Dermatophytes

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Introduction

  • Dermatophytes are fungi that infect skin, hair, and nails.
  • They grow only on keratinized tissues of the body.
  • Keratin is a hard protein present in outer skin, hair, and nails.
  • These fungi use keratin as their source of nutrition.
  • Dermatophytes cause superficial fungal infections called dermatophytosis.
  • These infections are commonly known as ringworm or tinea infection.
  • The name ringworm is used because lesions often appear ring-shaped, but no worm is involved.
  • Dermatophyte infection is usually limited to superficial tissues and does not invade deep organs.
  • The infection is very common in warm and humid climates.
  • Sweating, poor hygiene, crowded living conditions, and close contact increase infection risk.
  • Dermatophyte infections occur in all age groups.
  • Children commonly develop scalp infection, while adults more often develop skin and nail infection.
  • These fungi spread from infected persons, animals, soil, clothing, towels, and contaminated surfaces.
  • Dermatophytes produce enzymes that digest keratin and help them grow on the body surface.
  • They are important causes of superficial mycosis in medical microbiology.

Classification of Dermatophytes

Dermatophytes belong to three major genera:

1. Trichophyton

This is the most common genus infecting humans.

Important Features

  • Infects skin, hair, and nails.
  • Produces numerous microconidia.
  • Macroconidia are usually scanty.

Important Species

  • Trichophyton rubrum
  • Trichophyton mentagrophytes

2. Microsporum

Important Features

  • Infects skin and hair.
  • Usually does not infect nails.
  • Produces large spindle-shaped macroconidia.

Important Species

  • Microsporum canis
  • Microsporum audouinii

3. Epidermophyton

Important Features

  • Infects skin and nails.
  • Does not infect hair.
  • Produces club-shaped macroconidia.
  • Microconidia are absent.

Important Species

  • Epidermophyton floccosum

Classification According to Habitat

Anthropophilic Dermatophytes

  • These fungi primarily infect humans.
  • Spread occurs from person to person.
  • Infections are usually chronic and less inflammatory.

Zoophilic Dermatophytes

  • These fungi primarily infect animals.
  • Human infection occurs through contact with infected animals.
  • Lesions are usually more inflammatory.

Geophilic Dermatophytes

  • These fungi are found in soil.
  • Human infection occurs through contact with contaminated soil.

Morphology of Dermatophytes

Macroscopic Appearance in Culture

On culture media, dermatophytes form colonies that may appear:

  • Powdery
  • Velvety
  • Cottony
  • Granular

Colony color may vary from white to cream, yellow, brown, or reddish depending on species.

Microscopic Appearance

Under microscope, dermatophytes show:

  • Septate branching hyphae
  • Macroconidia
  • Microconidia

Macroconidia

  • Large multicellular spores
  • Thick or thin-walled depending on species

Microconidia

  • Small unicellular spores
  • Important for identification of species

Pathogenesis

Dermatophytes infect the body through the following process:

  • Fungal spores reach skin surface.
  • They adhere to keratinized tissue.
  • Germination occurs.
  • Hyphae penetrate superficial keratin layer.
  • Keratinase enzyme digests keratin.
  • Infection spreads centrifugally producing characteristic lesions.

Clinical Manifestations

Dermatophyte infections are named according to body site involved.

Tinea Capitis

Infection of scalp and hair, common in children.

Tinea Corporis

Infection of body skin presenting as circular ring lesions.

Tinea Cruris

Infection of groin region, commonly called jock itch.

Tinea Pedis

Infection of foot, especially between toes.

Tinea Unguium (Onychomycosis)

Infection of nails causing thickening and discoloration.

Tinea Barbae

Infection of beard area.

Tinea Faciei

Infection of face.

Tinea Manuum

Infection of hand.

Laboratory Diagnosis

Specimens

The type of specimen collected for diagnosing dermatophyte infections depends on the location and type of infection. Common specimens include:

  • Skin Scraping: Skin scrapings from the affected area are commonly obtained for superficial infections. These samples should include infected and healthy-looking skin to ensure fungal elements are present.
  • Hair Samples: In cases of tinea capitis, hair samples, including broken hair shafts or scalp scrapings, may be collected.
  • Nail Clippings: For onychomycosis (nail infection), nail clippings or scrapings from the affected nails are the specimen of choice.
  • Biopsy: In more severe cases or when the infection involves deeper tissues, a biopsy may be performed. The tissue sample is then examined for fungal elements.
  • Swabs: Swabs can collect samples from the infected sites for moist areas like the groin or between the toes.

Direct Microscopic Examination

Direct microscopic examination is a key diagnostic tool for identifying dermatophyte infections. Common techniques used for this purpose include:

  • KOH (Potassium Hydroxide) Preparation:
    • A skin scraping, hair sample, or nail clipping is treated with KOH, which dissolves keratin, allowing the fungal elements to be more visible under the microscope.
    • Dermatophytes can be identified as septate hyphae (branching filaments) and occasionally as arthroconidia (spores).
    • Hair specimens often show infected hair shafts with spores attached.
  • Wood’s Lamp Examination:
    • The Wood’s lamp emits ultraviolet light, which causes certain dermatophytes, such as Microsporum canis, to fluoresce with a characteristic greenish-yellow color.
    • This method is particularly useful for diagnosing tinea capitis.
  • Gram Stain: Although not as commonly used as KOH preparation, a Gram stain can also reveal fungal elements in clinical samples. Dermatophytes appear as Gram-positive, septate hyphae or conidia.
  • India Ink Staining: This method can be used in suspected fungal infections involving deeper tissues, though it is not commonly used for dermatophytes.
  • Lactophenol Cotton Blue Staining: This is a fungal-specific stain used for confirming the identity of dermatophytes in culture. It stains the fungal structures, allowing them to be visible under a microscope.

Culture and Identification

Culturing dermatophytes is essential for confirming the diagnosis and identifying the species involved, which helps guide treatment decisions. The following methods are commonly used for culturing dermatophytes:

  • Culture Media: Dermatophytes are typically cultured on Sabouraud dextrose agar (SDA) or Dermatophyte test media (DTM). This selective medium contains antifungal agents to inhibit the growth of bacteria and other fungi. On DTM, dermatophytes typically produce a color change in the medium (from yellow to red) due to alkaline byproducts of fungal growth.
  • Incubation Conditions: Dermatophyte cultures are incubated at room temperature (25-30°C) for 1-3 weeks, growing relatively slowly. Fungal growth is observed as colonies that may be creamy, white, or yellow, and some species may have a characteristic appearance, such as the “cottony” texture of Trichophyton rubrum.
  • Colony Morphology: The morphological features of the colony, such as its size, texture, color, and the presence of a red or yellow pigment, can help differentiate dermatophyte species.
  • Microscopic Examination: After colony growth, a small piece of the colony is placed in lactophenol cotton blue or potassium hydroxide (KOH) preparation to examine the fungal structures. Depending on the species, dermatophytes typically produce septate hyphae, macroconidia, and microconidia.
  • Biochemical and Molecular Identification: Some advanced methods, including DNA sequencing or PCR assays, can identify dermatophyte species based on genetic material. This can provide a more precise identification, especially in unclear morphological characteristics.

Other Laboratory Tests

Other laboratory tests may be used to complement the diagnosis of dermatophyte infections, particularly when culture results are unavailable or inconclusive:

  • Wood’s Lamp Examination: This technique is especially useful for detecting infections caused by Microsporum canis, which exhibits fluorescence under UV light.
  • Histopathology: In certain cases, skin biopsies or tissue samples may be examined histologically for the presence of fungal elements. Specialized stains such as PAS (Periodic Acid-Schiff) or GMS (Gomori’s Methenamine Silver) may enhance fungal hyphae’s visibility in tissue samples.
  • Serological Tests: Although not routinely used, serological tests detecting antifungal antibodies or antigens can be useful in some cases, particularly for dermatophyte infections associated with systemic involvement.
  • PCR-based Assays: PCR tests can detect the DNA of dermatophytes in clinical specimens, which is particularly useful for identifying species that are difficult to culture or when the infection is deep or chronic.

Treatment 

The treatment of dermatophyte infections depends on the location and severity of the infection and the species of dermatophyte involved. Treatment options include:

  • Topical Antifungal Agents: For localized and superficial infections, over-the-counter antifungal creams, ointments, or powders are commonly used. These may include:
    • Terbinafine (Lamisil)
    • Clotrimazole (Lotrimin)
    • Miconazole (Micatin)
    • Ketoconazole

Topical treatment is typically effective for infections like tinea corporis, tinea pedis, and tinea cruris.

  • Oral Antifungal Agents: For more extensive or recalcitrant infections, oral antifungal medications may be necessary. Common oral antifungals include:
    • Terbinafine
    • Griseofulvin (especially for tinea capitis or onychomycosis)
    • Fluconazole
    • Itraconazole

Oral treatment is especially important for nail infections (onychomycosis) or widespread tinea infections.

  • Nail Infections: Onychomycosis may require prolonged treatment (3-6 months) with oral antifungals like terbinafine or itraconazole.
  • Hygiene and Prevention: To prevent recurrence and the spread of infection, patients should maintain good hygiene, avoid sharing personal items like towels or combs, and wear loose-fitting clothing. For athletes, changing socks frequently and using antifungal powders can help prevent tinea pedis.

Prevention

Personal Hygiene

  • Maintain regular bathing and cleanliness.
  • Dry skin properly after washing.

Avoid Sharing Personal Articles

  • Do not share towels, combs, clothes, footwear, or nail cutters.

Control Moisture

  • Wear loose cotton clothing.
  • Keep skin folds dry.

Foot Care

  • Change socks regularly.
  • Use clean footwear.

Animal Care

  • Treat infected pets or domestic animals.

Environmental Cleanliness

  • Clean floors, bathrooms, and personal surroundings regularly.

Early Treatment

  • Early diagnosis and treatment prevent spread to others.
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