Microsporum canis Antifungal Susceptibility Testing: Methods, Variability, and Clinical Implications Original paper
Researched by:
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Karen Pendergrass
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Karen Pendergrass
Karen Pendergrass is a microbiome researcher specializing in microbiome-targeted interventions (MBTIs). She systematically analyzes scientific literature to identify microbial patterns, develop hypotheses, and validate interventions. As the founder of the Microbiome Signatures Database, she bridges microbiome research with clinical practice. In 2012, based on her own investigative research, she became the first documented case of FMT for Celiac Disease—four years before the first published case study.
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Karen Pendergrass
Researched by:
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Karen Pendergrass
ID
Karen Pendergrass
Microbiome Signatures identifies and validates condition-specific microbiome shifts and interventions to accelerate clinical translation. Our multidisciplinary team supports clinicians, researchers, and innovators in turning microbiome science into actionable medicine.
Karen Pendergrass
What was reviewed?
This review article evaluated both the conventional therapeutic approaches and Microsporum canis antifungal susceptibility testing methods. Microsporum canis is a zoonotic dermatophyte responsible for superficial skin infections in animals and humans. The review examined the in vivo efficacy of oral and topical antifungal agents, including griseofulvin (GRI), terbinafine (TER), and azoles such as fluconazole (FLZ), itraconazole (ITZ), and ketoconazole (KTZ). The article also critically assessed in vitro antifungal susceptibility testing methodologies—broth microdilution, E-test, and disk diffusion—highlighting methodological inconsistencies and their influence on minimum inhibitory concentration (MIC) values, which complicate clinical interpretation and standardization.
Who was reviewed?
The review synthesized findings from studies involving both human and animal subjects infected with M. canis, alongside dermatophyte strains tested in vitro. Microsporum canis antifungal susceptibility testing data were drawn from clinical trials, interlaboratory susceptibility testing evaluations, and experimental infections in cats. The authors also referenced resistance data across various fungal genera (e.g., Trichophyton rubrum, T. mentagrophytes) for comparative insights, but maintained a focus on studies explicitly testing M. canis strains under varied laboratory and therapeutic conditions.
Most important findings
Clinical studies and in vitro analyses underscore the importance of Microsporum canis antifungal susceptibility testing in guiding effective treatment. In human cases, terbinafine (TER) and itraconazole (ITZ) have demonstrated superior clinical efficacy over griseofulvin (GRI) and fluconazole (FLZ), especially for recalcitrant tinea capitis. Similarly, systemic TER and ITZ led to rapid lesion resolution in feline infections, while GRI produced more inconsistent outcomes. These findings highlight the growing concern over clinical resistance, now reported in up to 40% of treated cases, often due to poor pharmacokinetics, compliance, or emerging resistance, such as documented TER resistance in cats.
Laboratory Microsporum canis antifungal susceptibility testing further reveals significant variability in minimum inhibitory concentrations (MICs), particularly in broth microdilution assays due to inconsistent parameters like inoculum type, size, and media. MIC-0 (100% inhibition) yielded the most reproducible results. E-test data showed ITZ as the most active drug and FLZ the least, though variability in MIC reading, such as double growth zones, compromised interlaboratory agreement for some azoles. Disk diffusion methods also confirmed TER and voriconazole as promising agents based on inhibition zones, but high variability in test conditions complicates data interpretation. These inconsistencies reinforce the need for standardized Microsporum canis antifungal susceptibility testing protocols to improve diagnostic precision and therapeutic outcomes.
| Drug | Activity Against M. canis | Limitations |
|---|---|---|
| Terbinafine (TER) | High | Documented resistance emerging in cats |
| Itraconazole (ITZ) | High | Moderate reproducibility in E-test |
| Griseofulvin (GRI) | Low–Moderate | High MICs; not recommended for long-term use |
| Fluconazole (FLZ) | Low | Consistently poor in vitro activity |
| Voriconazole (VOR) | Promising | Inhibition zones large in DD, but few studies |
Key implications
The review emphasizes the urgent need for standardized antifungal susceptibility testing protocols tailored to dermatophytes, particularly M. canis. The variability in MIC values across different methodologies hinders the development of clinical breakpoints and impairs resistance surveillance. Microsporum canis antifungal susceptibility testing should be emphasized due to the low efficacy of GRI and FLZ, combined with emerging resistance to TER, which underscores the importance of methodical susceptibility testing prior to initiating long-term therapy. TER and ITZ remain the most reliable options based on current in vitro and in vivo evidence. The review also calls for improved standardization in inoculum preparation, incubation parameters, and endpoint definitions to enhance interlaboratory reproducibility and clinical translation.