Thrush is a degeneration of the frog with secondary anaerobic bacterial infection that begins in the central and collateral sulci. The central sulcus is more commonly involved if the horse has sheared heels; the lateral sulci are primarily involved in most cases of thrush (without sheared heels). The affected sulci are moist and contain a black, thick discharge with a characteristic foul odor; the borders of the frog are commonly necrotic. These signs alone are sufficient to make the diagnosis. Although many describe the primary etiology as a moist environment with poor hygiene, it is more likely caused by poor foot conformation or trimming and lack of exercise (thought to help “clean out” the sulci when the weight of the horse pushes down on the frog and surrounding structures) than from lack of hygiene in the stall. However, a moist environment should be avoided in animals with thrush.
Treatment should begin by providing dry, clean flooring and thorough debridement of the frog and sulci. Additionally, the foot needs to be balanced, and affected horses placed on a regular exercise schedule in a dry area. An astringent solution (eg, copper sulfate solution) may be applied with daily hoof cleaning. Commercial equine foot formulations that produce chlorine dioxide can also be used. If granulation tissue or sensitive tissue is exposed, astringent solutions should be avoided; a paste made of metronidazole tablets can be applied instead to the affected areas in combination with foot bandaging. The prognosis is usually favorable with appropriate changes in shoeing and exercise.