Effect of Avermectin and Tetracycline on the Microbiome and Resistome of Beef Cattle

Background

Feedlots commonly use in-feed macrolide and tetracycline antibiotics to control liver abscesses. It turns out that the avermectins used for parasite control are also in the macrolide family [ivermectin (Ivomec, Bimectin, Bovimectin, Ivermectin, Solmectin), doramectin (Dectomax), eprinomectin (Eprinex, Longrange), selamectin (pets), and moxidectin (was Cydectin)]. To this point, no one’s looked at whether avermectins contribute to macrolide antibiotic resistance.

Objectives

• Assess the effect of LongRange® eprinomectin and tetracycline separately and in combination, on rumen and fecal microbiota and AMR and investigate cross-resistance and co-selection of AMR genes in the bacterial population.

What they Did

This team previously collected 200 rumen and 200 fecal samples through another project to assess the effect of LongRange® eprinomectin and tetracycline on the rumen microbiome, dung-feeding insects and the soil microbiome. In this study, they examined the impact of the commercially available long-acting anthelmintic product eprinomectin, as well as the antimicrobial chlortetracycline, on the rumen and fecal microbiota of cattle at different sampling times. Specifically, they used a subset of 100 rumen and 100 fecal samples across treatments and characterized rumen and fecal microbiota using 16S rRNA amplicone sequencing. Furthermore, they performed shotgun metagenomic sequencing of 72 fecal samples from control and treated animals to characterize the microbiota and the antimicrobial resistome.

What They learned

The study found that the use of LongRange® (eprinomectin) and tetracycline, either individually or together, did not significantly alter the diversity or composition of microbial communities in the rumen or feces of beef cattle. This suggests that these treatments do not disrupt the normal balance of bacteria in the cattle’s digestive system. While the overall resistome (the collection of antimicrobial resistance genes) did not show significant differences between treatment groups, there was an observed increase in tetracycline resistance genes in cattle treated with tetracycline, an expected outcome of antibiotic exposure. The study also highlighted differences between the results obtained using 16S rRNA sequencing and shotgun metagenomic sequencing. While both methods largely agreed on the stability of microbial communities, they differed in the proportion of specific bacterial groups, like Proteobacteria. This points to ongoing challenges in choosing the most appropriate method for microbiome analysis.

What It Means

The study suggests that the use of certain antibiotics and parasiticides in beef cattle, such as tetracycline and LongRange®, does not have as detrimental effect on the cattle’s microbiome as previously feared. This is significant for livestock producers, as it supports the continued use of these treatments without the risk of significantly disrupting the animals’ microbial health. The increase in tetracycline resistance genes in treated cattle is a reminder of the potential for AMR development with the use of antibiotics. However, the study indicates that the use of LongRange® alone does not seem to specifically contribute to the promotion of AMR and macrolide resistance. Overall, this research underscores the importance of ongoing monitoring of AMR, particularly with regard to the types of antibiotics used. The study’s methodological insights can help guide future research in this area, ensuring more accurate and reliable profiling of both the microbiome and the resistome.