Primary cutaneous blastomycosis in a Chinese-Canadian girl following an insect bite

BACKGROUND: Blastomycosis is a disease caused by the dimorphic fungus Blastomyces species. This infection occurs most frequently among individuals residing in the midwestern, southeastern, and south-central United States, as well as in Canadian provinces bordering the Great Lakes and the St. Lawrence Seaway. Sporadic cases have been reported in China, the diagnosis of imported cases remains a significant challenge. CASE PRESENTATION: We report a diagnostically challenging case of primary cutaneous blastomycosis in a Chinese-Canadian girl, presenting as two nodules of different sizes on her left lower leg and left arm. The patient was initially diagnosed with insect bite dermatitis in Canada, which showed poor response to treatment. Direct microscopic examination revealed large spherical yeast cells, some with broad-based budding, initially interpreted as protothecosis or coccidioidomycosis. However, metagenomic next-generation sequencing (mNGS) indicated Blastomyces dermatitidis, which was subsequently confirmed by fungal culture and ITS sequencing. The patient was prescribed oral itraconazole at 50 mg/day and has since returned to Canada to continue treatment. CONCLUSIONS: For diagnosing infections caused by highly pathogenic pathogens such as imported blastomycosis, the combination of metagenomic next-generation sequencing (mNGS) and direct microscopic examination represents a valuable diagnostic approach.