Fecal Microbiota Transplantation in Axial Spondyloarthritis

Although biologic therapy have revolutionized the treatment of Spondyloarthrtitis (SpA), many patients do not experience complete relief of SpA related complaints. It has been established that patients with SpA have an altered composition of microorganisms (microbiota) in the gut compared to healthy controls, and that this correlates to disease activity and respons to therapy. The goal of this randomized double-blind study is to evaluate the efficacy of fecal microbiota transplantation (FMT) in patients with axial SpA with a suboptimal effect of biologic therapy. The main questions it aims to answer are: * Can FMT reduce disease activity in axial SpA? * Can FMT alleviate pain and reduce fatigue in axial SpA? * Is the composition of microorganisms restored to normal in patients with SpA after a treatment with FMT? Participants will receive a single treatment in the form of an enema with either donor FMT or placebo at baseline. The primary endpoint will be evaluated after 90 days, but efficacy and safety will be monitored from baseline until 365 days.