Ringworm" is the popular term for superficial fungal
infection of the skin. In the UK it is the only fungal
skin disease that is seen commonly. The fungi that
cause skin infections in horses are called
dermatophytes and hence the correct term for
ringworm is dermatophytosis.The term ringworm
presumably derives from the characteristic rings
that develop with infection,however the term is
misleading as worms or parasites are not
responsible for the condition. There are 2 genera of
dermatophytes that may infect horses; Microsporum
and Trichophyton. In each genera there are multiple
species that may affect horses however the vast
majority of cases are caused by only a few species.
Tricophyton equinum is the most common cause of
dermatophytosis followed by Trichophyton
mentagrophytes, Tricophytonverrucosum,
Microsporum equinum and Microsporum gypseum.
Dermatophytes invade hair follicles where they feed
on keratin and produce vast numbers of potentially
infective spores. Infection usually requires some
existing disruption of the skin surface to enable
penetration into the hair follicle. Once infection is
established it will remain and spread until the
infected hairs cease actively growing or until an
immune response develops and the fungi are
eliminated. The infection is always self-limiting in
healthy animals and eventually resolves. Following
infection clinical signs may be seen with in 1 to 6
weeks and the lesions often continue to enlarge for
a few months before they eventually resolve.