Veterinary session

Pasteurella multocida, a facultative pathogenic bacterium, which is a natural inhabitant of mucous membranes, can cause many diseases worldwide in mammals. It is one of the main bacterial pathogens of bovine respiratory disease complex (BRDC), and with other pathogens and predisposing factors, can cause severe lesions both in intensively kept dairy and beef herds. Its main effects are fibrinous pleuropneumonia and bronchitis in calves, and more rarely in adult cattle. Antimicrobial agents play a major role in the fight against this disease; however, the inappropriate use of these medicines have increased the antibiotic resistance that threatens humanity. Therefore, it is important to monitor continuously antibiotic sensitivities, in order to slow and reduce the ever-increasing resistance using appropriate treatments.

The aim of our study was to determine the antibiotic susceptibility of Pasteurella multocida bacterial strains isolated from cattle in 50 different Hungarian settlements using different methods. The minimum inhibitory concentration was determined for 10 different antibiotics (penicillin, amoxicillin, ceftiofur, florfenicol, spectinomycin, oxytetracycline, tulathromycin, tilmicosin, enrofloxacin and danofloxacin) using broth microdilution method based on CLSI regulations. As a result, the bacterial strains were the most sensitive to florfenicol and ceftiofur, only 3 strains showed resistance to ceftiofur and 1 to florfenicol. Several strains were moderately susceptible and resistant to other antibiotics. The highest rate of resistance (42%) was found to amoxicillin and oxytetracycline. The highest MIC values, which was more than 256 µg/ml, were obtained for spectinomycin, tulathromycin and tilmicosin. An outlier MIC value was also observed for amoxicillin, at 64 µg/ml. The MIC50 and MIC90 values were then determined from the obtained results, and we also compared them with the international trends.

Our other method was the disk diffusion test, in which all the strains were found susceptible to florfenicol, ceftiofur, amoxicillin and penicillin. Using this method, we found the highest resistance to oxytetracycline (30%) and to macrolide antibiotics (tulathromycin 18% and tilmicosin 22%). Since the results obtained from the two methods in the case of certain antibiotics did not show complete correlation, we further examined some of the strains using a third method, the E-test. The difference in amoxicillin values based on the MIC results may explained by partial resistance, which is not visible with the disk diffusion test.