Antibiotic sensitivity of Haemophilus parasuis plus Actinobacillus pleuropneumoniae and other respiratory pathogens

Antimicrobial resistance in porcine respiratory pathogens has been shown to exist inmany countries. However little is known about the variability in antimicrobial resistancewithin a population of one bacterial respiratory pathogen on a pig farm. This studyexamined the antimicrobial sensitivity of Actinobacillus pleuropneumoniae using multipleisolates within a pig and across the pigs at three different sampling times. Threeantimicrobial agents examined were tilmicosin, tetracycline and amoxicillin. A total of367 isolates of A. pleuropneumoniae were tested. The isolates were identified,serotyped and a subsample genotyped. All isolates were tested for their antimicrobialsensitivity. All isolates were identified as A. pleuropneumoniae serovar 1 with the samegenetic profile. The three batches were found to consist of a mix of resistant andsusceptible isolates against tilmicosin and there was a shift from the susceptible wildtype towards a resistant type observed that was statistically significant in the secondsampling for tilmicosin and amoxicillin. There was considerable variance in the secondsampling period and the zone diameter across the isolates towards the threeantimicrobials varied considerably. The results support the hypothesis that onepopulation of a porcine respiratory pathogen can consist of both susceptible and resistanttypes, with the prevalence of resistant types varying across time.