Milmedika

Microsporum genus and the importance of M. canis

The Microsporum genus was first described in 1843 by David Gruby, a pioneer of medical mycology. This genus includes about twenty species, with M. audouini, M. gypseum, and M. canis being the most well-known. In the Mediterranean region, including Montenegro, the most common cause of fungal infections in untreated skin is Microsporum canis.

Microsporum canis is a zoophilic fungus that causes infections in dogs, cats, and other pets (such as hamsters and rabbits). These animals often show no visible skin changes but can transmit the fungus to humans. Transmission occurs through:

• Direct contact with infected animals or people.
• Indirect contact with contaminated objects like combs, brushes, hats, toys, or bedding.

M. canis can survive in the environment for up to 15 months, increasing the risk of infection, especially with damaged skin.

Clinical picture of infections

The skin changes caused by M. canis are erythematous, round, and have raised, hyperemic borders. These changes often have scales and small papules. The popular name for this type of infection is "ringworm", as the appearance of the lesion resembles a ring.

These infections most often appear on exposed areas of the body, such as the trunk and extremities, with mild itching. On the scalp (capillitium), infections occur more frequently, and less commonly on the chin or eyebrows. The changes include hair thinning and breakage ("alopecia"), and a secondary bacterial infection with pustules and crusts is common. Nail infections (onychomycosis) caused by M. canis are possible but rare.

Microbiological diagnosis

To diagnose dermatophyte infections, samples are taken from the skin lesions using a sterile scalpel (scrapings from the skin) or fallen hairs. The samples are then analyzed as follows:

• Direct microscopic examination: Scrapings are treated with a 20% KOH solution to look for fungal elements like septate hyphae and spores.
• Culturing: Samples are cultured on specific media such as DTM (Dermatophyte Test Medium), SDA (Sabouraud Dextrose Agar), and Sabouraud broth.

In direct preparations from untreated skin, septate hyphae are usually visible, while spores are rare. The final diagnosis is confirmed after fungal growth on culture media, but the presence of fungal elements in the direct sample is enough to begin therapy.

Culturing and morphology

The material is cultured on nutrient media and incubated at 25-30°C. Colony growth is typically visible within a few days, with full development taking 10-15 days.

• On SDA media, colonies are cotton-like with a central fold and a yellowish border. Pigment on the underside of the colony is clearly visible.
• On DTM agar, colonies change the color of the medium to reddish-brown due to the presence of a phenol red indicator.
• On Sabouraud broth, the fungus forms a yellowish membrane on the surface of the medium.

In culture preparations, M. canis produces numerous spindle-shaped macroconidia with thick walls and bent tips ("club-shaped"). Microspores are rare.

Therapy

Antifungal therapy begins immediately after fungal elements are observed in scraping preparations. Treatment usually lasts several weeks and may include:

• Topical antifungals applied directly to the affected areas.
• Systemic antifungals if the infection is severe or widespread.

Proper diagnosis and prompt treatment are crucial for successful therapy and preventing the spread of the infection.