Ringworm on a human leg.
|Classification and external resources|
The fungi that cause parasitic infection, collectively dermatophytes, feed on keratin, the material found in the outer layer of skin, hair, and nails. These fungi thrive on warm and moist skin, but may also survive directly on the outsides of hair shafts or in their interiors. In pets, the fungus responsible for the disease survives in skin and on the outer surface of hairs. It is especially common among people who play sports involving skin-to-skin contact, wrestling in particular. Wrestlers with ringworm may be withheld from competition until their skin condition is deemed noninfectious by the proper authorities.
Currently, up to 20% of the population may be infected by dermatophytes at any given time. The most common term for the infection, "ringworm", is a misnomer, since the condition is caused by fungi of several different species and not by parasitic worms.
A number of different species of fungi are involved in dermatophytosis. Dermatophytes of the genera Trichophyton and Microsporum are the most common causative agents. These fungi attack various parts of the body and lead to the conditions listed below. The Latin names are for the conditions (disease patterns), not the agents that cause them. The disease patterns below identify the type of fungus that causes them only in the cases listed:
- Tinea pedis (athlete's foot) – fungal infection of the feet
- Tinea unguium – fungal infection of the fingernails and toenails, and the nail bed
- Tinea corporis – fungal infection of the arms, legs, and trunk
- Tinea cruris (jock itch) – fungal infection of the groin area
- Tinea manuum – fungal infection of the hands and palm area
- Tinea capitis – fungal infection of the scalp and hair
- Tinea barbae – fungal infestation of facial hair
- Tinea faciei (face fungus) – fungal infection of the face
- Other superficial mycoses (not classic ringworm, since not caused by dermatophytes)
Signs and symptoms
Infections on the body may give rise to typical enlarging raised red rings of ringworm. Infection on the skin of the feet may cause athlete's foot and in the groin, jock itch. Involvement of the nails is termed onychomycosis, and they may thicken, discolour, and finally crumble and fall off. They are common in most adult people, with up to 20% of the population having one of these infections at any given moment.
Animals such as dogs and cats can also be affected by ringworm, and the disease can be transmitted between animals and humans, making it a zoonotic disease.
Specific signs can be:
- red, scaly, itchy or raised patches
- patches may be redder on outside edges or resemble a ring
- patches that begin to ooze or develop blister
- bald patches may develop, when the scalp is affected
- nails may thicken, discolour or begin to crack
Fungi thrive in moist, warm areas, such as locker rooms, tanning beds, swimming pools, and skin folds; accordingly, those that cause dermatophytosis may be spread by using exercise machines that have not been disinfected after use, or by sharing towels, clothing, or footwear, such as rental bowling shoes, ski boots, ice skates or roller skates.
Advice often given includes:
- Avoid sharing clothing, sports equipment, towels, or sheets.
- Wash clothes in hot water with fungicidal soap after suspected exposure to ringworm.
- Avoid walking barefoot; instead wear appropriate protective shoes in locker rooms and sandals at the beach.
- Avoid touching pets with bald spots, as they are often carriers of the fungus.
As of 2016, no approved human vaccine exist against Dermatophytosis. For horses, dogs and cats there is available an approved inactivated vaccine called Insol Dermatophyton (Boehringer Ingelheim) which provides time-limited protection against several trichophyton and microsporum fungal strains.
Antifungal treatments include topical agents such as miconazole, terbinafine, clotrimazole, ketoconazole, or tolnaftate applied twice daily until symptoms resolve — usually within one or two weeks. Topical treatments should then be continued for a further 7 days after resolution of visible symptoms to prevent recurrence. The total duration of treatment is therefore generally two weeks, but may be as long as three.
To prevent spreading the infection, lesions should not be touched, and good hygiene maintained with washing of hands and the body.
Misdiagnosis and treatment of ringworm with a topical steroid, a standard treatment of the superficially similar pityriasis rosea, can result in tinea incognito, a condition where ringworm fungus grows without typical features, such as a distinctive raised border.
Dermatophytosis has been prevalent since before 1906, at which time ringworm was treated with compounds of mercury or sometimes sulfur or iodine. Hairy areas of skin were considered too difficult to treat, so the scalp was treated with X-rays and followed up with antiparasitic medication. Another treatment from around the same time was application of Araroba powder.
Ringworm caused by Trichophyton verrucosum is a frequent clinical condition in cattle. Young animals are more frequently affected. The lesions are located on the head, neck, tail, and perineum. The typical lesion is a round, whitish crust. Multiple lesions may coalesce in "map-like" appearance.
Clinical dermatophytosis is also diagnosed in sheep, dogs, cats, and horses. Causative agents, besides Trichophyton verrucosum, are T. mentagrophytes, T. equinum, Microsporum gypseum, M. canis, and M. nanum.
Ringworm in pets may often be asymptomatic, resulting in a carrier condition which infects other pets. In some cases, the disease only appears when the animal develops an immunodeficiency condition. Circular bare patches on the skin suggest the diagnosis, but no lesion is truly specific to the fungus. Similar patches may result from allergies, sarcoptic mange, and other conditions. Three species of fungi cause 95% of dermatophytosis in pets: these are Microsporum canis, Microsporum gypseum, and Trichophyton mentagrophytes.
Veterinarians have several tests to identify ringworm infection and identify the fungal species that cause it:
Woods test: This is an ultraviolet light with a magnifying lens. Only 50% of M. canis will show up as an apple-green fluorescence on hair shafts, under the UV light. The other fungi do not show. The fluorescent material is not the fungus itself (which does not fluoresce), but rather an excretory product of the fungus which sticks to hairs. Infected skin does not fluoresce.
Microscopic test: The veterinarian takes hairs from around the infected area and places them in a staining solution to view under the microscope. Fungal spores may be viewed directly on hair shafts. This technique identifies a fungal infection in about 40%–70% of the infections, but cannot identify the species of dermatophyte.
Culture test: This is the most effective, but also the most time-consuming, way to determine if ringworm is on a pet. In this test, the veterinarian collects hairs from the pet, or else collects fungal spores from the pet's hair with a toothbrush, or other instrument, and inoculates fungal media for culture. These cultures can be brushed with transparent tape and then read by the veterinarian using a microscope, or can be sent to a pathological lab. The three common types of fungi which commonly cause pet ringworm can be identified by their characteristic spores. These are different-appearing macroconidia in the two common species of Microspora, and typical microconidia in Trichophyton infections.
Identifying the species of fungi involved in pet infections can be helpful in controlling the source of infection. M. canis, despite its name, occurs more commonly in domestic cats, and 98% of cat infections are with this organism. It can also infect dogs and humans, however. T. mentagrophytes has a major reservoir in rodents, but can also infect pet rabbits, dogs, and horses. M. gypseum is a soil organism and is often contracted from gardens and other such places. Besides humans, it may infect rodents, dogs, cats, horses, cattle, and swine.
Treatment requires both systemic oral treatment with most of the same drugs used in humans—terbinafine, fluconazole, or itraconazole—as well as a topical "dip" therapy.
Because of the usually longer hair shafts in pets compared to those of humans, the area of infection and possibly all of the longer hair of the pet must be clipped to decrease the load of fungal spores clinging to the pet's hair shafts. However, close shaving is usually not done because nicking the skin facilitates further skin infection.
Washing of household hard surfaces with 1:10 household sodium hypochlorite bleach solution is effective in killing spores, but it is too irritating to be used directly on hair and skin.
Pet hair must be rigorously removed from all household surfaces, and then the vacuum cleaner bag, and perhaps even the vacuum cleaner itself, discarded when this has been done repeatedly. Removal of all hair is important, since spores may survive 12 months or even as long as two years on hair clinging to surfaces.
In bovines, an infestation is difficult to cure, as systemic treatment is uneconomical. Local treatment with iodine compounds is time-consuming, as it needs scraping of crusty lesions. Moreover, it must be carefully conducted using gloves, because of a possible infestation of the worker.
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