Fungal diseases of the skin

THE skin covers the entire body and protects it from environmental factors - ultraviolet rays, chemicals and pathogens. Above all, it plays a major role in the appearance of a person, as it is true that "beauty is skin deep". The skin can be divided into two layers - the upper epidermis and lower dermis. The epidermis is multi-layered, composed of the Stratum basale that cements the epidermis with the dermis. This is the dividing cell in the epidermis. Next are the Malpigian (Pricle) layer, Stratum granulosum and finally the uppermost, non- nucleated, non-living cells, the Stratum corneum. The three major functioning cells such as the Langherhans cells, Merkel cells and the Melanocytes are in the epidermis.

The Langherhans cells are otherwise known as dendritic cells. These cells act in primary immune surveillance and as antigen presenters. The main function of the Merkel cells is to govern the response to any stimuli on the skin. The Melanocytes determine skin colour as they produce the pigment, melanin. The Stratum basale gives genesis to the various layers of epidermal cells and subsequently replenishes the layers when they are removed during wear and tear.

The dermis forms the deeper layer of the skin and is connected with blood vessels and capillaries.

Normal skin is impenetrable to microorganisms. However, some microbes have developed the ability to destroy the upper layer of the skin to enable their colonisation. Among the various infections of skin, infections caused by fungi (mycoses) pose a major challenge. Poor personal hygiene, overcrowding and warm and humid weather are ideally suited for the growth of fungi. The most common infections that affect man are dermatophytosis, dandruff and Pityriasis versicolor, Piedra and Tinea nigra palmaris.

Dermatophytoses

Dermatophytoses is the infection of keratinised tissues such as the skin, hair and nail caused by closely related fungi called- dermatophytes. Among the various mycotic infections of man, dermatophytoses is the most common infection. The disease is otherwise called "Ringworm" or "Tinea". The infection usually appears as an annular and erythmatous lesion with a raised scaly margin. Severe itching is the most common symptom. Usually the lesion responds to treatment but can often recur with the cessation of therapy or may persist despite treatment.

* Clinical types of dermatophytoses

Any part of keratinised tissues in the body can be infected by dermatophytes. Depending upon the anatomical site of the lesion, the diseases are classified as tinea corporis (infection of the glabrous skin), tinea cruris jock itch or dhobies itch (infection of the crural and gluteal region), tinea pedis or athletes foot (infection of the inter trigenal folds of the feet), tinea unguium (infection of the nail) and tinea capitis (infection of the hair follicles and scalp region).

* Who is susceptible

Dermatophytes are considered to be true pathogenic fungi. Hence any individual in a given community is susceptible to dermatophytoses. However, several underlying diseases disorders such as diabetes mellitus, atopy, congenital ichthyosis, immunosuppressive drug therapy and HIV are known to predilect a chronic course of dermatophytoses. Further, subjects with blood group 'A' are also at a higher risk of developing chronic dermatophytoses, as shown in a research study (Arun Mozhi Balajee et al., 1996). Perhaps the cross reactivity between the fungal glycoprotein with human isoantigen A1 and A2 may be the reason for chronic progression of the disease in these subjects. Subjects who are on steroid (immunosuppressive drug) therapy for treatment are also at risk of developing dermatophytoses as most steroids besides immunosupression also delay the stratum corneum turnover. The retention of stratum corneum provides an ideal environment for the organism to sustain on skin for a longer period of time, thereby causing chronic disease. Organ transplant subjects who are on steroid therapy in order to prevent the rejection of the transplanted organ are also at risk.

It is noted that the incidence of disease is relatively high in males when compared to females. The effect of hormones (estrogen and testosterone) in the predilection of the disease cannot be ruled out. The male hormone, testosterone, has been reported to enhance the growth of certain dermatophyte species in vitro, whereas estrogen was found to have an inhibitory effect. Interestingly a high incidence of chronic dermatophytoses in females has been recorded especially after menopause.

* HIV and dermatophytoses

Varying clinical manifestations of dermatophytoses have been recorded in HIV positive subjects. Atypical/incognito type of manifestation of the disease with minimal or no pruritus, minimal diffused scaling and absence of erythema have been noticed in HIV subjects contrary to the usual occurence of typical lesions in non-HIV subjects. HIV subjects can act as potential reservoirs of dermatophytes as they are presented with atypical manifestations of the disease which usually go unnoticed or ignored.

* Environmental factors

A warm and humid weather and high temperature in the tropics are ideal for the growth of dermatophytes. Poor personal hygiene and overcrowding also favour spread of the disease. Occlusion due to tight clothing and sweating forms an ideal environment. This is the reason why the infection is most common in the crural, gluteal, waist regions and the soles of feet (in the case of those who wear shoes). Dermatophytes can exist in the soil as saprophytes. These organisms can also cause infection.

* Animals host

Dermatophytes do infect animals (both pet and wild animals). It has been proven that infected animals spread the disease to man either during direct contact as in the case of pet animals or act as a reservoir of these pathogens. Pets even in posh residences may harbour dermatophytes without having any symptoms. Wild animals are also known to play a role in the spread of human ringworm disease (Ranganathan et al., (1996).

Dandruff and Pityriasis versicolor

Dandruff and Pityriasis versicolar is yet another type of fungal infection of the skin caused by Pityrosporum ovale, a lipophilic fungus. The disease appears as scaly, discrete macules and patches with slight discoloration/hypopigmentation. The disease is usually mild, asymptomatic and chronic. The problem may be restricted to a localised region or may extend further to other sites. The problem poses a major cosmetic concern both in developed and developing countries. Most people may ignore or neglect the disease, as it usually does not give any discomfort to the sufferer. However, when it extends to exposed areas of the body, patients may seek medical aid due to cosmetic reasons. Pityrosporum ovale can also cause disease of the scalp region popularly referred to as dandruff. It is established that this organism exists on the scalp region as commensal flora. Some unknown host factor may trigger its rapid multiplication and becomes a potential pathogen. Another clinical entity this fungus can cause is Seborrheic dermatitis. This problem is very common in neonates.

Tinea Nigra palmaris

Tinea Nigra palmaris is a superficial asymptomatic fungal infection of the Stratum corneum, characterised by brown to black non-scaly macules. The palmer surface is often the most affected site. The causative fungi are Exophila werneckii and perhaps other species of dematiaceous fungi. Lesions are neither painful nor elevated or scaly. It can be misdiagnosed as malignant melanoma.

Piedra

Piedra is a fungal infection of the hair shaft characterised by the presence of firm irregular nodules. These are composed of fungal elements cemented together anywhere along the hair shaft. Multiple infection of the same strand is also common. Two varieties of piedra are recognised - white piedra casued by Trichophyton beigelii and black piedra caused by Piedraia hortae. The nodules of white piedra are relatively soft when compared to the black piedra.

Candidiasis

Candidiasis is the infection caused by the species of the genus Candida, a yeast-like fungus. This organism is widely considered as normal flora inhabiting the body especially in the oral cavity and gut mucosa. This organism can cause an array of clinical diseases in man especially when the immune system is disturbed either due to an infection (HIV) or due to some immunosuppressive drugs.

Oral thrush is the most common infection encountered in HIV subjects and which is considered to be the marker of HIV progression. Subjects with diabetic mellitus are also prone to develop various diseases due to candida. Nail infection (Paranychia) is another common problem, especially in housewives. Candidiasis is one of the most common opportunistic fungal diseases, which has emerged dramatically with the advent of AIDS.

Strategies and challenges in the management of fungal diseases

It is said, "prevention is better than cure". Treatment of fungal diseases is very difficult, as fungi are eukaryotes. Targetting a eukaryote (fungi) in a eukaryotic system (human and animals) is very difficult. As a result, the side-effects produced by most systemic antifungal agents are innumerable. Further, some anti- fungal drugs can cause drug-drug interaction. Hence their use is restricted. (Fluconazole is not given for the treatment of fungal infection to subjects who are on Rifambicin for TB).

The drug of choice for dermatophytoses is griseofulvin, as it is fungi static and proven to be of little value in the management of chronic dermatophytoses. Further, drug resistance to griseofulvin is also very common. Localised lesions can be treated with various topical azole preparations. Oral azole drugs (immidazole and triazole) are used in the case of chronic dermatophytoses. In the case of dandruff, antifungal shampoos are used. Topical antifungal agents are sufficient for pityriasis versicolor. Seborrhoeic dermatitis in neonates usually requires no treatment. In the case of candidasis, the treatment may vary with severity, site of infection and underlying diseases/disorders of the subject. In the case of oral thrush, topical antifungal preparations, and in the case of severe infection topical application along with oral preparations may be required.

DR. S. RANGANATHAN S. GOKUL SHANKAR M. S. RANJITH

The writers are with the Department of Microbiology, The New College, Chennai.

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