Mycoplasma hyosynoviae-associated arthritis in finisher pigs showing signs of lameness in Southwestern Norway.

Lameness in pigs is a major welfare and economic concern, with infectious arthritis being an important contributor. Mycoplasma hyosynoviae is a recognized cause of arthritis in finisher pigs globally, yet its occurrence and clinical relevance in Norway have not been systematically investigated. This small-scale study aimed to elucidate the aetiology of acute lameness in finisher pigs in southwestern Norway through systematic necropsies and microbiological analyses. Between September 2020 and April 2022, ten farms in Rogaland County with recurrent episodes of acute lameness were enrolled. Farm data on housing, lameness history, and antimicrobial use were collected. Thirty-five pigs (mean ± SD weight: 50.7 ± 16.0 kg) were euthanized and subjected to necropsy. Synovial fluid and tissues were sampled for bacterial culture, histopathology, and multiplex PCR (targeting M. hyosynoviae, Mycoplasma hyorhinis, Glaesserella parasuis, and Streptococcus suis). The finishing farms produced 384–2,100 finishers annually; lameness typically began 0–3 weeks after arrival. Antimicrobials commonly used included benzylpenicillin procaine and tiamulin. Of 35 pigs examined, M. hyosynoviae was detected as the sole agent in 17 cases (16 non-purulent arthritis, 1 serous tenosynovitis) originating from eight farms. Other findings included Streptococcus dysgalactiae ssp. equisimilis (n = 3), Trueperella pyogenes (n = 1), and a co-infection with Actinobacillus pleuropneumoniae and M. hyosynoviae (n = 1). In several herds, benzylpenicillin procaine was used despite M. hyosynoviae’s intrinsic resistance to beta-lactams. This study provided evidence of M. hyosynoviae-associated arthritis in Norwegian finisher pigs after systematic investigation, highlighting the need for diagnostic investigations to guide appropriate antimicrobial therapy. The findings confirmed a common relationship between M. hyosynoviae and lameness in finishers pigs and suggest that undiagnosed mycoplasmal arthritis may contribute to welfare and treatment challenges.