Thrush is a mixed bacterial infection in the grooves along the side of the frog, in the central cleft of the frog and/or in the crease between the heel bulbs. It usually involves the organism Spherophorus necrophorus (aka Fusobacterium necrophorus), which requires a very low oxygen environment to grow. Although this organism is capable of invading skin, it usually needs other bacteria to do so effectively. Exactly how this works is incompletely understood, but one theory is that the other bacteria will keep the oxygen level low enough for S. necrophorus to survive.
Contrary to popular belief, thrush does not involve infection with a fungus.
Manure can serve as a source of bacteria, including S. necrophorus, and manure packed into the bottom of the foot also creates a low oxygen environment. However, the horse does not have to be standing around in manure to get thrush. Muddy spring 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. Pain or lameness elsewhere in the leg often results in thrush because the horse does not bear full weight and therefore does not force material out of the bottom of the foot. Finally, injury to the tissues from embedded stones or other materials will break through the normal barriers and allow bacteria easy access.
The trim matters significantly. Medial lateral imbalance in the heels causes shearing forces that can damage tissue and allow thrush to start in the heel cleft.
Before any product can help, it must come in contact with the organisms. Step one in treatment is to thoroughly clean out the black material. Pick out the foot, then wash using a stiff brush until thoroughly clean. Trim back any overhanging flaps of frog.
Once the foot has been cleaned and thoroughly dried, apply your treatment. With liquids, packing the sides of the frog with cotton soaked in the product and pushed firmly into place with a hoof pick will give prolonged treatment. With all treatments, keeping the environment dry and the feet clean is essential to success.