Dermatophytes

Dr. Rubaiya Binte Kabir

M.Phil : Part - 1

General Characteristics

• Dermatophytes are cutaneous fungi which infect only

the keratinized tissues by liberting keratinase enzyme

which helps them to invade into keratinized tissue

like stratum corneum layer of skin, hair and nail.

• It is a group of about 40 related fungi that belong to

three genra : 1. Microsporum

2. Trichophyton

3. Epidermophyton.

• They are restricted to non-viable skin because most

are unable to grow at 37˚C or in the presence of

serum .

• Many species have particular keratinase, elastase and

other enzymes which make them host specific.

• Several are capale of sexual reproduction – produce

ascospore. Thus belongs to genus Arthroderma.

Ascospore (sexual spores)

• In skin, they produce hyaline, septate, branching

hyphae, or chains of arthoconidia.

• E. floccosum is the only pathogen in this genus which

produces macroconidia.

Arthroconidia (asexual form)

• They are highly contagious and frequently

transmitted by exposure to shed skin scale, nails,

hairs cantaing hypae and conidia.

• They remail viable for long periods on fomites.

Classification

• According to shape and site of infection:

1. Microsporum : spindle shaped; infect skin and hair.

- M. canis

- M. gypseum

- M. gallinae

- M. nanum.

2. Trichophyton : pencil shaped; infect skin, nail, hair.

- T. rubrum

- T. tonsurans

- T. mentagrophytes.

3. Epidermophyton : club shaped; infect skin and nail.

- E. floccosum.

• According to habitat :

1. Antropophilic : habitat in human body.

- some Trichophyton species

- E. floccosum.

2. Geophilic : usually habitat in soil.

- M. gypseum.

3. Zoophilic : usually habitat in animal.

- M. canis (dogs and cats)

- M. gallinae (fowl)

- M. nanum (pigs)

- T. equinum (horses)

- T. verrucosum (cattle).

Immunity

• Trichophytid reaction : trichophytin is a crude antigen

preparation that can be used to detect immediate or delayed type

hypersensitivity.

- chronic, noninflammatory dermatophyte infection

poor cell mediated immune response to dermatophyte antigen.

Immediate type hypersensitivity with elevated IgE.

Allergic reaction- dermatophytid (usuallly vesicle and often in

hand)

Trichophytid reaction : markedly positive

Clinical findings

• Dermatophytosis is classified according to site of involvement :

1. Tinea capitis : infection in scalp and hair.

2. Tinea barbae : infection in beard area.

3. Tinea corporis : infection in the trunk.

4. Tinea cruris (jock itch): infection in groin/ inguinal region.

5. Tinea manum : infection in hand.

6. Tinea unguium (onychomycosis): infection in nail.

7. Tinea pedis (athlete’s foot) : infection in foot.

• Tinea Capitis :

- Dermatophytosis or ringworm of the scalp and hair is called

Tinea capitis.

- Caused by Trichophyton or Microsporum.

- Pathogenesis :

Hyphal invasion of the skin of scalp

Subsequent spread down the keratinized wall of hair follicle

Infection begins just above hair follicle, grow downwards on

noninvolving area as the hair grows upwards.

Production of dull grey, circular patches of

alopecia, scaling and itching.

As the hair grows out of the follicle--

1.Microsporum : Hypae produce chain of spores

forming sheath around the hair shaft- ectothrix.

2. Trichophyton : hypae produce spores within the hair

shaft - endothrix.

- Hair become weakened; typically break easily at the

follicle opening.

A- ectothrix; B endothrix

3. Zoophilic species : induce combined inflammatory

and hypersensitivity reaction – kerion.

4. Trochophyton schoenleinii : acute inflammatory

reation of hair follicle leading to formation of scutula

(crust)- favus.

• Tinea Barbae: highly inflammatory reaction

resembling pyogenic infecion.

• Tinea Corporis : annular lesion of ringworm with a

clearing, scaly center surrounded by a red advancing

border that may be dry or vescicle.

- Pathogenesis :

Fungal metabolites, enzymes, antigens diffuse through

the viable layers of the epidermis

Erythema, vesicle formation, pruritus

Lesion expand centrifugally and active hyphal growth at

periphery.

• Tinea cruris : mostly occurs in men.

- dry, itchy lesions often start at the scrotam and

spread to the groin.

• Tinea manus : dry, scaly lesion may involve one or

both hands, single finger, or two or more fingers.

• Tinea Unguium / Onychomycosis :

- May affect toe nails or finger nails, but toe nail

infection is particularly common which follows tinea

pedis.

- Caused by : Trichophyton and Epidermophyton.

- Risk factors :

1. Diminished blood supply.

2. Humid, moist environment.

3. Perspiring heavily.

4. Bare foot in dump places.

5. DM, immunosuppression.

- Pathogenesis :

With hyphal invasion

Nails become yellow, brittle, thickened and crumbly.

• Tinea Pedis / Athlete’s foot :

- Chronic infection of toe webs. May be vesicle,

ulcerative and moccasin types, hyperkeratosis of the

sole.

• Coarse of disease :

Itching between toes.

Development of small vesicles.

Rupture of vesicle and discharge of a thin fluid.

Skin of the toe webs become macerated and peels.

Cracks appear and secondary bacterial infection develops.

When become chonic- peeling and cracking are accompanied by pain and pruritus.

Laboratory diagnosis

• Sample :

1. Skin scrapping.

2. Nail scrapping.

3. Hair plucking.

• Collection of samples :

1. Skin : from the margin of the lesion, with the scalpel.

2. Nail : deeper part is collected and superficial part is discarded.

3. Hair : plucked by fine forceps.

• Wood’s lamp test :

ectothrix of Microsporum species impart a greenish

to silvery fluerescence when examined under Wood’s

light.

• Microscopic examination :

1. KOH preparation of skin or nail : branching hypae

or chains of arthoconidia are seen.

Unstained microscopic

KOH prep. of scraping from

a ring worm showing

arthrospore (asexual spores).

Multicellular macroconidia

with echinulate wall of M.

canis

Club shaped

macroconidia

with thin and

smooth wall

arise in small

clusters of E.

floccosum.

Small and

piriform

microconidia of

T. rubrum

Elongated microconidia of T.

tonsurans

Abundant grape

like clusters of

spherical

microconidia on

terminal branch

of T.

mentagrophytes.

2. KOH preparation of hair : ectothrix and endothrix are

seen.

Left - ectothrix (arthospore outside hair shaft);

Right – endothrix (arthospore inside hair shaft.)

• Culture :

- Incubation period : 1-3 weeks.

- Incubation temparature : 25˚ C.

- Media used :

1. Sabouraud’s dextrose agar media.

2. Dermatophyte test media : Sabouraud’s dextrose agar + cyclohexamide + chloramphenicol + phenol red.

3. Malt agar.

- Colony morphology :

T. rubrum : White cottony

surface and a deep red

nondiffusible pigment from

reverse side

T. tonsurans : flat, powdery,

velvety colony.

T. metagrophytes : cottony

to granular colony

Microsporum : white cottony

sarface with deep yellow from

revese.

Epidermophyton : flat, velvety

with a tan to olive green tinge.

Dermatophyte test media

• Others :

1. PCR : species specific identification.

2. Nutritional test and growth at 37˚ C.

3. In vitro hair perforation : placing an organism in a

petri dish- water, yeast extract, hair.

Treatment

• Therapy consists of thorough removal of infected and dead

epithelial structures and application of a topical antifungal drug.

1. Tinea capitis :

- oral Griseofulvin or terbinafin several weeks.

- topical shampoo and miconazole cream several weeks.

- alternative : itraconazole, ketokonazole.

2. Others :

- oral itraconazole and terbinafine.

- topical miconazole, tolnafate, clotrimazole 2-4 weeks.

- troublesome cases : oral griseofulvin.

3. Tinea unguium :

- orat itraconazole or terbinafine for months with surgical removal

of the nail.

- topical imidazole, luciconazole,.