Injectable antimicrobials in commercial feedlot cattle and their effect on the nasopharyngeal microbiota and antimicrobial resistance

Citation

Holman, D.B., Timsit, E., Booker, C.W., Alexander, T.W. (2018). Injectable antimicrobials in commercial feedlot cattle and their effect on the nasopharyngeal microbiota and antimicrobial resistance. Veterinary Microbiology, [online] 214 140-147. http://dx.doi.org/10.1016/j.vetmic.2017.12.015

Plain language summary

In North America, beef cattle are typically transported to a feedlot where they are finished on a high-grain diet until slaughter. Bovine respiratory disease (BRD), also called shipping fever, remains the most common cause of morbidity and mortality after feedlot placement, resulting in significant economic losses. Cattle that are deemed to be at high-risk of developing BRD early in the feeding period (recently weaned, light weighted, commingled, auction market derived, etc.) are often given, at feedlot placement, a single injection of an antibiotic such as florfenicol, tilmicosin, tulathromycin, or oxytetracycline to control BRD. Given that antimicrobial use can lead to the development of resistance, it is important to evaluate the effects of antimicrobials in order to maintain effective disease management. Therefore, the objective of this study was to investigate the effect of a single antibiotic injection at feedlot placement on the nasopharyngeal microbiota of beef cattle in a commercial feedlot system during the first 60 d of production. We also determined the effect that these antimicrobials had on antimicrobial Resistance determinants. We found that administration oxytetracycline or tulathromycin altered the nasopharyngeal microbiota of feedlot cattle. Cattle receiving oxytetracycline or tulathromycin at feedlot entry also had a lower relative abundance of BRD pathogens at exit (≥60 d). Lastly, administration of oxytetracycline increased the proportion of the resistance gene tet(H) in the nasopharyngeal microbiota. These results demonstrate that in feedlot cattle receiving subtherapeutic in-feed antimicrobials, the administration of a single injection of either oxytetracycline or tulathromycin resulted in measurable changes to the nasopharyngeal microbiota during the first 60 d following feedlot placement.

Abstract

Beef cattle in North America that are deemed to be at high risk of developing bovine respiratory disease (BRD) are frequently administered a metaphylactic antibiotic injection to control the disease. Cattle may also receive in-feed antimicrobials to prevent specific diseases and ionophores to improve growth and feed efficiency. Presently, attempts to evaluate the effects that these medications have on antibiotic resistance in the bovine nasopharyngeal microbiota have been focused on culturable bacteria that are associated with BRD. Therefore, we assessed the effects of injectable antibiotics on the nasopharyngeal microbiota of commercial feedlot cattle in Alberta, Canada, through the first 60 d on feed. Although all cattle in the study were also receiving in-feed chlortetracycline and monensin, the administration of a single injection of either oxytetracycline or tulathromycin at feedlot placement altered the nasopharyngeal microbiota in comparison with the cattle receiving only in-feed antibiotics. Oxytetracycline significantly (P < 0.05) reduced the relative abundance of Mannheimia spp. from feedlot entry to exit (≥60 d) and both oxytetracycline and tulathromycin treated cattle had a significantly lower relative abundance of Mycoplasma spp. at feedlot exit compared with the in-feed antibiotic only group. The proportion of the tetracycline resistance gene tet(H) was significantly increased following oxytetracycline injection (P < 0.05). Oxytetracycline also reduced both the number of OTUs and the Shannon diversity index in the nasopharyngeal microbiota (P < 0.05). These results demonstrate that in feedlot cattle receiving subtherapeutic in-feed antimicrobials, the administration of a single injection of either oxytetracycline or tulathromycin resulted in measurable changes to the nasopharyngeal microbiota during the first 60 d following feedlot placement.