Authors: van den Wollenberg L, van Maanen C, Buter R, Janszen P, Rey F, van Engelen E
Journal: Frontiers in veterinary science
Summary
# Editorial Summary Whilst *Streptococcus equi* subsp. *equi* remains the primary bacterial cause of equine head lymph node abscessation, three unrelated cases presented with culture-negative results for strangles, prompting investigation into an alternative pathogen. Van den Wollenberg and colleagues employed MALDI-TOF mass spectrometry, antimicrobial susceptibility testing, and whole genome sequencing to fully characterise the causative organism, ultimately identifying *Arcanobacterium denticolens* in all three isolates with high confidence through multilocus sequence typing analysis. The three cases were epidemiologically unrelated, as evidenced by substantial genetic divergence (10,170 to 36,058 single nucleotide polymorphisms between isolates), despite 95% nucleotide sequence identity at the genus level; virulence factor profiling revealed only four known virulence-associated genes, and notably, no antibiotic resistance determinants were detected, correlating with susceptibility patterns observed in antimicrobial testing. This work establishes the first draft genome for *A. denticolens* from equine sources and warrants inclusion of this organism in the differential diagnosis for submandibular and other cranial lymph node infections when conventional strangles diagnostics prove negative—a clinically significant distinction given the organism's apparent antimicrobial susceptibility and lower virulence profile compared to *S. equi* subsp. *equi*.
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Practical Takeaways
- •When submandibular abscessation in horses tests negative for strangles (S. equi), consider Actinobacillus denticolens in your differential diagnosis
- •A. denticolens appears to respond well to antibiotics based on susceptibility testing, though clinical treatment response data are needed
- •These cases appear unrelated, suggesting A. denticolens may be opportunistic rather than contagious, but transmission routes warrant further investigation
Key Findings
- •Actinobacillus denticolens identified as causative agent in 3 cases of equine submandibular abscessation previously negative for Streptococcus equi
- •Three isolates showed 95% nucleotide sequence identity but were not clonal (10,170-36,058 SNPs), indicating epidemiologically unrelated cases
- •A. denticolens isolates demonstrated high antibiotic susceptibility with only 4 known virulence-related genes detected and absence of known antibiotic resistance genes
- •First draft genome of A. denticolens from equine source reported