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ASM case reports2026; 2(3); e00211-25; doi: 10.1128/asmcr.00211-25

Septic arthritis caused by Streptococcus equi subspecies zooepidemicus: a case report.

Abstract: subspecies (SESZ) is an uncommon zoonotic pathogen in humans but can cause severe joint infections. Reported cases are often linked to exposure to horses, dairy products, or raw pork, though infections may occur without an identifiable animal source. Unassigned: An 84-year-old man with multiple comorbidities presented with acute left shoulder pain, fever, and chills. Synovial fluid aspiration yielded turbid fluid containing calcium pyrophosphate dihydrate crystals, while Gram stain was negative. Despite empiric broad-spectrum antibiotics, his symptoms persisted, requiring surgical irrigation and debridement. Cultures from the joint aspirate and drain revealed β-hemolytic, mucoid colonies of gram-positive cocci (Lancefield group C), identified as SESZ by Vitek 2, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, and whole-genome sequencing. Interestingly, the isolate had an atypical biochemical phenotype, i.e., it could not ferment sorbitol. Antimicrobial therapy was changed to intravenous ceftriaxone according to susceptibility testing, resulting in subsequent clinical improvement. Unassigned: Although rare, SESZ should be considered in the differential diagnosis of septic arthritis, particularly in elderly patients, even in the absence of clear zoonotic exposure. Timely recognition, adequate surgical drainage, and prolonged targeted antimicrobial therapy are essential for favorable outcomes.
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Publication Date: 2026-02-27 PubMed ID: 42094989PubMed Central: PMC13141944DOI: 10.1128/asmcr.00211-25Google Scholar: Lookup
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Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

Septic arthritis caused by Streptococcus equi subspecies zooepidemicus (SESZ) is rare but can cause serious joint infections. This case report describes an elderly man with SESZ septic arthritis successfully treated with surgery and targeted antibiotics despite no obvious animal contact.

Background

  • Streptococcus equi subspecies zooepidemicus (SESZ) is a zoonotic bacterium, meaning it can spread from animals to humans.
  • SESZ infections in humans are uncommon and typically linked to contact with horses, consumption of dairy products, or raw pork.
  • Infections can sometimes occur without a clear animal source, complicating diagnosis.
  • SESZ can cause severe joint infections such as septic arthritis, which involves inflammation and infection of a joint space.

Case Presentation

  • The patient was an 84-year-old man with multiple underlying health conditions.
  • He presented with sudden onset of left shoulder pain, fever, and chills, symptoms indicative of joint infection.
  • Synovial fluid from the shoulder joint was turbid and contained calcium pyrophosphate dihydrate crystals, often associated with crystal-induced arthritis, but Gram stain for bacteria was negative initially.
  • Despite starting broad-spectrum antibiotics, the patient’s symptoms did not improve, prompting surgical drainage and debridement of the joint.

Microbiological Findings

  • Joint fluid and drainage cultures revealed β-hemolytic, mucoid colonies of gram-positive cocci categorized as Lancefield group C streptococci.
  • Advanced identification methods—Vitek 2, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), and whole-genome sequencing—confirmed the pathogen as SESZ.
  • The isolate exhibited an atypical biochemical profile, most notably an inability to ferment sorbitol, which is unusual for SESZ and may complicate laboratory diagnosis.

Treatment and Outcome

  • Based on antibiotic susceptibility testing, antimicrobial therapy was switched to intravenous ceftriaxone.
  • The patient showed subsequent clinical improvement following targeted therapy combined with surgical intervention.
  • Effective management combined timely pathogen identification, surgical control of infection, and prolonged administration of appropriate antibiotics.

Clinical Significance

  • SESZ, although rare in humans, should be considered in septic arthritis cases, especially in elderly or immunocompromised patients.
  • Lack of identifiable animal exposure does not exclude SESZ infection, so clinicians should maintain suspicion when clinical signs point to unusual bacterial causes.
  • Correct microbial diagnosis may require advanced laboratory techniques due to atypical characteristics of some SESZ strains.
  • Prompt surgical drainage alongside targeted prolonged antibiotic therapy is critical for successful outcomes.

Summary

  • This case underscores the importance of considering zoonotic pathogens like SESZ in joint infections, even when typical exposure histories are absent.
  • It highlights the role of comprehensive microbiological analysis and tailored treatment strategies in managing complex septic arthritis cases.

Cite This Article

APA
Wang T, San Juan MR, Choi H, Hwang M, Jinadatha C, Navarathna DH. (2026). Septic arthritis caused by Streptococcus equi subspecies zooepidemicus: a case report. ASM Case Rep, 2(3), e00211-25. https://doi.org/10.1128/asmcr.00211-25

Publication

ASM case reports
ISSN: 2996-2684
NlmUniqueID: 9919031604206676
Country: United States
Language: English
Volume: 2
Issue: 3
PII: e00211-25

Researcher Affiliations

Wang, Tengfei
  • Department of Pathology and Laboratory Medicine Services, Central Texas Veterans Health Care System, Temple, Texas, USA.
San Juan, Ma Rowena
  • Department of Pathology and Laboratory Medicine Services, Central Texas Veterans Health Care System, Temple, Texas, USA.
Choi, Hosoon
  • Department of Research, Central Texas Veterans Health Care System, Temple, Texas, USA.
Hwang, Munok
  • Department of Research, Central Texas Veterans Health Care System, Temple, Texas, USA.
Jinadatha, Chetan
  • Department of Medicine, Central Texas Veterans Health Care System, Temple, Texas, USA.
Navarathna, Dhammika H
  • Department of Pathology and Laboratory Medicine Services, Central Texas Veterans Health Care System, Temple, Texas, USA.

Conflict of Interest Statement

The authors declare no conflict of interest.

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