Case Report: Equine allogeneic umbilical cord blood mesenchymal stromal cells (CB-MSC) as adjunctive therapy in a foal with septic arthritis and osteomyelitis.

Mesenchymal stromal cells (MSCs) are recognized for their potent anti-inflammatory, antibacterial, and immunomodulatory properties, making them a promising therapeutic option for combating antibiotic resistance and biofilm-associated infections. This report describes the successful treatment of septic arthritis and osteomyelitis in a foal using equine allogeneic cord blood-derived MSCs (CB-MSCs) in combination with antibiotic therapy. An 8-day-old Thoroughbred filly initially presented with septic arthritis of the right tibiotarsal joint, pneumonia, and omphalophlebitis/arteritis. Subsequently, the filly developed septic arthritis of the left elbow joint, epiphysitis, and osteomyelitis of the ulna, which progressed to an aggressive pathological fracture. Chloramphenicol was instituted based on the bacterial culture and susceptibility. Due to limited clinical and cytological improvement following needle lavage, arthroscopic lavage, and intra-articular antibiotic administration, the left elbow joint and fracture site were treated three times with 15 million TLR3-activated CB-MSCs in combination with meropenem (7.25 mg/kg IA). Additionally, the filly received twice systemic treatment with non-activated CB-MSCs (1 million cells/kg IV). The treatment resulted in complete resolution of both septic arthritis and osteomyelitis. At a 12-month follow-up, the filly remained sound, and radiographic re-evaluation showed significant remodeling of the ulna. This case describes the successful use of equine allogeneic cord blood-derived mesenchymal stromal cells (CB-MSCs), administered locally and systemically in combination with antibiotic therapy, to manage a refractory intra-synovial and osseous septic process in a foal. The use of TLR3-activated CB-MSCs may have supported antimicrobial treatment, highlighting the potential antimicrobial, anti-inflammatory, and immunomodulatory properties of CB-MSCs.