Antimicrobial resistance in bacteria associated with porcine respiratory disease in Australia

The porcine respiratory disease complex greatly affects the health and production of pigs. While antimicrobial agents are used to treat the respiratory infections caused by bacterial pathogens, there is no current information on antimicrobial resistance in Australian pig respiratory bacterial isolates. The aim of this study was to determine the antimicrobial resistance profiles, by determining the minimum inhibitory concentration of nine antimicrobial agents for 71 Actinobacillus pleuropneumoniae, 51 Pasteurella multocida and 18 Bordetella bronchiseptica cultured from Australian pigs. The majority of A. pleuropneumoniae isolates were resistant to erythromycin (89%) and tetracycline (75%). Resistance to ampicillin (8.5%), penicillin (8.5%) and tilmicosin (25%) was also identified. The P. multocida isolates exhibited resistance to co-trimoxazole (2%), florfenicol (2%), ampicillin (4%), penicillin (4%), erythromycin (14%) and tetracycline (28%). While all the B. bronchiseptica isolates showed resistance to beta-lactams (ampicillin, ceftiofur and penicillin), some were resistant to erythromycin (94%), florfenicol (6%), tilmicosin (22%) and tetracycline (39%). The incidence of multiple drug resistance (MDR) varied across the species - in B. bronchiseptica, 27.8% of resistant isolates showed MDR, while 9.1% of the resistant isolates in A. pleuropneumoniae, and 4.8% in P. multocida showed MDR. This study illustrated that Australian pig strains of bacterial respiratory pathogens exhibited low levels of resistance to antimicrobial agents commonly used in the pig industry.