The thrush is well known disease of the hoof demonstrating itself in a decay of the frog. Commonly occurs in horses that spend a great deal of time in the stables and it afflicts one or more feet.
Thrush begins mostly in the center groove (Fig. 2/a) of the frog in a form of smeary dough like mass carrying a foul odor. In the advance stage of this disease the entire frog is soft and apparently damaged (Fig. 1). Horses for most part do not go lame.
The cause of this disease is not known. According to some authors, the pathological process begins in the new hoof growth near the laminae. Henceforth, the primal cause is believed to be of internal origin and not of the external elements
Most authors disagree with the above theory and are suggesting that the cause is in the external elements and the environment in which the horse lives. They believe that the process of the disease begins in the periphery of the frog and spreads inward. According to these folks, thrush comes mainly from unclean, unhygienic stable environment in which the horse stands (mainly too much moisture to the frog), from an infection (Bac. putrificus verrucosus) and/or from a too long hoof growth
It is however obvious that without a certain internal influence, these above mentioned environmental impurities would hardly cause the disease by themselves.
In my 40 years experience with several hundred cases of this disease, I would have to favor the first theory. I believe that this disease is of internal origin associated with (insufficient, unnatural or uneven) impact (pressure) to the hoof, it’s frog (Fig.2/C, C1, a, D), sole (Fig.2/A, A1, A2). The unhealthy foot will often contract this disease and the unhygienic (usually, but not necessary wet) environment then contributes to the growth and spread of the thrush. From the second theory I will accept that the abnormal and too long hoof growth will contribute to the development of this diseases because of it’s influence on the inner health of the foot.
I have often noticed that a change in shoeing/trimming of the foot often got rid of the diseases without any medication, change of environment or a change in the horses routine. This is not to insinuate in anyway, that your farrier is to be blamed for the thrush! He is blamed for enough already.