Cat-transmitted Sporotrichosis: A Clonal Household Outbreak with Atypical Human and Canine Disease Due to Sporothrix brasiliensis

The epidemiology of sporotrichosis, a neglected mycosis caused by Sporothrix species, has shifted in South America from classic sapronosis to urban zoonosis driven by feline transmission of the hypervirulent Sporothrix brasiliensis. However, the fine-scale transmission dynamics of this epidemic, particularly within households, remain poorly understood. Here, we dissect an intrafamilial outbreak in Brazil involving a cat (the index case), a dog, and three humans. High-resolution genotyping using 15 microsatellite markers revealed a clonal transmission event, with isolates from the index cat and human patients being genetically indistinguishable (100% similarity). Multiple bioinformatic analyses, including principal component analysis, minimum spanning tree, and self-organizing maps, corroborated these findings. Strikingly, this outbreak strain produced markedly divergent clinical phenotypes, including Parinaud's oculoglandular syndrome in an 8-year-old girl, a fixed cutaneous lesion in her 5-year-old brother, and a disseminated maculopapular exanthem in her 28-year-old mother. This clinical pleomorphism from a genetically invariant pathogen unequivocally demonstrates the pivotal role of host factors, such as age, immune response, and inoculation site, in dictating disease manifestation. Furthermore, all outbreak isolates belonged to the MAT1-1 idiomorph, a finding that challenges the paradigm of a monolithic epidemic dominated by MAT1-2 strains and suggests a more complex, polycentric expansion of S. brasiliensis. Overall, this study provides strong evidence of zoonotic transmission and highlights how host determinants, not pathogen variability, shape the clinical diversity of sporotrichosis.