Dermatophytosis Flashcards
Importance Dermatophytosis
Dermatophytosis is a common contagious disease caused by fungi known as dermatophytes. Dermatophytes belong to a group of organisms that are able to break down the keratin in tissues such as the epidermis, hair, nails, feathers, horns and hooves. Most of these fungi reside in the soil and are involved in decomposition; however, the dermatophytes can infect living hosts. Some dermatophytes (anthropophilic species) are adapted to humans, and are usually transmitted from person to person. Others (zoophilic species) are adapted to animals. A few (geophilic) species normally live in the environment, but occasionally act as parasites. The zoophilic and geophilic species are sometimes transmitted from animals to people. It is also possible for humans to transmit anthropophilic dermatophytes to animals, although this seems to be uncommon.
Serious consequences are uncommon and infections can be self-limiting. However, the illness may be disfiguring and uncomfortable, especially when the lesions are widespread. Economic effects, such as damage to hides, are also important in livestock. Infrequently, dermatophytes may invade subcutaneous tissues and (very rarely) other sites, especially in immunocompromised hosts.
Etiology Dermatophytosis
Dermatophytosis is caused by pathogenic, keratin-digesting fungi in the genera Microsporum, Trichophyton and Epidermophyton. Members of Microsporum and Trichophyton cause illness in both humans and animals. E. floccosum is the only species of Epidermophyton known to cause disease, and it usually affects only people.
Zoophilic Microsporum species
- Microsporum canis,
- M. gallinae
- M. persicolor
Zoophilic members of Trichophyton
- Trichophyton equinum,
- T. bullosum,
- members of the T. mentagrophytes complex:
- T. simii
- T. verrucosum
Transmission Dermatophytosis
People and animals become infected by dermatophytes after contact with spores (conidia). Dermatophytes growing in a vertebrate host normally form only arthrospores (arthroconidia), asexual spores that develop within the hyphae.
In the environment (e.g., in laboratory culture), they can also produce microconidia and macroconidia, asexual spores that develop outside the hyphae.
Initially, the dermatophyte infects a growing hair or the stratum corneum of the skin. Hyphae spread in the hairs and keratinized skin, eventually developing infectious arthrospores. Anthropophilic and zoophilic dermatophytes are mainly transmitted between hosts by arthrospores in hairs or skin scales. Other asexual or sexual spores formed by the environmental stages may also be infectious.
Fomites such as brushes and clippers are important in transmission.
Spores may remain viable in suitable environments for up to 12-20 months, and some spores were also reported to persist for at least a year in salt water. Certain types of spores (e.g., microconidia) might be dispersed by airborne means.
Disinfection Dermatophytosis
Dermatophyte spores are susceptible to benzalkonium chloride, dilute chlorine bleach (1% sodium hypochlorite), enilconazole (0.2%), formaldehyde and some strong detergents. (e.g. Virkon-S®). Dermatophytes are also reported to be susceptible to iodophors, glutaraldehyde and phenolic compounds; however, some agents may have limited efficacy in “real life” environmental disinfection.
The mechanical removal of any material containing keratin, such as shed skin and hairs, facilitates disinfection. Vacuuming is considered to be the best method in many cases. Dusting may also be appropriate. After mechanical removal, washable surfaces should be cleaned thoroughly with detergent and water.
Dermatophytes are susceptible to high heat. Moist heat of 121°C, applied for at least 20 minutes, or dry heat of 165-170°C for 2 hours, are reported to be effective
Incubation Period Dermatophytosis in humans
The incubation period in humans is usually 1 to 2 weeks
Types of humans dermatophytoses
In humans, dermatophytoses are referred to as “tinea” infections, and are named according to the area of the body involved. Tinea capitis Tinea corporis Tinea faciei and tinea barbae Tinea cruris Tinea pedis and tinea manuum Tinea unguium
Tinea capitis
Tinea capitis, most often seen in children, is a dermatophyte infection of the hair and scalp. M. canis, a zoophilic species, is often isolated from tinea capitis cases in continental Europe; however, the anthropophilic dermatophyte T. tonsurans is currently responsible for most cases in the U.S. and the U.K.
Tinea corporis
Tinea corporis, or ringworm, occurs on the trunk and extremities. Anthropophilic organisms that cause tinea corporis include T. rubrum and E. floccosum, which infect the skin but not the hair, as well as dermatophytes also found in tinea capitis, such as M. audouinii, T. schoenleinii, T. tonsurans and T. violaceum. Various zoophilic organisms such as M. canis, T. verrucosum, Tequinum, T. mentagrophytes and M. persicolor, as well as the geophilic organisms M. gypseum and M. nanum can also cause this form of tinea.
Tinea faciei and tinea barbae
Tinea faciei and tinea barbae are dermatophyte infections occurring on the face. These infections are often acquired from pets or livestock, but they can also be caused by anthropophilic dermatophytes that originally affected other parts of the body such as the scalp or torso. The zoophilic organisms T. verrucosum (associated with cattle) and T. mentagrophytes can cause a very inflammatory form of this disease, with pustular folliculitis or kerions. Some other species that may be involved include M. canis, which is zoophilic, and the anthropophilic organisms T. tonsurans, T. megninii and T. violaceum Tinea faciei is seen on the nonbearded parts of the face. It can be caused by a number of anthropophilic and zoophilic organisms including T. rubrum, T. tonsurans, T. schoenleinii, T. mentagrophytes, M. canis and T. erinace
Diagnostic Tests Dermatophytosis
Dermatophytosis is usually diagnosed by a combination of direct microscopic examination, culture and Wood’s lamp examination. Biopsy (histopathology) tends to be used mainly when the presentation is unusual. A Wood’s lamp examination for fluorescence can be helpful in detecting some species of dermatophytes, such as M. canis and T. quinckeanum. Not all strains of these organisms exhibit fluorescence. Certain topical preparations may mask the fluorescence, and alcohol can either suppress it or cause non-specific fluorescence. Microscopic examination of skin scrapings or plucked hairs may reveal hyphae or arthroconidia. Hyphae rounding up into arthroconidia are diagnostic; however, hyphae alone could be caused by other fungi, including contaminants. Samples should be selected from the margins of active lesions, or from the entire lesion if there is no inflammatory margin. The best hairs to select are those that fluoresce under a Wood’s lamp, or are broken or scaly. Samples are usually cleared with potassium hydroxide (KOH) to help visualize the organism, although other clearing agents may be used. A longer clearing time can be helpful when the hair is thicker and more heavily pigmented, or if the sample is taken from a thick, crusted lesion. Various stains such as chlorazol black E, Parker blue-black ink, Swartz-Lamkin stain, Congo red stain or Giemsa can aid the visualization of fungal structures. In practices where fluorescence microscopy is available, calcofluor white staining can be used. Skin scrapings or plucked hair samples for culture should be taken from active lesions, as for microscopic examination. Nail beds and claws are cultured in cases of onchomycosis. Swabbing dermatophyte lesions first with alcohol may decrease contaminants, especially in livestock. Additional collection methods, which are especially helpful in asymptomatic animals suspected of being carriers, including brushing the fur with a disinfected toothbrush or other small brush, or rubbing it with a sterile piece of carpet
Treatment Dermatophytoses in humans
Dermatophyte infections are treated with a variety of topical and oral antifungal drugs. In immunocompetent patients, topical agents are usually effective in cases that are limited to glabrous skin (e.g., tinea corporis, tinea cruris, and tinea pedis). Systemic (oral) antifungal drugs may be necessary in severe cases, or if the infection does not respond to treatment or reappears. Topical agents are ineffective against organisms that infect the hairs. These infections are usually treated with systemic antifungals, although topical lotions or shampoos are sometimes used concurrently to decrease shedding of fungi and spores, or to help treat kerions. Topical agents may also be used to treat asymptomatic carriers or prevent reinfection. Tinea capitis is reported to be more difficult to treat when it is caused by M. canis than Trichophyton spp. and may not respond as well to some drugs. Dermatophyte infections of the nails (tinea unguium) are usually treated with oral antifungal drugs. Concurrent therapies may include debridement of the nail or nail avulsion.
Reptiles affected by dermatophytes
Reptiles are not usually affected by the dermatophytes of mammals or birds; however, rare clinical cases associated with Trichophyton spp. have been reported in lizards, snakes (green anacondas, Eunectes murinus) and an olive ridley sea turtle (Lepidochelys olivacea)
Zoophilic dermatophytes
Microsporum canis (Microsporum equinum) Microsporum gallinae Microsporum persicolor Trichophyton bullosum Trichophyton equinum Trichophyton langeronii Trichophyton mentagrophytes T. mentagrophytes var. erinacei (or T. erinacei) T. mentagrophytes var. granulosum T. mentagrophytes var. quinckeanum has traditionally been associated with mice. However, it is considered to be a camel-associated isolate in the ITS genetic taxonomy. Trichophyton sarkisovii seems to be adapted to camels. Trichophyton simii affects nonhuman primates, but some authors believe the primary host is a grounddwelling animal. It has also been reported from other mammals and birds. Trichophyton verrucosum, which is adapted to cattle, is the most important dermatophyte in this species. T. verrucosum readily infects other hosts, especially sheep, goats, South American camelids and camels, but it can also be found occasionally in other species such as horses, donkeys, pigs, dogs and rabbits. T. verrucosum var. autotrophicum affects sheep
