Ringworms (cutaneous mycoses), Biology

Ringworms (cutaneous mycoses)


Cutaneous mycoses – also called dermatomycoses, occur worldwide and represent the most common fungal diseases in humans. The dermatophytes are the fungi that infect only the epidermis and its appendages (hair and nails), i.e., structures in which keratin are abundant. The skin lesions are usually circular, tend to expand equally in all directions, and have raised borders. They are, therefore, thought in ancient times to be due to worms or lice, and they are still called ringworms or tinea (worms or insect larvae). The names are usually qualified by the area of skin involved, e.g., ringworm of scalp (tinea capitis), of the body (tinea corporis), of the groin (tinea cruris) and of the feet (athete’s foot, tinea pedis).


Epidemiology:

Three genera of dermatophytes are involved in ringworm:
Epidermophyton, Microsporum and Trichophyton.

Several species of dermatophytes are primarily found in human skin (anthrophilic), some are indigenous in domesticated and wild animals (zoophilic), and a few are free living and are isolable from soil (geophilic).


Infection is transmitted from man-to-man or animal-to-man or vice versa by direct contact or by contact with infected hairs and epidermal scales. The reservoir of animal infection is large. Dogs and cats are commonly infected with Microsporum canis, which is a frequent cause of ringworm in children also.


The incidence of different dermatomycoses varies with age. For example, athlete’s foot is common in adults but rare in children, whereas the opposite is true for ringworm of the scalp. Resistance of adults to scalp infection has been linked to the increases secretory activity of the sebaceous glands at puberty and the antifungal activity of unsaturated fatty acids in sebum. Most dermatophytes have a worldwild distribution.


Clinical features:
The dermatophytes infect skin, hair, nail and scalp, and the clinical features vary with the site of infection. When skin is infected, there is irritation, erythema, oedema and some vesiculation, especially at the spreading edge, and this

 

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irregular pink periphery gives rise to the name ringworm. Secondly infection or vigorous treatment may give rise to formation of vesicle, pustules and ulceration.Infection of the nails renders them irregular, discoloured and friable. The fungal grows deep into the substance of the nail. When the scalp is infected, the fungus grows in the horny layer of the epidermis and down into the hair follicles. The hyphae surround and invade the hair shaft. Some species of dermatophytes grow only within the shaft (endothrix infections) but others more commonly on the outside (ectothrix infections). After 2 to 3 weeks’ growth the weakened hair breaks off, leaving a black dot at the follicle mouth as in endothrix infections or a grey spore-covered stump in the ectothrix infections.


Diagnosis: Diagnosis of ringworm is based on direct demonstration of fungal hyphae and arthrospores in the infected tissue by microscopy and by culturing the fungus in the laboratory.


Control and prevention: Dermatophyte infections are not easy to prevent. They produce arthrospores which are very resistant to environmental conditions and are transmitted by direct or indirect contact or through the hair. The infections due to dermatophytes, however, can be reduced by raised standards of hygiene in the home, at the school and hair dressing establishments by keeping feet dry and clean, and by avoiding the sharing or exchanging of caps, socks and underclothes.

Posted Date: 9/20/2012 3:10:47 AM | Location : United States







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