Thrush is a mixed bacterial infection in the grooves along the side of the frog and in the central cleft of the frog. Thrush looking bacteria in the crease between the heel bulbs is often not Thrush rather Inerbulbular Dermititis (see article).
It usually involves organisms which requires a very low oxygen environment to grow.
Thrush is an aerobic necrotic process.
Think of Thrush as a loss of tissue as opposed to Canker which is a growth of tissue.
Manure can serve as a source of bacteria and manure packed into the bottom of the foot also creates a low oxygen environment. Muddy conditions also favor thrush because the organisms can survive in moist soil. Mud packed into feet also seals them off from the oxygen in air.
Improperly trimmed frogs grow flaps which cover the grooves, lowering oxygen and trapping moisture.
Feet and heels that are too long, with the frog out of ground contact also pre-dispose to thrush because the grooves become very deep and the hoof does not self-clean efficiently.
Treatment
Debriding and exposure to more oxygen along with a good antibacterial are key to treatment.
Possible environment change may be needed.
Correctly trim the frog and hoof. If owner compliance an issue or more severe, perhaps copper alloy (Kawell USA) horseshoes may be recommended.
