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Epidemiological cut off values (ECVs) for species of Sporothrix - a step to defining resistance

September 26 2017

Sporotrichosisis is a widespread endemic fungal infection of the tropical and subtropical regions affecting both humans and animals. It is particularly prevalent in Brazil, India, Mexico, Peru, the United States, Japan and South Africa.

Sporotrichosis is usually responsive to itraconazole or amphotericin B, but expansion of species beyond Sporothrix schenckii to include S. globosa, S. brasiliensis, S. mexicana and S. luriei (Sporothrix complex), raises the possibility of resistance in certain species.

In a recent study by Ana Espinel-Ingroff et al, in vitro susceptibility testing by broth microdilution M38-A2 method for 875 strains of S. schenckii, S. brasiliensis, S. globosa and S.mexicana collected from 17 independent laboratories was done. The CLSI M38-A2 is a reference method for testing the susceptibility of filamentous fungi that cause invasive and cutaneous fungal infections to antifungal agents. The wild type susceptibility (MIC/MEC) distributions for 5 antifungals in clinical use were determined and the ECVs were obtained.

In rank order, terbinafine had the lowest ECVs, then itraconazole, posaconazole and amphotericin B with voriconazole having the highest, consistent with intrinsic resistance.

The species specific CLSI ECVs for S. schenkii and S. brasiliensis respectively, were proposed to be 4 and 4 µg/ml for amphotericin B, 2 and 2µg/ml for itraconazole, 2 and 2 µg/ml for posaconazole and 64 and 32µg/ml for voriconazole. The ketoconazole and terbinafine ECVs for S. brasiliensis were 2 and 0.12µg/ml respectively. The data was found to be insufficient for the calculation of ketoconazole and terbinafine ECVs for S. schenkii as well as for the ECVs for S. globosa and S. mexicana versus any of the stated antifungal agents

Sporotrichosis mainly occurs following a traumatic inoculation of the dermis with spores or mycelial fragments of the fungus Sporothrix complex, or contact with cats. Direct infection occurs in humans through wounding caused by plants or indirectly through contact of an existing wound with contaminated soil. The typical clinical manifestation include one or more nodular ulcerous scabby lesions on the limbs, sometimes on the face or body.

Sporotrichosis in a cat: Picture courtesy of Dr Flavio de Queiroz Telles.