Background -Interleukin (IL)-2 stimulates antitumour immunity and is successfully used for the treatment of different neoplasias.Hypothesis/Objectives -Canarypox virus locally expressing feline IL-2 is safe and can be used to treat equine sarcoids.Animals -Twenty horses of different breeds with a median age of eight years (interquartile range 6.0-13.3 years) and a total number of 59 sarcoids were included in the study.Methods -In this prospective clinical trial, sarcoids were injected twice seven days apart, with a recombinant canarypox virus expressing feline IL-2. Complete blood counts (CBC) and fibrinogen levels were measured before treatment and on days 1, 2, 7 and 8.Results -Complete regression was achieved in eight horses (40%) and partial regression in two horses (10%). No change in sarcoid size was observed in two horses (10%) and the disease progressed in five horses (25%). Sarcoids of three horses (15%) showed initial response followed by tumour growth. There were no significant changes in CBC and fibrinogen levels after either injection. One horse developed a mild fever the day after each injection, which subsided without treatment the following day.Conclusions -Treatment of equine sarcoids with recombinant canarypox virus expressing feline IL-2 seems to be a safe therapy option. Although the expression of IL-2 after vector injection and its biological activity in horses were not proven in this study, the treatment resulted in regression and partial regression in 50% of the cases. Further studies are necessary to verify these findings and to establish a treatment protocol.

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