Antimicrobial susceptibility of Haemophilus parasuis isolates from Germany by use of a proposed standard method for harmonized testing

Haemophilus parasuis-related infections, especially among weaners are responsible for major economic losses on pig farms. A method for broth microdilution susceptibility testing of this fastidious organism has recently been developed, but the suitability of this method needs to be validated in a large collection of current field isolates. Using the proposed method, this study tested 123 H. parasuis isolates from different geographic regions in Germany (including five isolates from the Netherlands and Belgium) against a panel of 24 antimicrobial agents and antimicrobial combinations. The isolates were collected between 2013 and 2016. As there are no H. parasuis specific breakpoints available, the tested isolates could not be classified as susceptible, intermediate or resistant. Bi- or multi-modal distributions of minimum inhibitory concentration (MIC) values were observed for some antimicrobial agents (e.g. aminoglycosides, β-lactams, fluoroquinolones and tetracyclines), indicative of non-wild type populations of H. parasuis. Susceptibility testing revealed broad distributions of MIC values for various antimicrobials (e.g. neomycin, streptomycin, tetracycline, tiamulin, tilmicosin, trimethoprim/sulfamethoxazole and tulathromycin). The lowest MIC90 (i.e. the concentration at which 90% of isolates were inhibited) was obtained for cefotaxime (≤0.015 μg/ml), and the highest MIC90 (512 μg/ml) was obtained for streptomycin. This study tested a large set of current field isolates and included the most common serovars (serovars 4 and 5). The results point to the suitability of the broth microdilution susceptibility testing method proposed previously for determining H. parasuis MIC values. In addition, the study provides a reliable overview of the susceptibility status of H. parasuis at present in Germany.