Long-term serovar distribution and toxin gene profiling of Actinobacillus pleuropneumoniae in Korea (2015–2025), with identification of an apxIICA-deficient serovar 15 profile

Importance: Actinobacillus pleuropneumoniae (APP) is a bacterial pathogen that causes porcine pleuropneumonia and contributes to respiratory disease complexes in swine.Shifting serovars, toxin-gene profiles, and antimicrobial resistance patterns complicate disease control strategies and can affect vaccine performance.Objective: To provide a decade-long molecular and phenotypic characterization of APP isolates from Korean pig farms, focusing on the serovar distribution, apx-based toxingene profiles, and antimicrobial susceptibility.Methods: Between 2015 and 2025, 1,215 pneumonic lung samples from 965 pig farms yielded 132 APP isolates.The species identity was confirmed by 16S rRNA sequencing.The serovars were determined using capsule polysaccharide (CPS) gene-based multiplex polymerase chain reaction (PCR).Toxin genes (apxIA-apxIVA) were profiled, and the antimicrobial susceptibility to 29 agents was assessed by broth microdilution according to the Clinical and Laboratory Standards Institute guidelines.Results: Serovar 1 was predominant (66.7%), followed by serovars 5 (17.4%) and 2 (6.8%).CPS multiplex PCR identified three isolates (2.3%) as serovar 15, which was detected intermittently and displayed heterogeneous toxin gene profiles, including apxIICA-deficient profiles.Most isolates exhibited classical repeats-in-toxin operon arrangements, suggesting ongoing diversification of the toxin gene profiles.The resistance to older antimicrobial classes, particularly tetracyclines and phenicols, remained prevalent and frequently cooccurred in recurrent multidrug-resistant (MDR) combinations.Conclusions and Relevance: Serovar 1 is dominant in Korea, but the emergence of atypical toxin gene profiles in serovar 15 may carry immunological implications.Persistent resistance to older drug classes underscores the necessity for long-term molecular surveillance, evaluation of vaccine coverage against evolving strains, and enhanced antimicrobial stewardship to strengthen the control efforts for porcine pleuropneumonia in Korea.