Lyme arthritis: Demographic characteristics and Borrelia ospA genospecies in synovial fluid. A 17-year cohort study in Sweden.

Lyme arthritis (LA) is a recognized manifestation of Lyme borreliosis in Sweden, but data on the causative Borrelia species are limited. This study aimed to describe the demographics of LA and identify Borrelia burgdorferi sensu lato (B. burgdorferi s.l.) genospecies present in synovial fluid (SF). Patient data were collected from the test requisition form accompanying consecutive SF samples submitted to the Clinical Microbiology Laboratory in Lund, Sweden, for Borrelia DNA detection between 2007 and 2023. Detection was performed using 16S rDNA PCR, and genospecies were determined through ospA PCR followed by sequencing. Borrelia DNA was detected in 323 samples, corresponding to 283 unique patients, from a total of 2353 samples. Species characterization was successful in 213 cases. LA was diagnosed year-round, with approximately half of the cases occurring in youth (0-19 years). Males predominated among adults (≥20 years), though less so among youth. Two species were most common: B. afzelii (104 cases, 49 %) and B. burgdorferi sensu stricto (s.s.) (70 cases, 33 %). B. garinii, B. bavariensis, and B. spielmanii were identified in 21 (9 %), 14 (7 %), and 4 (2 %) cases, respectively. In conclusion, B. afzelii was the predominant species in SF. B. burgdorferi s.s. accounted for one-third of LA cases, despite being rarely found in ticks or humans in Sweden. B. spielmanii was detected in four cases, representing the first identification of this species in humans in Sweden and, more broadly, in synovial fluid.