Submitted to: Veterinary Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/25/2016
Publication Date: 8/9/2016
Citation: DeDonder, K.D., Harhay, D.M., Apley, M.D., Lubbers, B.V., Clawson, M.L., Schuller, G., Harhay, G.P., White, B.J., Larson, R.L., Capik, S.F., Riviere, J.E., Kalbfleisch, T., Tessman, R.K. 2016. Observations on macrolide resistance and susceptibility testing performance in field isolates collected from clinical bovine respiratory disease cases. Veterinary Microbiology. 192:186-193.

Interpretive Summary: There are a number of viral and bacterial pathogens, and environmental stressors that contribute to bovine respiratory disease. Major bacterial pathogens that cause bovine respiratory disease include Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni. In this study, 180 cattle were purchased from sale barns in three U.S. states, transported to Kansas, either given a macrolide antibiotic known as gamithromycin, or a saline control, and introduced into a feedlot. They were monitored over time for the development of respiratory disease. Nine cattle given gamithromycin and seventeen given saline went on to develop respiratory disease. The 26 diseased cattle received a treatment of gamithromycin following their diagnosis. Only four subsequently failed to improve symptomatically and required further aid. M. haemolytica, P. multocida, and H. somni were isolated from the sick animals, and tested for macrolide resistance. With low numbers of sick cattle, there was not a statistical difference in the likelihood of isolating macrolide-resistant M. haemolytica and P. multocida, between calves that received gamithromycin at the time of their arrival to the feedlot and those that received saline. Statistical modeling for H. somni could not be conducted due to the small number of isolates obtained from the study. A significant source of macrolide-resistant M. haemolytica isolates in this study was cattle that originated from the sale barn of one of the three states. Through whole genome sequencing of the M. haemolytica isolates, a single genetic subtype, or strain of M. haemolytica was found to harbor macrolide resistance genes. Positivity for the resistance genes and resistance to gamithromycin, which is a macrolide antibiotic, was highly specific and sensitive. The subtype harboring the resistance genes was isolated from two of the four gamithromycin-treatment failures, which was not significant. These results suggest that factors preceding the introduction of cattle to the feedlot may influence the frequency of a specific M. haemolytica genetic subtype with macrolide resistance genes.

Technical Abstract: The objectives of this study were; first, to describe gamithromycin susceptibility of Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni isolated from cattle diagnosed with bovine respiratory disease (BRD) and previously treated with either gamithromycin for control of BRD (mass medication=MM) or sham-saline injected (control=CON); second, to describe the macrolide resistance genes present in genetically typed M. haemolytica isolates; third, use whole-genome sequencing (WGS) to correlate the phenotypic resistance and genetic determinants for resistance among M. haemolytica isolates. M. haemolytica (n=276), P. multocida (n=253), and H. somni (n=78) were isolated from feedlot cattle diagnosed with BRD. Gamithromycin susceptibility was determined by broth microdilution. Whole-genome sequencing was utilized to determine the presence/absence of macrolide resistance genes and to genetically type M. haemolytica. Generalized linear mixed models were built for analysis. There was not a significant difference between MM and CON groups in regards to the likelihood of culturing a resistant isolate of M. haemolytica or P. multocida. The likelihood of culturing a resistant isolate of M. haemolytica differed significantly by state of origin in this study. A single M. haemolytica genetic subtype was associated with an over whelming majority of the observed resistance. H. somni isolation counts were low and statistical models would not converge. Phenotypic resistance was predicted with high sensitivity and specificity by WGS. Additional studies to elucidate the relationships between phenotypic expression of resistance/genetic determinants for resistance and clinical response to antimicrobials are necessary to inform judicious use of antimicrobials in the context of relieving animal disease and suffering.