【Pathology】Review on dermatomycosis: pathogenesis and treatment 笔记

Review on dermatomycosis: pathogenesis and treatment 

Content:

1. Dermatophytes
2. ECOLOGICAL CLASSIFICATION
3. Trichophyton
4. Microsporum
5. Epidermophyton
6. DISTRIBUTION FREQUENCY OF DERMATOPHYTES AND DERMATOPHYTOSIS
7. PATHOGENESIS AND CLINICAL PRESENTATION

1. Dermatophytes

a group of keratinophilic fungi thriving on the keratin substrate are the etiological agents responsible for causing cutaneous infections. 

Dermatomycoses are infections of the skin, hair and nail caused as a result of colonization of the keratinized layers of the body.

• This colonization is brought about by the organisms belonging to the three genera namely 
• Trichophyton
• Microsporum
• Epidermophyton
• Infection may also be caused rarely by 
• the members of the genus Candida
• non-dermatophytic moulds belonging to the genera Fusarium, Scopulariopsis and Aspergillus

2. ECOLOGICAL CLASSIFICATION

In the course of evolution these pathogens have developed host specificity. 

• ascribed to the difference in the composition of keratin 

Based on their host specificity dermatophytes are classified into three ecological groups namely 

• geophiles (soil)
• generally saprophytic and derive nutrients from keratinous substrates
• anthropophiles (man)
• The primary hosts of anthropophilic species are human beings but they may also cause infection in animals.
• Transmission of infection is from man to man. 
• zoophiles (animals)
• pathogens with only one animal host and grow as saprophytes on animal materials. 
• Human beings acquire the infection from infected animals. 
  

Examples (Soil) include 

• Trichophyton ajelloi
• Trichophyton terrestre
• Microsporum fulvum
• Micropsorum gypseum
• Microsporum cookie
• Epidermophyton stockdaleae

Examples (animal) include 

• Trichophyton simii (monkeys)
• Trichophyton mentagrophytes (rodents)
• Trichophyton equinum (horses)
• Microsporum canis (cats)
• Micropsorum nannum (pigs)

Examples (human) include 

• Trichophyton rubrum
• Trichophyton kanei
• Trichophyton schoenleini
• Trichophyton concentricum
• Trichophyton tonsurans
• Micropsorum gypseum
• audouinii
• Microsporum ferrugineum
• Epidermophyton floccosum

3. Trichophyton

• The genus Trichophyton includes 24 species. 
• The colonies on agar media are powdery, velvety or waxy. 
• The predominant spore type is micro conidia with sparse macro conidia. 
• Reverse side pigmentation is characteristicof the species and is used for the identification of the species within the genus. 
• The macro conidia are thin walled with smooth surface and variable
• shape. 
• Some of the Trichophyton species are fastidious in their requirement for amino acid as nitrogen source. 
• Trichophyton tonsurans requires ornithine, citrul-line and Arginine 
• Trichophyton mentagrophytes requires methionine. 
• This nutritional specificity has been used by many authors in the identification of the Trichophyton species

4. Microsporum

• The genus Microsporum includes 16 species. 
• The colony morphology of Microsporum species on agar surface is
• either velvety or powdery with white to brown pigmentation
• Both macro and micro conidia are produced but the predominant conidial structures are macro conidia.
• Micro conidia are less abundant. 
• The macro conidia are multi septate with thick wall and rough surface
• Rarely some species produce neither micro nor macro conidia 
• They do not have any special nutritional requirements.

5. Epidermophyton

• The genus Epidermophyton includes only 2 species. 
• The colonies are slow-growing, powdery and unique brownish yellow in colour. 
• This genus is devoid of micro conidia.
• Macro conidia are abundant and produced in clusters
• These macro conidia are thin walled with smooth surface.

6. DISTRIBUTION FREQUENCY OF DERMATOPHYTES AND DERMATOPHYTOSIS

The predominant cause of dermatophytic infections is Trichophyton followed by Epidermophyton and Microsporum. 

Within the genus Trichophyton, Trichophyton rubrum is the predominant etiological agent accounting for 69.5% followed by 

• Trichophyton mentagrophytes
• Trichophyton verrucosum
• Trichophyton tonsurans

According to the World Health Organization (WHO) survey on the incidence of dermatophytic infection, about 20% the people world wide present with cutaneous infections. 

• The disease does not spare people of any age
• Among the tinea infections, the most predominant type of infection is tinea corporis or tinea circinata
• followed by tinea cruris, tinea pedis and Onychomycosis.
• Tinea corporis accounts for about 70% of the dermatodermatophytic infection

7. PATHOGENESIS AND CLINICAL PRESENTATION

• The possible route of entry for the dermatophytes into the host body is injured skin, scars and burns. 
• Infection is caused by arthrospores or conidia
• Resting hairs lack the essential nutrient required for the growth of the organism.
• Hence these hairs are not invaded during the process of infection 
• The pathogen invades the uppermost, non-living, keratinized layer of the skin namely the stratum corneum, produces exo-enzyme keratinase and induces inflammatory reaction at the site of infection
• The customary signs of inflammatory reactions, such as 
• redness (ruber)
• swelling (induration)
• heat and alopecia (loss of hair) are seen at the infection site. 
• Inflammation causes the pathogen to move away from the site of infection and take residence at a new site. 
• This movement of the organism away from the infection site produces the classical ringed lesion

The infections caused by dermatophytes are commonly referred to as “tinea” or “ring-worm” infections due to the characteristic ringed lesions. 

Based on the site of infection,

• tinea capitis (scalp)
• tinea corporis or tinea circinata (non-hairy, glaborous region of the body)
• tinea pedis (“Athletes’ foot”; foot)
• tinea ungium (“Onychomycosis”; nail)
• tinea mannum (hands)
• tinea barbae (“Barbers’ itch”; bearded region of face and neck)
• tinea incognito (steroid modified)
• tinea imbricata (modified form of tinea corporis)
• tinea gladiatorium (common among wrestlers’) 
• tinea cruris (“Jocks’ itch”; groin)