Dermatophytosis is a fungal infection affecting the superficial layer of the epidermis and keratinized skin structures (i.e., nails and hair), caused by a group of fungi known as “dermatophytes.” The fungal group responsible for dermatophytosis includes three genera: Microsporum, Epidermophyton, and Trichophyton.
Dermatophytosis in cats is usually caused by three species of dermatophytes: Microsporum canis (98% of cases), Microsporum gypseum, and Trichophyton mentagrophytes. The significance of dermatophytosis in cats—particularly those caused by species of Microsporum—is not only related to its high incidence and frequent misdiagnosis, but also to the potential risk of transmission to humans.
Factors Predisposing Cats to Dermatophytosis
Several factors can predispose cats to infection by dermatophytes. These factors can be categorized as (1) environmental factors, such as hot and humid environments, and (2) subjective factors, including young age, genetic predisposition, and any condition that weakens the body’s defense mechanisms (e.g., malnutrition, parasitic diseases, chronically debilitating conditions, and virus-caused immunodeficiency). Cats with long hair are also more likely to get infected, possibly due to the difficulty in removing fungal spores through self-licking. Additionally, cats that are frequently bathed, which removes sebum with antifungal properties, are also at higher risk.
To understand the possible origin of an infection, it’s important to remember that, as a general rule, Microsporum canis is a “zoophilic” fungus (literally, an animal-loving fungus), while Microsporum gypseum is a “geophilic” fungus (literally, a soil-loving fungus). Thus, an infection with M. canis is more likely to be transmitted through direct contact with infected or carrier animals, while M. gypseum infections occur more commonly following contact with contaminated environments, especially contaminated soil. Regardless of the source of infection, once the fungal spores adhere to the skin or hair, they can germinate and invade the keratinized layers of the skin, nails, and hair. The invasion is supported by the production of keratinolytic enzymes by the fungal hyphae (growing vegetative fungal stages). The infection continues until the fungus reaches the keratin-producing layer of the epidermis, where a balance between fungal growth and keratin production forms. Typically, this infection is self-limiting, as the fungus is eliminated when infected hairs fall out or the infected skin exfoliates. Additionally, the infection may be limited or even eliminated if the host’s inflammatory response is particularly effective.
Dermatophytosis in cats can present with very different clinical signs, depending on the interaction between the host and the fungus. A common symptom is hair loss or weakening, often accompanied by skin areas with fragile or broken hairs or areas of alopecia, with or without skin erythema. Skin exfoliation is generally pronounced, characterized by “ash-like” scales, and may sometimes be accompanied by the formation of crusty material. Commonly affected areas include the muzzle, head, and limbs. When the inflammatory reaction is particularly intense, pimples and pustules may develop. Pruritus (itchiness) can vary, and secondary bacterial infections are uncommon in cats. A particular clinical manifestation of dermatophytosis in cats is pseudomycetoma, which appears as a non-painful, hard lump affecting the deeper layers of the skin (dermis and hypodermis), usually along the torso or base of the tail. This type of lesion typically develops in cats with compromised immune systems, such as those affected by FIV or FeLV.
Treatment and Management of Dermatophytosis
Once properly diagnosed, the therapeutic protocol for dermatophytosis in cats should include systemic drugs (e.g., griseofulvin, terbinafine, itraconazole) in combination with topical treatments (e.g., clotrimazole, sulconazole, chlorhexidine). Systemic treatment, usually administered orally, should continue for two weeks after the disappearance of clinical signs and until culture tests return negative. As a general rule, if clinical lesions persist after eight weeks of treatment, the possibility of a drug-resistant fungus or an underlying primary disorder must be considered. Topical treatments, which should always accompany systemic treatments to be effective, can be applied in various formulations such as solutions, gels, ointments, and creams.
Additional measures may include clipping, especially around the affected area, as well as certain appropriately prepared vaccines that can reduce and significantly prevent the appearance of symptoms, although they do not provide full protection from infection.
The therapeutic protocol for dermatophytosis should also involve an environmental decontamination program with suitable disinfectants. It is crucial to remember that the environment can serve as both a source of infection and re-infection for cats, as well as for humans.
Unfortunately, dermatophytosis in cats is often misdiagnosed, poorly treated, and underestimated in its potential zoonotic risk (risk of transmission to humans). Therefore, it is the veterinarian’s responsibility to emphasize the importance of this risk and to develop effective prophylactic and/or therapeutic plans. It is also the cat owner’s responsibility to follow these plans and strictly adhere to the veterinarian’s instructions.
