Autochthonous Blastomyces dermatitidis, India

Abstract

Blastomyces spp. fungi, the causal agent of blastomycosis, are common in North America but do occur in other areas of the world. The most prevalent pathogen in the genus is B. dermatitidis. Most B. dermatitidis isolates originate from North America, but there are sporadic reports of B. dermatitidis recovery from Africa and Asia. High-quality reports that incorporate genetic information about the fungus outside North America have been rare. Genome sequencing of 3 fungal isolates from patients in India with chronic respiratory diseases revealed that the isolates belong to a genetically differentiated lineage of B. dermatitidis. Because the patients had no history of traveling outside of Asia, blastomycosis was most likely autochthonously acquired, which suggests a local population of B. dermatitidis. Our results suggest the endemic range of B. dermatitidis is larger than previously thought, calling for a reassessment of the geographic range of different agents of endemic mycoses.

Keywords: Blastomyces; Blastomyces dermatitidis; India; blastomycosis; fungal infections; fungi; phylogenetics.

Figure 1

Autochthonous Blastomyces dermatitidis biopsy and culture findings from a patient in India. Microscopic characteristics are consistent with Blastomyces dermatitidis. A) Gomori’s methenamine silver–stained section of a subcutaneous nodule biopsy obtained from sternum region showing a large, thick-walled broad-based budding yeast cell (white arrows) typical of Blastomyces. B) Lactophenol cotton blue mount of the 6-day-old yeast form on pea seed agar at 37°C, showing numerous large thick-walled and broad-based budding yeast cells (white arrow). Original magnification is ×100.

Figure 2

Genetic differentiation between 3 Blastomyces dermatitidis from India and other dimorphic fungi. A) PC analyses showing the 3 isolates from India (light blue) clustering with B. dermatitidis (dark blue) in PC 1 and PC2. The broken stick model (inset) demonstrates the first 3 PCs are significant. B) PC analyses showing the 3 isolates from India (light blue) clustering with B. dermatitidis (dark blue) in PC 32 and PC 3. C) Rooted phylogram for autochthonous B. dermatitidis isolates from India and the genetic relationships with other Blastomyces fungi. A maximum-likelihood tree derived from genomewide concatenated markers using the B. dermatitidis reference genome suggests a close phylogenetic relationship between B. dermatitidis and the 3 isolates from India. The star shows the node leading to the B. dermatitidis lineage from India. Scale bar represents the number of substitutions per site. PC, principal component.