The frog is one of the simplest yet most effective structures in the horse’s body. This rubbery, triangular wedge can be considered a marvel of technical engineering, with the capacity to perform a multitude of functions.
Its roles range from shock absorber and support to a hinge that holds the heels together. The frog also provides grip, acts as a vent for the sole and creates some of the hydraulic pressure needed for blood circulation. It was thought the frog was the “pump” that drives deoxygenated blood back towards the heart. This is partly true — it does play a part in this “venous return system”, but is a facilitator of the pumping action rather than the pump itself.
The frog’s versatility is down to its elastic structure and location, sitting snugly between the heels. As each limb bears weight, the pastern drops. The digital cushion, a fatty, fibrous mass situated inside the hoof capsule, is then squeezed by the descending bodyweight and expands to press sideways but also downwards. This pushes the frog onto the ground, giving the foot traction.
This downwards pressure is met by an upwards force, called the ground reaction force. As the two collide, energy is dissipated sideways, leading to full expansion and contraction of the heels. The natural angle of the sole — ideally concave — begins to distort under this descending weight and becomes much shallower in its natural arch. This is facilitated by the softer and more flexible rear half of the foot, primarily the frog, which is further driven downwards into contact with the ground to give extra “anchor”.
A healthy frog is key to promoting the horse’s ability to land and load through each digit. Just as roof guttering channels rainwater away, the frog vents the underside of the hoof.
Three grooves in its surface, the “sulcuses”, help dispel water or mud from the gap between the sole and ground, or cause dry, sandy material to pack into the hollows. The frog and hoof rim can then lock onto the surface and won’t slip as the horse takes his opposite foot off the ground.
Weighty issues
Breed and conformation have a huge influence on frog size and shape. Warmbloods typically have an upright shoulder and pastern angle, compared with the thoroughbred, therefore a taller hoof capsule and less width between the heels. The warmblood frog is usually narrower and longer in proportion, with deeper sulcuses than those seen in the more open-heeled breeds.
An older horse lacking muscle tone, especially behind, may develop a shallower pastern angle and consequently a frog flatter in angle and shape. Obesity can also affect the feet. The digital cushion, similar in composition to human breast tissue, is one of the body’s main shock absorbers. If continuously overloaded, it can become deflated and lose elasticity, which is unlikely to be regained.
The frog is more rigid than the digital cushion in structure and grows continuously, so it can be altered in shape and “reinflated”. Once the internal structures of the hoof have taken a bashing from excess bodyweight, however, the functionality of the foot is permanently affected. Packing the hoof with gel or dental impression material can help, but will not return it to brand-new status.
Where there’s muck
The frog has the highest moisture content of all the horny structures in the foot, at up to 50%. Its permeability allows moisture to come and go through changing environments and seasons: it will absorb water if the hooves stand in it and can become parched and cracked in dry conditions.
Because the frog is flexible, moist and — thanks to a healthy blood supply — always warm, any muck in the sulcuses will provide ideal conditions for infection to take hold. And the use of rubber matting and increased stabling of horses can present the perfect storm for thrush bacteria. Horses that spend most of their time indoors or standing in mud or faeces will often have some type of thrush present in the frog clefts.
Long-term thrush will contribute to lameness issues and can reach the point where the frog is eaten away and drops off, like a damaged fingernail. The key to treatment is appropriate trimming of the frog, according to foot shape and conformation, so that the clefts are left clear and well-vented and any necrotic (dead) material is removed.
Once your farrier has prepared the foot, scrub the sole with a solution of warm water and Hibiscrub, or very salty water, and leave the foot to dry. A topical treatment, as recommended by your vet, can then be applied, followed 15 minutes later by a coating of quality hoof oil which will act as a barrier to protect the soft and permeable horn.
Repeat the process daily until improvement is seen, then gradually reduce it to a regular treatment to keep infection at bay. A “maintenance approach” of an anti-bacterial treatment and a light hoof oil dressing, once or twice a week, is the best way to keep frogs healthy and infection-free. Persistent infection may call for a tougher course of treatment.
Canker is a debilitating hoof disease that can occur even in a well-managed environment. The infected frog excretes a foul-smelling substance and develops cauliflower-like lesions, requiring an aggressive veterinary treatment plan aimed at debriding the area back to healthy tissue.
Clean, dry conditions underfoot will go a long way towards preventing frog problems. Straw creates a comfortable and cost-effective bed but will not absorb urine as well as shavings, so a two-tier approach may be needed for the horse who turns his stable into a swamp.
Tip two or three buckets of fine shavings over areas that are typically wet, before laying straw over the top. This should keep the horse from standing directly in the saturated areas.
Ref Horse & Hound; 27 June 2019
