Thrush is a common bacterial infection of the horse's hoof, specifically the frog. The bacterial organism responsible for causing the infection, Fusobacterium necrophorum, is found commonly in mud and dirty, unsanitary conditions. Horses become infected with the organism by standing for long periods of time in such environments. It will actively gravitate towards the horse's frog because it prefers to live in environments with low-oxygen, such as what the clefts in the horse's frog provides.

Thrush is characterized by the presence of foul-smelling black material within the sulci of the frog. The frog surface is often white and crumbly. Thrush is often over diagnosed in horses, especially by horse owners, as the early stages of canker often greatly resemble clinical signs seen in horses with thrush. If any bleeding is observed with manipulation of any of the tissues, lesions extend outside of the frog area, or the horse is not responding to thrush treatment, it is usually indicative of canker.