Dermatophytes

• Dermatophytes are molds that infect only keratinized epidermal structures such as superficial skin, hair, feathers, horns, hooves, claws, and nails.
• Infections caused by dermatophytes are referred to as ringworm or dermatophytosis.
• Other yeast and saprophytic fungi can cause cutaneous infections mimicking infections caused by dermatophytes, hence the generic term dermatomycosis is used to represent all fungal infections of skin.
• Dermatophytes produce septate, branching hyphae collectively called mycelium in the parasitic state.
• The asexual reproductive units (conidia) of dermatophytes are found in the aerial mycelium.
• Hair is not attacked, there is no fluorescence under Wood's lamp.
• Reports of infections in domestic animals are essentially nonexistent, but there are some of isolations from mice and from a dog.
• Colonies are usually greenish yellow, olive, or khaki, powdery, with radial furrows and a yellow to tan reverse.
• Dermatophytes can develop microconidia and macroconidia.
• Hyphal characteristics of dermatophytes include spirals, nodules, rackets, chandeliers, and chlamydioconidia (chlamydiospore).
• Dermatophytes have three important genera: Microsporum, Trichopyton in animals, and Epidermophyton primarily in humans.
• Traditional culture medium for dermatophytes is Saboroud’s dextrose agar.
• Some dermatophytes in skin and hair produce green fluorescence visible under ultraviolet light (366nm, Wood’s light) (Microsporum canis).
• Transmission of dermatophytes can be both direct and indirect.
• Pathogenesis of dermatophytes involves proteolytic enzymes (elastase, collagenase, keratinase) and inflammation with high iron influx, iron is available for fungi growth.
• Infectious unit (Conidium) of dermatophytes is a defect in stratum corneum, germ tube, hyphae branch development, hyphae develops into arthroconidia, growth pattern in hairless skin, hair invasion (ringworm), germination with spore formation, hyphal strands growth along the hair.
• Microsporum canis produces a yellow-greenish fluorescence in hair, and it is associated with small ectothrix spores.
• Humoral immunity has a limited role in protection against dermatophytes, while cellular immunity is decisive in protection and recovery.
• Treatment for dermatophyte infections can involve topical and systemic medications such as miconazole, econozole, ketoconazole, itraconazole, and thiobendazole.
• Accumulation of arthroconidia inside the hair shaft is referred to as endothrix.
• Microsporum gypseum is a geophilic dermatophyte that often attacks humans and animals.
• Accumulation of arthroconidia outside the hair shaft is referred to as ectothrix.
• Laboratory diagnosis of dermatophyte infections can involve testing for fluorescence (Wood’s lamp), observing hyphae and arthroconidia in skin scrapings and hair samples, using lactophenol cotton blue stain, and culturing on Sabouraud’s dextrose agar.
• A vaccine for Microsporum verrucosum, a type of dermatophyte, is used in Europe in cattle.
• Antibody-mediated and cellular-mediated hypersensitivity may occur in dermatophyte infections.
• Crowding of animals increases the prevalence of dermatophyte infections.
• Microsporum canis can infect numerous animals including bats, canaries, cattle, chimpanzees, chinchilla, donkeys, foxes, gibbons, goats, gorillas, guinea pigs, horses, jaguars, lions, lynx, monkeys, orangutans, pigs, rabbits, sheep, tigers, and others.
• Betadine and chlorhexidine are used as lotions and ointments for control of dermatophyte infections.
• Dermatophytes have a global distribution.
• Young animals are more susceptible to dermatophyte infections.
• The head is the most common site of infection with Microsporum canis, causing areas of alopecia around the nose, eyes, and ears.
• Microsporum canis is the etiologic agent of roughly 98% of the cases of feline ringworm and about 70% of the cases of canine ringworm in North America.
• Dermatophytes, such as Microsporum canis, normally regress spontaneously, unless secondary bacterial infections is present.
• Microsporum canis is generally regarded as a zoophilic dermatophyte, but it often attacks humans.
• Trichophyton verrucosum produces very large ectothrix spores in chains.
• Trichophyton equinum is a zoophilic species that rarely attacks humans; it causes ringworm in horses and donkeys, and occasionally, in dogs.
• Trichophyton has been isolated from baboons, buffaloes, cats, cattle, chickens, chimpanzees, chinchillas, dogs, fowl, guinea pigs, horses, leopards, monkeys, mice, parrots, rabbits, rats, squirrels, tapirs, tigers, and other species.
• Trichophyton verrucosum is a zoophilic dermatophyte often attacks humans; it has been isolated from buffalo, canaries, cats, cattle, dogs, donkeys, dromedaries, fowl, goats, horses, mules, pigs, sheep, and zebu.
• Infected hair in humans is not fluorescent, but some fluorescence has been noted in cattle.
• Epidermophyton floccosum is the only widely accepted species in this genus, E. floccosum infects only skin and nails, and it is virtually confined to humans.