Piedraia
Piedraia is a genus of fungi notable for its association with black piedra, an infection affecting human scalp hair, primarily caused by the species Piedraia hortae. This fungus thrives in humid tropical regions, with a significant presence in Central and South America, as well as parts of Southeast Asia and Africa. Black piedra manifests as small, dark nodules on the hair, which can give a gritty feel or produce a metallic sound when brushed. Unlike some other fungal infections, it typically does not cause itching or discomfort, which can lead to underreporting of cases. Transmission often occurs through shared hair-care tools, and the infection is more prevalent among young adults, particularly males.
Another species within the genus, Piedraia quintanilhae, has been found in the hair of chimpanzees but is not known to cause human infections. Treatment for black piedra typically involves shaving or clipping the infected hair, although topical antifungal agents may also be employed. The lack of significant symptoms may result in individuals remaining untreated for extended periods. Despite its relatively benign nature, in immunocompromised individuals, the infection can lead to more serious complications. Understanding the characteristics and transmission of Piedraia is essential for managing and preventing its spread, particularly in tropical areas where it is more common.
Piedraia
- TRANSMISSION ROUTE: Direct contact
Definition
Piedraia is a genus of fungi of which one species, P. hortae, causes black piedra, an infection of hair on the scalp.
Natural Habitat and Features
Piedraia is found in soils worldwide but most often in humid tropical regions of Central America, South America, Southeast Asia, and Africa. The highest concentration is in South America. In Brazil, it is found almost exclusively in regions with a mean average temperature of 78.8° Fahrenheit (26° Celsius), average yearly rainfall of 99 inches (30 meters), and average humidity of 80 percent. International travel and migration have led to reports of isolated detection and sporadic outbreaks of infection in nontropical areas.
Piedraia is a saprotrophic mold, meaning it lives on decaying material and dead tissue. It is also found in organisms in stagnant water and on crops.
There are two species of Piedraia. Quintanilhae has been isolated from the hair of chimpanzees in central Africa, but no cases of human infection have been reported. Hortae causes an infection of human scalp hair and can occur anywhere on the scalp. It occurs most often on the front and top of the scalp, near the forehead. Infection of hair elsewhere on the body, such as beard, mustache, or pubic hairs, almost never occurs.
Transmission of hortae and its infection between persons occurs primarily through the use of shared haircare tools and products. In the tropics of Brazil, some indigenous people use plant oils to dress the hair, which can introduce hortae to the scalp and encourage its transmission. Infection is most common among young adults, with a slightly higher preponderance among males.
In persons infected with hortae, the scalp reveals small, firm, tightly packed, irregular (oval or elongated), dark brown to black nodules that are composed of fungal matter attached to the sides and tip of hairs. The genus name derives from the Spanish word piedra, which means “stone.” The nodules look and feel like tiny stones. Each infected hair contains from four to eight or more nodules, each 0.03 to 0.07 inches (1 to 2 millimeters) in diameter. Infected hair may have a gritty feel or give off a metallic sound when brushed. The fungus remains on the hair surface without penetrating the hair. Unlike other fungal scalp infections, such as tinea capitis, Piedraia-infected hairs usually do not break off.
Hortae is cultured in a standard media such as Sabouraud’s agar at room temperature for two to three weeks. The colonies are distinguished by thick-walled, branched, dark-colored septate hyphae. The front is folded and velvety with a flat margin. A colony may remain glabrous (smooth and hairless) or become covered with short, light, airy hyphae. From the reverse, the colony is black.
Under microscopic examination, colonies of hortae reveal packed masses of dark septate hyphae (segmented filaments). Also seen are asci, which are thin-walled sacs that contain sexual spores called ascospores. The ascospores of hortae are hyaline (transparent), one-celled, and tapered toward both ends to form whiplike appendages. The ascospores of quintanilhae do not have these appendages.
Pathogenicity and Clinical Significance
Hortae is the cause of black piedra, an infection of scalp hair. White piedra, in contrast, is caused by five species of the fungus Trichosporon and infects hair on the scalp, face, and pubic regions. Black piedra does not cause itching or any other symptoms. Because of this, and because the nodules, although plentiful, are small, the infection may be underreported. Persons may be infected for many years without seeking treatment.
Drug Susceptibility
There is scant data on drug susceptibility to infection caused by hortae. No standard method for testing or comparison has been developed, although terbinafine has been shown to be effective. Shaving or clipping infected hair may resolve the infection, and it is the standard treatment of choice. In persons who are reluctant to have hair removed, a topical antifungal agent (cream, ointment, or solution) may be used instead. A topical agent may also be applied to the scalp in conjunction with hair removal. Common choices of topical agents are a salicylic acid preparation or an imidazole cream. Oral ketoconazole or terbinafine may also be used. Medical treatment without hair removal has a greater risk of relapse than does hair removal or combination therapy alone. In immunocompromised persons, infection may spread and produce purplish hemorrhagic nodules on the skin.
Bibliography
Berger, T. G. “Dermatologic Disorders.” In Current Medical Diagnosis and Treatment 2011, edited by Stephen J. McPhee and Maxine A. Papadakis. 50th ed. New York: McGraw-Hill Medical, 2011.
Goldberg, Lynne. "Piedra." UpToDate, 24 Jan. 2023, www.uptodate.com/contents/piedra/print. Accessed 5 Feb. 2025.
Richardson, Malcolm D., and David W. Warnock. Fungal Infection: Diagnosis and Management. New ed. Malden, Mass.: Wiley-Blackwell, 2010.
Schwartz, R. “Superficial Fungal Infections.” The Lancet 364 (2004): 1173-1182.