A new study from Kansas State University on the treatment of non-responding cases of bovine respiratory disease (BRD) sheds light on the relationship between drug treatments and the emergence of antimicrobial resistance.

The study -- "Association Between Antimicrobial Drug Class for Treatment & Retreatment of Bovine Respiratory Disease (BRD) & Frequency of Resistant BRD Pathogen Isolation from Veterinary Diagnostic Laboratory Samples" -- was conducted by Hans Coetzee, professor and head of the anatomy and physiology department at the Kansas State College of Veterinary Medicine, and his collaborators from Iowa State University. Results were published in the December 2019 issue of the journal PLOS ONE.

"Bovine respiratory disease is one of the most important diseases facing beef cattle producers in the U.S., with economic losses estimated to approach $1 billion a year," Coetzee said. "Antibiotics are critical to minimize losses associated with BRD caused by bacterial infections."

Antibiotics used to treat BRD are broadly classified into two groups: those that prevent bacterial growth (bacteriostatic), and those that kill the organism (bactericidal), Kansas State said.

Although 90% of BRD relapses are reported to receive retreatment with a different class of antimicrobial, the impact of antibiotic selection — bactericidal or bacteriostatic — on disease outcomes and the emergence of antimicrobial resistance has not been investigated, according to Coetzee.

The focus of the study was determining the association between antimicrobial class selection for treatment and retreatment of BRD relapses and antimicrobial susceptibility of Mannheimia haemolytica, Pasteurella multocida and Histophilus somni.

Pathogens were isolated from samples submitted to the Iowa State University Veterinary Diagnostic Laboratory from January 2013 to December 2015. A total of 781 isolates with corresponding animal case histories, including treatment protocols, were included in the analysis, the announcement said.

"Our overall interpretation of the data suggests that there is direct association between the number of treatments to which an animal was exposed and the emergence of antibiotic resistance in samples submitted to a veterinary diagnostic laboratory for analysis," Coetzee said. "In addition, these exploratory data suggest that BRD treatment protocols involving first-line treatment with a bacteriostatic antibiotic, followed by second-line treatment with a bactericidal antibiotic, may increase the probability of isolating BRD bacteria that are resistant to antibiotics."

While this observation suggests that consideration should be given to the antibiotic's mechanism of action when selecting drugs for retreatment of non-responding cases of BRD, Coetzee said further research is needed to determine the clinical relevance of this finding in livestock production systems.