In human medicine, onychomycosis is a fungal disease of the toe- and fingernails. In horses, various species of fungi are often found in the crumbly, disintegrating horn typical of white line disease and seedy toe (and the hoof wall separation which results). It is as yet not certain whether these fungi are the primary cause of the disease, or are opportunistic pathogens than ‘join in’ after other factors or pathogens have caused the initial damage.

Onychomycosis is sometimes used as a synonym for one or both of the above mentioned disorders, but this is rather misleading, as it is unlikely that a fungal origin alone is responsible for the damage to the inner layers of the hoof wall.

A disease affecting the horn in the region of the white line. The horn in the white line is generally softer than the horn in the adjacent hoof wall and it may be additionally weakened by diseases or disorders of the hoof such as laminitis. These factors contribute to the increased potential for the horn of the white line to become damaged due to mechanical factors (e.g., small stones that become lodged in the horn) or invading micro-organisms (e.g., bacteria or fungi) that lead to disintegration of the horn. White line disease is a progressive disorder that can lead to damage in the stratum internum or the innermost layers of the stratum medium of the hoof wall proximally to the white line. Damage can become extensive and cavities can be formed within the hoof wall. This results in an undermined hoof wall, a condition very similar to and often confused with seedy toe.