Fungal diseases in Man, Biology tutorial

Introduction:

Fungi cause diseases termed as mycosis both in the man and in animals. Most of the fungi are moulds; however there are a number of pathogenic yeasts and most of them are dimorphic. Dimorphic fungi generally suppose the mould form when growing as saprophytes in nature and as yeast form when causing the infection.

Some of the fungi are capable to establish infection in all exposed individuals whereas others are opportunistic that cause diseases just in immunosuppressed individuals or host. Fungi cause three kinds of diseases, namely:

1) Superficial mycosis

2) Subcutaneous mycosis and

3) Systemic (deep) mycosis

Superficial Mycoses:

This is as well termed as dermatomycoses. They are diseases of the hair, skin, nail and mucous membranes and are the most general of all fungi infections and encompass a global distribution. Superficial mycoses comprise ringworm.

Ringworm:

Ringworm infections are very common diseases of the stratum corneum of the skin, hair and nail; they are as well termed to as dermatophytosis or tinea, a name that is qualified by the site influenced, example: Tinea capitis comprises the scalp, Tinea pedis comprises the feet, Tinea cruris comprises the groin and Tinea corporis comprises other areas.

The infections are caused by around 20 species of dermatophyte fungi that are grouped into three genera, Microsporum, Trichophyton and Epidermophyton. Ringworm infections are spread through direct or indirect contact by an infected individual or animal.

Indirect transfer might take place through the floors of swimming pools and showers or on combs, brushes, towels and animal grooming tools. A few abnormalities of the epidermis, like slight peeling or minor trauma, are probably essential for the establishment of infection. The infective particle is generally a fragment of keratin having viable fungus.

Scalp ringworm is mainly a disease of children, and foot ringworm a disease of adults, specifically in adult males.

The hyphae grow into latest differentiated keratin as it is formed, keeping pace along with the keratin growth. In tissue, fungi take the form of branching hyphae, which might necessarily break up into athroconidia, specifically in infected hair.

Ringworm lesions differ considerably according to the site of the infection and species of fungus comprised. At times there is only dry scaling or hyperkeratosis. However more generally there is irritation, oedema, erythema and a few vesiculation. More inflammatory lesions having weeping vesicles, pustules and ulceration are generally caused through the zoophidic species of dermatophyte.

In the skin infection of body, face and scalp, spreading annular lesions with a raised, inflammatory border are generated. Lesion in groins, tend to spread outwards from the flexures. In foot ringworm, infection is frequently confined to the toe clefts; however it can spread to the sole. Infected nails become discolored, thickened, raised and friable.

In scalp infection, the fungus invades the hair shaft and then the hyphae break-up by chains of arthroconidia. The prototype of hair invasion influences the clinical appearance of the lesion. In endothrix infection, the hair breaks off at or just beneath the mouth of the follicle, which then becomes plugged by dirt and seburn to give what is termed as black dot ringworm, however in extothrix infection, the hair generally breaks off 2 to 3 mm above the mouth of the follicle resultant in favus. Fungal growth by the favus is minimal and the hair remains intact however intense fungal growth in and around the hair follicle generates a waxy, honeycomb-like crust on the scalp.

Tropical agents like Whitfield's ointment and tolnafotate are employed for the treatment of most dermatophyte infections of skin however these have been largely superceded through terbinafine and azole compounds.

Oral griseofulvin is helpful for scalp, skin and fingernail infections apart from for toe-nail infection. Terbinafine and itraconazole are now employed for all forms of ringworm infection as they give much better cure rates having shorter periods of treatment and lower relapse rates. Control of the spread of ringworm is via enhanced living conditions and standard of hygiene.

Superficial candidiasis:

Candida albicans accounts for 80 to 90 percent of superficial Candida infection that comprises the mucous membrane of the mouth and vagina (that is, thrush), the skin or nails. Other species like C. tropicalis, C. krusei and C. parapsilosis might as well be involved. C. albicans forms around 20% of the normal flora of the mouth, gastro-intestinal tract, vagina and skin. The carriage rate tends to rise with age, and is higher in the vagina throughout pregnancy. Yeast overgrowth and infection take place if the normal microbial flora of the body is modified or if the host resistance to infection is lowered by the disease.

Mucosal infections are the very common form of superficial candidiasis. They are characterized by the growth of discrete white patches on the mucosal surface which, might build up to form a curd resembling pseudomembrane.

In the mouth or oral candidiasis white flecks come out on the buccal mucosa and the hard palate, the surrounding mucosa is red and sore and the infection might spread to the tongue. Infection takes place most often in infancy and old age, or in harshly immune-compromised individual or patients, comprising those with AIDS.

In vaginal candidiasis, usual white lesions on the epithelial surface of the vulva, vagina and cervix are accompanied through itching, soreness and a non-homogenous white discharge. At times the mucosa simply appears friable and inflamed. The peri-vulval skin might become sore and small satellite pustules might appear around the perineum and natal cleft. Vaginal candidiasis is much common, particularly throughout pregnancy; most women will have at least one episode throughout their lifetime and some suffer recurrent attack.

The other site of infection is the skin and nail. Skin infection takes place at moist sites like the groins, axillae, perineum, sub-mammary folds and toe clefts. In infants, Candida species are often comprised in napkin dermatitis.

Infection of the finger webs, nail folds and nails is the occupational disease of housewives, nurses and barmaids, who often immerse their hands in water. Superficial infections occasionally take place on the penis after intercourse with females having vaginal thrush. The yeast might as well influence the outer ear.

In the treatment of superficial candidiasis, the predisposing factor has to be taken to the consideration in addition to the use of nystatin and amphotericin B. Azoles are more efficient. In oval candidiasis, nystatin, amphotericin B and miconazole can be employed in gel or lozenge form. The application of an azole derivative or by oral therapy having fluconazole or itraconazole for 1 day can be utilized to treat vaginal candidiasis successfully.

The treatment of chronic paronychia comprises a combination of antifungal therapy, nail care and avoidance of the prolonged exposure to water. Patients must dry their hands cautiously after washing. Regular application of an azole lotion or an azole given orally is suitable, however it might take some months to cure the condition; antifungal creams or ointment are less efficient.

Tinea nigra:

It is an asymptomatic skin disease caused due to black mould Exophida werneckii, characterized through the pigmented macules of variable size, generally on the palms and soles. The disease is not contagious however is contracted by contact having the fungus in the soil. Treatment is with the utilization of keratolytic agents like Whitfield's ointment.

Subcutaneous Mycoses:

It is a disease of the skin, subcutaneous tissues and bone. They outcome from the infection of saprophytic fungus which grows in the soil, on living or decaying plant tissues, into the wounds, where it grows and spreads all along the lymphatic channels, generating subcutaneous nodules that at times drain to the skin. Subcutaneous mycoses comprise mycetoma, chromomycosis and sporotrichosis.

Mycetoma:

Mycetoma is a chronic, granulomatous infection of the skin, subcutaneous tissues, fascia and bone of the foot or the hand. It is caused through a member of the actinomycetes (or actinomycetoma) or moulds (or eumycetoma). Infection takes place if the organism is introduced into the subcutaneous tissue via thorns or sphisters from soil or vegetable source. Therefore, the disease is most general in male agricultural workers, who obtain minor skin injuries.

In host tissues, the organisms build up to form compacted colonies (or grains), 0.5-2 mm in diameter, whose color based on the organism responsible; for illustration: Madurella grains are black and Actinomadura pelletieri grains are red.

Localized swollen lesions that build up multiple draining sinuses, are generally found on the limbs, however infection takes place on other parts of the body. The prognosis differs according to the causal agent; therefore it is significant that its identity is established.

Actinomycetoma responds well to rifanipicin in combination by sulphonamide or cotrimoxazole. In eumycetoma, chemotherapy is useless, and radical surgery is generally essential.

Chromomycosis:

This disease, as well termed as chromoblastomycosis, is a chronic, localized disease of the skin and subcutaneous tissues. This is characterized through crusted, warty lesion in the limb. This is a tropical disease.

The causative agents comprise Fonsecaea pedrosoi, F. compacta, Phialophora vernicosa, Cladosporium carrianii. Itraconazole, either alone or in combination having flucytosine is employed for treatment. Early on, solitary lesions might be excised.

Sporotrichosis:

Sporotrichosis is a chronic, pyogenic granulomatous infection of the skin and subcutaneous tissues that might remain localized or show lymphatic spread. It is generally caused by Sporothrix schenckii, a saprophyte in nature.

S. schenkii is a dimorphic fungus. In nature and in culture at 25 to 30 percent, it develops as a mould with thin septate hyphae; spore-bearing hyphae carry clusters of oval spores. The yeast stage is formed in the tissue and in culture at 37°C, and is composed of the spherical cells. Sporotrichosis most often presents as a modular, ulcerating disease of the skin and subcutaneous tissues. Usually, the primary lesion is on the hand having secondary lesion extending up the arm. The primary lesion might remain localized or disseminate to comprise the joints, bones, lungs and in rare situations, the central nervous system. Disseminated disease generally takes place in debilitated or immunosuppressed individuals. Treatment by potassium iodide or itraconazole is efficient for the cutaneous form. In disseminated disease, intravenous amphotericin B is needed.

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