The frog is a “highly dynamic” structure that changes in response to terrain and other hoof demands, Taylor said. A frog’s width should be approximately 50 to 60% of its length, and the portion closest to its apex (point) should be substantial enough to touch the ground when the horse is bearing weight.

“If this portion of the frog does not engage the ground, fibrocartilage in the (rear) of the foot is hypothesized to develop poorly or atrophy, contributing to a weak heel,” she said.

A healthy frog has a shallow central sulcus, wide enough for a ring or index finger to fit, Taylor said. A common frog defect is a contracted (too narrow) central sulcus, which creates an anaerobic (lacking oxygen) environment ideal for thrush development. The central sulcus will remain contracted until the thrush resolves.

“When thrush gets in there, the horse may try to avoid using the back of his foot,” she said, creating a mechanical situation that can predispose the foot to lameness.