground when traveling on softer surfaces rather than pavement, as the hoof sinks in. If we need to encourage the frog to load, to stimulate frog growth and thus better health, a packing material and a flat pad or a frog support pad can aid this process.
There are also a few ways that the integrity of the frog can be challenged or damaged.
THRUSH!! Thrush is a bacterial infection that loves to eat frog tissue. The bacteria are anaerobic, meaning they thrive without oxygen. They love a damp, moist environment, and are encouraged by manure and urine and mud. Thrush can eat at the entire frog, but especially a frog that is under stress from contracted heels, as well as being stretched out forward and thin by long toes and long under run heels. This can also be referred to as LOF "Lack Of Farrier" disease.
Thrush can be chronic and mild, but if left unchecked it can eat right through the frog and break into the internal hoof capsule, as shown in the following photo:
This can cause most of all, lameness, but also abscesses, as well as poor hoof circulation. The horse will refrain from bearing weight on his heels, and carry more weight on the toes. This shift in weight from back to front can also put undue leverage on the laminae of the hoof wall, which leads to laminitis, inflammation of the laminae.
Thankfully, thrush is fairly easy to treat for the average horse owner or barn manager. There are many great thrush treatment products available for purchase. If you have questions, ask me! However of primary importance is pasture management and regular cleaning out of the hooves with a hoof pick. Make sure your horse has someplace dry to be at least most of the time. You can't stop the rain, but you can be sure to give him a clean dry stall or run in. Long term, proper drainage of pastures could be considered.
Treatment of severe thrush as pictured, can be done by "flossing". Simply soak some bandage material or Vetwrap or cut a clean rag into a strip, soak it in betadyne, push one end into the crevice and pull through the bottom edge. Repeat back and forth a few times to get the debris and gunk out and give the whole area a washing with the medication.
Diligent care and management will eliminate thrush. Proper trimming and movement will stimulate new, healthy frog growth. Better frog mechanics will ensure a healthy hoof and a happy horse! And that's a big deal!