A Rare Case of Streptococcus equi Infection: A Report and Brief Review of the Literature.

Streptococcus equi subspecies zooepidemicus (SEZ) infection is a very rare cause of prosthetic joint infection, and this article reports a case where SEZ caused bacteremia, septic arthritis of a prosthetic knee, and deep vein thrombosis in a patient with notable horse exposure. The case highlights the diagnostic challenges, treatment approach, and the importance of considering zoonotic infections in similar clinical scenarios.

Case Presentation and Background

• The patient was a 73-year-old male with pre-existing conditions including Parkinson’s disease, pulmonary embolism, and recent left knee total arthroplasty (prosthetic knee joint replacement).
• He developed fever and signs of sepsis, which are systemic responses to infection.
• Complications included septic arthritis specifically involving the prosthetic knee joint and deep vein thrombosis (DVT), a condition where blood clots form in the deep veins, often in the legs.
• Blood cultures from four bottles were all positive for Streptococcus equi subspecies zooepidemicus (SEZ), confirming bloodstream infection.
• Medical history revealed frequent exposure to horses, a key detail as SEZ is a zoonotic pathogen that can be transmitted from animals to humans.
• The infection likely spread hematogenously, meaning the bacteria traveled through the bloodstream to infect the prosthetic knee joint.

Challenges in Diagnosis

• Systemic computed tomography (CT) scans revealed abnormalities that complicated the overall picture.
• The simultaneous occurrence of DVT initially masked the source of infection, making it a diagnostic challenge to pinpoint the infectious origin.
• This complexity demonstrates the need for thorough clinical evaluation and consideration of zoonotic infections when patients have specific exposure histories.

Treatment Approach

• The patient initially received broad-spectrum antibiotic therapy with piperacillin-tazobactam for three days to cover a wide range of possible bacteria.
• After identifying SEZ, therapy was de-escalated to intravenous ceftriaxone for a duration of six weeks, targeting the specific bacteria with an effective treatment.
• This was followed by long-term oral antibiotic therapy using amoxicillin-clavulanate, initially planned for six months to ensure eradication of infection.
• Based on clinical response, the oral antibiotic treatment was extended for an additional 12 months, emphasizing the difficulty in managing prosthetic joint infections and the need for prolonged therapy.

Significance of the Case

• Prosthetic joint infections caused by SEZ are exceedingly rare, making this case a valuable contribution to medical literature and understanding of such infections.
• The role of SEZ as a zoonotic pathogen is underlined, particularly in patients with relevant animal exposures such as horses.
• The case points out that prosthetic joint infections can arise via hematogenous seeding from rare zoonotic bacteria, especially in immunocompromised individuals or those with comorbidities.
• It stresses the importance of a thorough patient history including environmental and animal exposures for accurate diagnosis.
• Clinical management requires prompt identification and appropriately prolonged antibiotic therapy to manage these unusual infections effectively.

Summary

• This article documents a rare bloodstream infection by SEZ resulting in prosthetic knee joint infection and DVT in an elderly patient with risk factors including immunocompromise and animal exposure.
• It demonstrates diagnostic complexity caused by overlapping conditions and the importance of considering zoonoses in such infections.
• The patient responded to a carefully managed, prolonged antibiotic regimen.
• The report encourages clinicians to consider rare zoonotic pathogens when faced with unexplained prosthetic joint infections, particularly in patients exposed to animals such as horses.