Novel Genotype of Streptococcus dysgalactiae subsp. equisimilis Associated with Mastitis in an Arabian Filly: Genomic Approaches and Phenotypic Properties.
Authors: Bustos Carla Paola, Retamar Gabriela, Leiva Romina, Frosth Sara, Ivanissevich Ana, Demarchi Maria Eugenia, Walsh Saoirse, Frykberg Lars, Guss Bengt, Mesplet Maria, Waller Andrew
Journal: Journal of equine veterinary science
Summary
# Editorial Summary Streptococcus dysgalactiae subsp. equisimilis (Sde) is typically a benign commensal in horses but can cause opportunistic infections, including mastitis in neonates—a rare but serious condition requiring accurate identification for effective treatment. Researchers isolated and sequenced an Sde strain from an 8-day-old Arabian filly with mastitis, identifying a novel emm-subtype (stC839.12) and sequence type (ST-670) that clustered distinctly within an equine-specific genotype group, suggesting this represents a previously undocumented pathogenic variant. The strain harboured multiple virulence-associated genes including those encoding the M-protein (emm), hyaluronic acid synthase (hyl), streptolysins (slo), and phosphoenolpyruvate phosphotransferase systems, alongside genes for efflux pumps (pmrA, bmrC, lmrP) that may facilitate antimicrobial tolerance despite no conventional antibiotic-resistance mechanisms being detected. Whilst this case represents a single clinical presentation, the identification of a novel equine-adapted genotype with enhanced virulence potential warrants heightened clinical awareness of Sde mastitis in neonatal foals and reinforces the value of genomic sequencing in diagnosing atypical streptococcal infections when conventional culture and susceptibility testing yield ambiguous results. Practitioners managing neonatal foal health should consider Sde as a differential diagnosis in mammary disease and consult genomic analysis when standard microbiological approaches fail to guide therapeutic decisions.
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Practical Takeaways
- •Neonatal mastitis in foals can be caused by opportunistic commensal streptococci with novel genotypes; consider Sde in differential diagnoses of filly mastitis even without obvious pathogenic bacteria
- •Genomic sequencing may be warranted for atypical or treatment-resistant cases in neonatal foals to identify emerging pathogenic strains
- •While no classical antibiotic resistance was detected, the presence of efflux pump genes suggests potential for reduced drug susceptibility; antimicrobial stewardship and susceptibility testing are important for treatment decisions
Key Findings
- •A novel emm-subtype stC839.12 and ST-670 multilocus-sequence type were identified in an Sde strain isolated from an 8-day-old Arabian filly with mastitis
- •The strain belonged to a horse-specific equine genotype group despite being originally characterized as a commensal bacterium
- •Multiple virulence factor genes (emm, cpa, fbp-2, adcA, hyl, htrA, tig, slo, ndk) and efflux pump genes (pmrA, bmrC, lmrP) were identified without classical antibiotic resistance mechanisms
- •This is the first reported case of equine neonatal mastitis caused by this novel genotype of Sde