Tinea Capitis Caused by Trichophyton rubrum Mimicking Favus

Comment

Favus, or tinea favosa, is a chronic inflammatory dermatophyte infection of the scalp, less commonly involving the skin and nails.² The classic lesion is termed a scutulum or godet consisting of concave, cup-shaped, yellow crusts typically pierced by a single hair shaft.¹ With an increase in size, the scutula may become confluent. Alopecia commonly results and infected patients may exude a “cheesy” or “mousy” odor from the lesions.³ Sabouraud¹ delineated 3 clinical presentations of favus: (1) favus pityroide, the most common type consisting of a seborrheic dermatitis–like picture and scutula; (2) favus impetigoide, exhibiting honey-colored crusts reminiscent of impetigo but without appreciable scutula; and (3) favus papyroide, the rarest variant, demonstrating a dry, gray, parchmentlike crust pierced by hair shafts overlying an eroded erythematous scalp.

Favus usually is acquired in childhood or adolescence and often persists into adulthood.³ It is transmitted directly by hairs, infected keratinocytes, and fomites. Child-to-child transmission is much less common than other forms of TC.⁴ The responsible organism is almost always Trichophyton schoenleinii, with rare cases of Trichophyton violaceum, Trichophyton verrucosum, Trichophyton mentagrophytes var quinckeanum, Microsporum canis, and Microsporum gypseum having been reported.^2,5,6 This anthropophilic dermatophyte infects only humans, is capable of surviving in the same dwelling space for generations, and is believed to require prolonged exposure for transmission. Trichophyton schoenleinii was the predominant infectious cause of TC in eastern Europe in the 19th and early 20th centuries, but its incidence has dramatically declined in the last 50 years.⁷ A survey conducted in 1997 and published in 2001 of TC that was culture-positive for T schoenleinii in 19 European countries found only 3 cases among 3671 isolates (0.08%).⁸ Between 1980 and 2005, no cases were reported in the British Isles.⁹ Currently, favus generally is found in impoverished geographic regions with poor hygiene, malnutrition, and limited access to health care; however, endemic foci in Kentucky, Quebec, and Montreal have been reported in North America.¹⁰ Although favus rarely resolves spontaneously, T schoenleinii was eradicated in most of the world with the introduction of griseofulvin in 1958.⁷ Terbinafine and itraconazole are currently the drugs of choice for therapy.¹⁰

Tinea capitis is the most common fungal infection in children, with 1 in 20 US children displaying evidence of overt infection.¹¹ Infection in adults is rare and most affected patients typically display serious illnesses with concomitant immune compromise.¹² Only 3% to 5% of cases arise in patients older than 20 years.¹³ Adult hair appears to be relatively resistant to dermatophyte infection, probably from the fungistatic properties of long-chain fatty acids found in sebum.¹³ Tinea capitis in adults usually occurs in postmenopausal women, presumably from involution of sebaceous glands associated with declining estrogen levels. Patients typically exhibit erythematous scaly patches with central clearing, alopecia, varying degrees of inflammation, and few pustules, though exudative and heavily inflammatory lesions also have been described.¹⁴

In the current case, TC was not raised in the differential diagnosis. Regardless, given that scaly red patches and papules of the scalp may represent a dermatophyte infection in this patient population, clinicians are encouraged to consider this possibility. Transmission is by direct human-to-human contact and contact with objects containing fomites including brushes, combs, bedding, clothing, toys, furniture, and telephones.¹⁵ It is frequently spread among family members and classmates.¹⁶

Prior to World War II, most cases of TC in the United States were due to M canis, with Microsporum audouinii becoming more prevalent until the 1960s and 1970s when Trichophyton tonsurans began surging in incidence.^12,17 Currently, the latter organism is responsible for more than 95% of TC cases in the United States.¹⁸Microsporum canis is the main causative species in Europe but varies widely by country. In the Middle East and Africa, T violaceum is responsible for many infections.

Trichophyton rubrum–associated TC appears to be a rare occurrence. A global study in 1995 noted that less than 1% of TC cases were due to T rubrum infection, most having been described in emerging nations.¹² A meta-analysis of 9 studies from developed countries found only 9 of 10,145 cases of TC with a culture positive for T rubrum.¹⁴ In adults, infected patients typically exhibit either evidence of a concomitant fungal infection of the skin and/or nails or health conditions with impaired immunity, whereas in children, interfamilial spread appears more common.¹¹