Background: 
Streptococcus equi subspecies zooepidemicus, traditionally classified under Group C Streptococci, is primarily a veterinary pathogen. Rarely, it may cause infections such as bacteremia, meningitis, endocarditis and pneumonia in humans. Musculoskeletal infections secondary to this pathogen are very uncommon. 
Clinical case : 
We report a case of a 74 year old farmer who had close contact with animal and ingestion of unpasteurized dairy products, was admitted to the hospital with a history of pain and swelling of the left knee and fever.  The clinical exam showed a warm and swollen left knee. Echography of the knee showed an infiltration of the soft tissues and a mild joint effusion with no sign of abcess, and the analysis of the synovial fluid revealed a purulent fluid with leukocytes clusters and the culture showed a growth of Gram-positive cocci in chains identified as Streptococcus equi subspecies zooepidemicus. The patient improved steadily under treatment by penicillin associated to ciprofloxacine.
Conclusion: 
Streptococcus equi is an uncommon cause of arthritis due to group C streptococci that should be considered in the differential diagnosis of arthritis in a single joint in patients who are in contact with animals, especially horses.

Septic arthritis secondary to Streptococcus Equi Subspecies Zooepidemicus: a case report