|Classification and external resources|
Ringworm on the arm
Ringworm (dermatophytosis) is an clinical condition caused by fungal infection of the skin in humans, companion animals such as cats, and domesticated animals such as sheep and cattle. It is caused by fungi of several different species. The fungi that cause parasitic infection (dermatophytes) feed on keratin, the material found in the outer layer of skin, hair, and nails. These fungi thrive on skin that is warm and moist, but may also survive directly on the outsides of hair shafts, or in their interiors. In pets, the fungi responsible for the disease survive in skin and on the outer surface of hairs.
This condition 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.
It has been estimated that in current times, up to twenty percent of the population is infected by ringworm or one of the other dermatophytoses. It is especially common among people who play sports, wrestling in particular. Wrestlers with ringworm may be disqualified.
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 will grow without typical features like a distinctive raised border.
Advice often given to prevent ringworm includes:
Ringworm in pets may often be asymptomatic, resulting in a carrier condition which either infects other pets, or shows disease only when the companion animal develops an immunosuppressive condition. Circular bare patches on the skin suggest the diagnosis but no lesion is truly specific to the fungus. Three species of fungi cause 95% of ringworm in pets. These are Microsporum canis, Microsporum gypseum, and Trichophyton mentagrophytes.
Veterinarians have several tests to identify ringworm infection and identify species:
Woods Test This is a black light (ultraviolet light) with a magnifying lens. Only 50% of Microsporum canis will show up as an apple-green fluorescence on hair shafts, under the black 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 vet 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 there is ringworm 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 vet 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. Microsporum 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. Trichophyton mentagrophytes has a major reservoir in rodents, but can also infect pet rabbits, dogs and horses. Microsporum gypseum is a soil organism and is often contracted from gardens and other such places. Besides humans, it may infects rodents, dogs, cats, horses, cattle, and swine. 
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.
In more severe cases or where there is scalp ringworm, systemic treatment with oral medications may be given.
To prevent spreading the infection, lesions should not be touched, and good hygiene maintained with washing of hands and the body.
Because of longer hair shafts in pets, the area of infection and possibly all longer hair of the pet must be clipped to decrease the load of fungal spores clinging to the pet's hair. However, close shaving is usually not done because nicking the skin facilitates further skin infection.
Washing of household hard surfaces with 1:10 household hypochlorite bleach solution is effective in killing spores.
Pet hair must be rigorously removed from all household surfaces, and then the vacuum cleaner bag (and often 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.
RINGWORM (or Tinea Tonsurans), a disease of the scalp (especially common within the tropics); it consists of bald patches, usually round, and varying in diameter from half. an inch up to several inches, the surface showing the broken stumps of hairs and a fine whitish powdering of desquamated epidermic scales. In scrofulous subjects matter is sometimes produced, which forms crusts, or glues the hair together, or otherwise obscures the characteristic appearance. The disease is due to a parasite, Trichophyton tonsurans, which exists mostly in the form of innumerable spores (with hardly any mycelium), and is most abundant within the substance of the hairs, especially at their roots. If a piece of the hair near the root be soaked for a time in dilute liquor potassae and pressed flat under a cover-glass, the microscope will show it to be occupied by long rows of minute oval spores, very uniform in size, and each bearing a nucleus.
The same fungus sometimes attacks the hairs of the beard, producing a disease called "sycosis." Sometimes it invades the hairless regions of skin, forming "tinea circinata"; circular patches of skin disease, if they be sharply defined by a margin of papules or vesicles, may be suspected of depending on the tinea-fungus. Interesting varieties of tinea are found in some of the Pacific and East Indian islands. Among the best remedial agents are various mercurial preparations. But in modern practice much success has been found in X-raying the patch in order to remove the dead and diseased hairs, thus leaving a free channel for the passage of antiseptic applications to the follicles. The exposures are followed by inunction of a mercurial preparation or of a lotion of tincture of iodine with methylated spirit.
See also Favus.