Livestock Antimicrobial Resistance


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What a recent study on antimicrobial resistance’s ability to diminish the effectiveness of antibiotics could mean for cattle producers.

Antimicrobial resistance threatens to diminish the effectiveness of antibiotics used in both human and veterinary medicine. Boehringer Ingelheim and Zoetis partnered to fund a study to understand the resistance prevalence among bacterial pathogens in beef cattle suspected to have bovine respiratory disease (BRD).

Vet-Advantage asked Nathan Meyer, DVM, Ph.D., Boehringer Ingelheim about this study and what the results mean for cattle producers treating animals for BRD.


Headshot of Nathan Meyer, DVM, Ph.D., Boehringer Ingelheim
Nathan Meyer, DVM, Ph.D., Boehringer Ingelheim


Q: Why is understanding antimicrobial resistance important?

Dr. Nathan Meyer: This is a very important topic in the industry. We want to make sure these products keep working because, as an industry, we’re not coming out with new antimicrobials any time soon.


Q: Tell us about how the study was set up.

Dr. Meyer: Boehringer Ingelheim, Zoetis and Production Animal Consultation (PAC) helped design the study, which was conducted and analyzed by PAC. Geographically, it was one of the largest antimicrobial resistance studies done in cattle originating throughout the U.S. in Nebraska, Kansas and Colorado feedyards. We wanted to look at cattle identified as having BRD in the feedyard – that were pulled and brought to the chute for nasal pharyngeal samples and then treatment like producers normally would administer.


Q: What was unique about this sample set?

Dr. Meyer: We collected more than 450 samples across 15 feedyards, five in each state. We wanted a large snapshot of what is going on with antimicrobial resistance. These cattle were identified by trained crews as morbid. When veterinarians get information from a diagnostic lab, those animals are not a random sample. Those are probably unique cases. This study gives us a snapshot of real-word conditions and gives us a good baseline.


Q: What were some of the main takeaways from the study?

Dr. Meyer: In about 70% of the samples, we identified bacteria associated with BRD. In the other 30% of cases, bacteria couldn’t be identified at all. Now, maybe our currently available lab methods aren’t good enough to detect bacteria deep in the lungs – these were upper airway samples. Or maybe we’re treating viral infections with antibiotics in some cases. Without a definitive diagnostic test, we are making educated guesses.

Results showed 70% of the cattle were positive for at least one of the three major players we expect to see in bacterial BRD cases: Histophilus somni, Mannheimia haemolytica or Pasteurella multocida.


Q: In the samples that were positive for BRD bacteria, what did the antimicrobial picture look like?

Dr. Meyer: About 34% of the H. somni samples were resistant to tetracyclines. About 9% of H. somni isolates were multidrug resistant. Resistance to macrolides was about 7% for H. somni and 2% for M. haemolytica. It was certainly a positive result that macrolide resistance was so low. We also didn’t see a difference in resistance between feedyard regions. These results serve as a good baseline for further research as antibiotic usage and resistance patterns change over time.


Q: How should cattle producers use this information?

Dr. Meyer: It’s difficult to understand resistance in terms of clinical outcomes. When a steer comes to the chute, there doesn’t seem to be a strong correlation between resistance patterns and how that drug is going to work in the animal. It’s a great reminder that resistance patterns are just one piece in the treatment protocol decision making process. Working closely with veterinarians researching this area will lead to better treatment outcomes.


Q: Are there future studies that build on these results?

Dr. Meyer: Since the study was conducted, there have been big shifts in antimicrobial usage as key products have gone generic. We’re pursuing another study soon that will look at any shifts in antibiotic use and see if that’s translated into resistance.

The topic of antimicrobial resistance is very complex. Pharmacologists specializing in this field are debating certain aspects of it. We believe all antibiotics have a place depending on how you use them.


Key Study Findings:

  • Bacterium prevalence include P. multocida (36.0%), M. haemolytica (32.7%) and H. somni (28.5%).
  • Of the Histophilus isolates, 39.5% were resistant to at least
    one antimicrobial, compared to 11.7% and 8.8% Pasteurella and Mannheimia, respectively.
  • Non-susceptibility across all organisms was 5.7 times more likely in animals that received metaphylaxis than those that did not.
  • 214 animals (67.1%) were positive for one organism, 89 (27.9%) tested positive for two organisms, and 16 (5%) tested positive for all three organisms.


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