The most common dermatophyte fungi responsible for causing onychomycosis, particularly in toenails, are Trichophyton rubrum and Trichophyton mentagrophytes. These two species are the predominant causative agents of fungal nail infections worldwide.
Trichophyton rubrum is generally considered the most common dermatophyte associated with onychomycosis. It accounts for a significant majority of cases, especially those involving toenails. Trichophyton mentagrophytes is also a common cause of onychomycosis and can infect both toenails and fingernails.
Other dermatophyte fungi, such as Trichophyton interdigitale and Epidermophyton floccosum, may also cause onychomycosis, but they are less frequently encountered compared to Trichophyton rubrum and Trichophyton mentagrophytes.
These dermatophyte fungi have a natural affinity for keratinous tissues, such as nails and skin, and they can invade and proliferate within the nail plate, leading to the characteristic symptoms of onychomycosis, including thickening, discoloration, and brittleness of the nails.