Dermatophytosis (ringworm)

An infection of the keratinized epithelium, claws, hair and stratum corneum caused by dermatophyte fungi of the species Microsporum, Trichophyton and Epidermophyton. The dermatophytes that most frequently infect animals are Microsporum and Trichophyton.

The most commonly isolated pathogenic species in our geographical area are Microsporum canis, Microsporum gypseum and Tricophyton mentagrophytes.

Microsporum canis is a zoophilic dermatophyte that causes most infections in dogs and cats and can also affect humans. Microsporum gypseum is a geophilic dermatophyte that is frequently isolated from the soil and can be found as a contaminant on the skin of animals. It occasionally causes infections in dogs, cats and humans. Trichophyton mentagrophytes has both a zoophilic and an anthropophilic form. The first is highly infectious and causes an intense inflammatory response in humans. It is more common in horses and laboratory animals than dogs and cats.

The arthrospores released when the hyphae break up are the infective form of the fungus, and pass from the infected animal to other individuals or to the environment, where they can remain active for many months.

Symptoms

The clinical picture is highly pleomorphic, and the appearance of the lesions can vary greatly. Dermatophytes develop in the keratin of the skin, hair and claws. As the fungus develops, hyperplastic changes appear on the skin accompanied by folliculitis and furunculosis with secondary bacterial contamination in some cases. The hair is invaded only in the anagen phase (growth phase), with fungal elements being deposited internally and externally (endotrix-ectotrix).

• Ringworm or classic dermatophytosis: one or more lesions appear, with varying degrees of scaling, crusting and hair loss. Hairs usually break at the point where they emerge from the skin. Over the course of the infection, healing tends to start in the centre of the lesion in some cases, with the skin regaining its normal appearance while the microorganism remains active in the periphery of the lesion. This forms the classic ring-shaped lesion. The lesions do not usually itch.
• Generalized form: affects large areas of the body in the form of extensive hair loss with scaly or oily seborrhoea. It appears to have a predilection for Yorkshire Terriers, and is almost always caused by Trichophyton mentagrophytes.
• Facial folliculitis-furunculosis: characterized by alopecia, erythematous and crusty areas accompanied by papules and pustules in the dorsal area of the nasal septum that can extend to the entire face. Typically caused by Tricophyton mentagrophytes and Microsporum persicolor.
• Kerion: term used to define a well-defined, nodular, alopecic, exudative lesion with fistulous tracts that are caused by staphylococci superinfection following dermatophytosis. Microsporum gypseum and Trichophyton mentagrophytes are isolated from these lesions.
• Onychomycosis: rare in dogs, it is characterized by onychodystrophy (changes in the claws) and perionychia (inflammation of the periungual tissue) as well as alopecia and peeling of the skin around the claws. Trichophyton mentagrophytes is isolated in most cases.

Analysis and sampling

The disease cannot be diagnosed on the basis of symptoms, because the appearance of the lesions is similar to other clinical entities:

• Circular seborrheic lesions.
• Demodicosis, particular the localized form.
• Contact dermatitis in the distal extremities.
• Histiocytomas, with kerion lesions.
• Staphylococcus aureus folliculitis
• Epidermal collarettes in some autoimmune diseases.

Sampling

• Hair: pluck hair from the periphery of lesions and erythematous or scaly areas using mosquito forceps. In the case of diffuse lesions, or to diagnose asymptomatic carriers, hair and keratin residue can also be collected by brushing the animal using a sterilised toothbrush (Mackenzie technique).
• Skin: gently clean the affected area with a gauze soaked in 70º alcohol and allow to dry. Then scrape the margins of the lesion.
• Claws: cut and discard the distal part of the claw. Cut a sample from the intermediate are for the purpose of analysis.

Specific tests

• Wood's lamp: The lamp emits ultraviolet light that can cause some dermatophyte strains (especially Microsporum canis) to fluoresce a yellowish-green colour. The lamp should be allowed to warm up for 3-5 minutes, following which the sample skin or hairs are illuminated in a darkened room.
• Direct microscopic examination: from scraping, and especially from hair. To improve observation, they are usually softened with 10% potassium hydroxide, with mineral oil for microscopy, or with calcofluor.
• Culture: the hair and scraping samples are carefully (loosely) scattered on the surface of the medium and incubated in the dark at room temperature using the following media: Sabouraud and DTM (Dermatophyte Test Medium).
• Biopsy: Not usually performed, but may be useful if culture is inconclusive or difficult to perform. Histological lesions are characteristic and fungal elements can be observed, especially with special stains (PAS, Gomori, Grocott).
• ELISA test for detecting anti-dermatophyte antibodies: Technically, this test can detect specific antibodies in the serum of infected animals, but the method still has limitations and is not used in routine clinical practice.
• PCR (polymerase chain reaction): The use of PCR to detect dermatophytes has been described, but is not routinely used.

Health measures

Not all animals exposed to the fungus will develop the infection. Young animals are more susceptible than adults, probably due to the immaturity of their immune system. Natural infection occurs by contact, and contaminated brushes, combs and bedding are important factors in transmission. Small skin lesions, high environmental humidity, and temperature, overcrowding and confinement predispose the animal to infection.

Dermatophytosis is generally considered a self-limiting infection, meaning that in immunocompetent individuals it will be eliminated within a matter of months by the immune response. This consists of antibody synthesis, which is not effective in eliminating the infection, and a cellular response consisting of T cell proliferation, which ultimately eliminates the infection. In cats, asymptomatic infections with subclinical or very subtle lesions, and even the phenomenon of carrier animals that are capable of transporting infectious spores without developing lesions are relatively common.

In some cases, owners may acquire the infection from infected or carrier pets, especially in the case of feline dermatophytosis. Airborne spores can remain infectious for 2 years, so it is extremely important to treat the infected animal’s habitat using an enilconazole spray, 1/10 bleach in hot water, 1% formaldehyde, or glutaraldehyde-based disinfectants.

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