Thrush is a disease of the foot that usually occurs within the frog (the wedge-shaped structure on the bottom surface of the foot) and its sulci (the grooves next to and in the middle of the frog). The disease is characterized by a very soft frog and sulci. When the frog is picked out, a thick, black discharge is present that is easily removed from the sulci. There also is a characteristically pungent odor. Once smelled, it will never be forgotten, and you have your diagnosis.
The disease is thought to be caused by a bacterium. Usually thrush does not cause lameness if the disease stays in the superficial external and non-sensitive area of the frog. However, if left unchecked, the disease can extend into the sensitive tissue of the frog and make horses quite sore. They might flinch as you clean the sulci. Thrush can progress to a severe lameness that can be seen at a walk–much like a hoof abscess. The infection in general leads to degeneration of the frog. Thrush also can cause enough degeneration of the frog that portions of that structure have to be removed by your veterinarian or farrier.
Thrush is most commonly associated with poor management practices or conditions. It can be seen in horses which are allowed to stand in paddocks or stalls that are full of wet manure, or just in horses which do not regularly have their feet cleaned. However, this disease is also seen in horses which are never allowed to stand in manure
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