Effect of in-feed tylosin use in feedlot cattle on dynamics of antibiotic resistant enterococci and resistance genes

Authors: Agga, Getahun; GALLOWAY, HUNTER - Western Kentucky University; DEGRAVES, FRED - Western Kentucky University; CONTE, ERIC - Western Kentucky University

Submitted to: Research Workers in Animal Diseases Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 10/1/2019
Publication Date: 10/29/2019
Citation: Agga, G.E., Galloway, H.O., Degraves, F., Conte, E.D. 2019. Effect of in-feed tylosin use in feedlot cattle on dynamics of antibiotic resistant enterococci and resistance genes. Research Workers in Animal Diseases Conference Proceedings. Paper No. P015.

Interpretive Summary:

Technical Abstract: Liver abscess is a primary cause of liver condemnation at slaughterhouses resulting in huge economic losses to the beef cattle industry. Tylosin is added to the feed of feedlot cattle for the prevention of liver abscesses during beef cattle production. However, this practice is scrutinized because of increasing concerns over antimicrobial resistance. We investigated the effect of continuous in-feed use of tylosin in feedlot cattle on concentrations of antibiotic resistant enterococci and resistance genes. A field trial was conducted with a cohort of weaned calves randomized to receive either tylosin-medicated feed (n=10) or nonmedicated feed (n=10). Fecal samples were cultured on media supplemented with erythromycin or tetracycline for the enumeration of erythromycin (ERY) resistant- and tetracycline (TET) resistant enterococci, respectively. Macrolide (ermA, ermB, ermC, ermF and msrC) and tetracycline (tetM) resistance genes were quantified by droplet digital PCR from metagenomic DNA extracted from the fecal samples. Data were analyzed by negative binomial regression. Overall mean concentration of ERY resistant enterococci was significantly higher in the tylosin group. ERY resistant enterococci concentrations in both treatment groups gradually increased after three weeks, peaking at Day 174 before gradually returning to baseline. In both treatment groups, after initial decline on Day 7 TET resistant enterococci concentrations increased followed by decline to baseline levels. While other resistance genes declined in the first three weeks followed by gradual increase in both treatment groups, ermB significantly increased and remained higher in the tylosin group for most of the feeding period. Tylosin feeding magnified correlations between the tetM and the erm genes, as well as among the erm genes themselves. In conclusion, continuous tylosin use increases antimicrobial resistance; withdrawal periods and animal manure treatments are required to reduce environmental and public health impacts.