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Clinical Mycology

                                               Medical Mycology

Clinical Mycology :                                                 

    Fungal infections:                                   Dr. Jawad Kadhum Tarrad                                                            

--------------------------------------------------------------------------------------------Human diseases caused by fungi known as (Mycoses) . The term mycosis was derived from Greek word mykes for mushroom.

The clinicians find value in classifying the fungi into five categories of mycoses:

Superficial mycoses:

Superficial mycoses are limited to outermost layer of skin or hair with little tissues damage and generally no inflammatory response. In other word, they are defined as infect only superficial keratinized tissue without noticeable invasion of living tissue and without apparently provoking immune response of host.

1-    Tinea versicolor (pityriasis versicolor, pityriais alba) :

Etiology;

The causative agent of pityriasis versicolor is thermal dimorphic fungus ,Malassezia( Pityrosporum), especially species, M. furfur ,              M. globosa . This fungus exist as yeast when grow on skin as normal flora ,or grow as mold at room temperature.

Transmission and epidemiology:

The fungus is part of normal flora and cause endogenous infection(autoinfection) and  it can be transmitted from person to other by direct contact. The tinea is common infection with worldwide distribution , especially occur more frequently in hot and humid weather, usually more occur in summer . The infection is  higher prevalence in tropical and temperate zones ,and it is more common in immunocompromised patients such as patients who have undergone renal transplant ,and  in patients with AIDS.

     Pathogenesis;

Malassezia species are a lipophilic yeast (it required lipid for growth)and infect the stratum corneum (keratinized layer , cornified is dead layer)of skin.

Pityriasis versicolor is not associated with host immune response , and no virulence factors are known.

Clinical findings :

The lesion is noticed as hypopigmented areas ,especially on tanned skin. The tinea is characterized by dry , scaling (exfoliative skin) , itching , depigmented maculae occur on skin ,usually of chest ,neck,  upper back ,arms, abdomen.

Species of this yeast have been implicated as a cause of contributor to seborrheic dermatitis, or dandruff.

Lab.Dx :

1.     direct microscopic examination of scaly lesion ,(scrapings of infected skin), treated with KOH or stained with calcofluor white , will be reveals as yeast or as hyphae.

2.      Culture is not usually done(no cultured so far).

Treatment and prevention : 

1.     this lesion is treated topically with selenium sulfide or with topical or oral miconazole.

2.     Removing infected stratum corneum mechanically or chemically

3.     Good hygiene is important in prevention.

2. Tinea nigra :

Etiology;

Tinea nigra is infection of keratinized layer of skin (stratum corneum), it is caused by Hortaea werneckii(formerly,Exophiala werneckii, Cladosporium werneckii) . The fungus is dimorphic,that  exist either as pathogenic yeast in tissue or grow as saprophytic mold in soil. The fungus is melanin-producing (so it called dematiaceous fungus).

Transmission and epidemiology:

  fungus is found in soil and transmitted via injury. Tinea nigra is infection of stratum corneum of skin , this condition is more prevalent in warm coastal regions and among young women . Tinea nigra is epidemic in tropics.

Pathogenesis;

  Tinea nigra does not elicit host immune response, and no virulence factors are known.

Clinical findings;

the lesion caused by this fungus which appears as brown to black spot discoloration ,these macular areas appears most common on  palms and soles .The lesion is characterized by flat spot , dark discoloration ,non scaly macula.

Lab.Dx:

1.     direct microscopic examination of scrapings in KOH preparation will reveals as mold with branched and septate hyphae , or reveals as budding yeast cells. walls.

2.     Culture  of skin scrapings is done at 25 C and at 37C . culture reveals black colonies with melaninized cell.

Treatment and prevention:

1.     infection is treated with topical keratolytic solutions such as salicylic acid , or azole antifungal drugs .

2.     Good hygiene is important preventive measure .

3. Tinea piedra(White piedra and black piedra):

Etiology;

  white piedra is caused by mold Trichosporon ashii , Black piedra is caused by mold Piedraia hortae .

  Transmission and epidemiology:

  both fungal pathogens are transmitted from person to person by infected hair on shared combs or hairbrushes . Both fungi are more common in young adults and endemic in tropical undeveloped countries .

Pathogenesis;

  White and black piedra do not elicit host immune response, and no virulence factors are known.

Clinical pictures:

·        White piedra is a nodular infection of hair shaft ( scalp , axillary, beard, pubic hairs), and it is characterized by soft ,white to light brown nodules which is formed around hair shaft.

·        Black piedra is nodular infection of scalp hair , and characterized by hard ,brown to black nodules attached to hair shaft.

Lab.Dx:

1.     direct microscopic examination of hairs in KOH preparation reveal dark pigmented nodules containing dark septate hyphae (black piedra), or white to brown septate hyphae (white piedra) on hair shaft.

2.     Culture on sabouraud agar at 25C for 3 weeks there appear shiny colonies which are cream colored and  wrinkled surface.

Treatment and prevention:

1.     Piedra is treated by removal of infected hairs by shaving the hairs, and application of topical antifungal agents .

2.     Good personal hygiene is very important in prevent  the infection.