November 2017

BRD Antimicrobial Resistance Trends are Updated!

By Dr. Brian Lubbers

Why summarize antimicrobial susceptibility test data?

Cumulative antimicrobial susceptibility test summaries (also known as antibiograms) provide practitioners with information on local antibiotic resistance patterns, as an aid to adjusting empiric therapy recommendations. KSVDL routinely publishes the antibiotic susceptibility test summaries for Mannheimia haemolytica, Pasteurella multocida and Histophilus somni, and these summaries have been updated on the KSVDL website to include data from 2015 and 2016.

What do the summaries show?

While there is significant value in the big picture of these summaries, it should also be understood that there is significant "noise" in the data. Geography, disease status and prior exposure to antimicrobials are all factors that can introduce bias worth considering when interpreting these summaries. So without over / under – stating the data, some points of note:

  • Antimicrobial resistance in Mannheimia haemolytica remains very high (≥50% of isolates) to almost all drugs used to treat BRD. Many of these isolates will be multi-drug resistant (MDR).

  • Antimicrobial resistance in Pasteurella multocida is: very low (<10% of isolates) for enrofloxacin, florfenicol and penicillin; moderate (10 -25% of isolates) for tilmicosin and tulathromycin; high (>30% of isolates) for oxytetracycline.

  • Antimicrobial resistance in Histophilus somni very low (<10% of isolates) for florfenicol and penicillin; moderate (10 -25% of isolates) for tilmicosin and tulathromycin; high (>30% of isolates) for enrofloxacin; and very high (≥50% of isolates) for oxytetracycline.

  • Ceftiofur is not included in the summaries. In vitro resistance to ceftiofur is rare in isolates recovered at KSVDL; however, there are clinical concerns with the established breakpoint. A recent publication* reported a significant number of ceftiofur-intermediate and ceftiofur-resistant isolates recovered from a group of stocker cattle. KSVDL will continue to closely monitor resistance to ceftiofur in the BRD pathogens.

  • Comparing antimicrobial resistance rates within the macrolide class (tilmicosin & tulathromycin) shows a very close correlation for all 3 BRD pathogens. Use of 2 macrolides in the same treatment protocol is NOT advised.

  • Some antimicrobials appear to show a decrease in resistance in the past few years; however, this should be interpreted VERY cautiously given the potential confounding factors associated with this data. There are occasional reports of decreasing antibiotic resistance in the human medical literature; however, I (BVL) am not ready to go there yet – if you have thoughts otherwise, give me a call....

Where can I find this information?

The KSVDL antimicrobial susceptibility test summaries can be accessed from our laboratory homepage by clicking "BRD Antimicrobial Resistance Trends" (under the "Search KSVDL website" box) or directly at If you have questions regarding the summaries, please contact Dr. Brian Lubbers at or 785-532-5650.

*Snyder E, Credille B, Berghaus R, Giguere S. (2017) Prevalence of multi drug antimicrobial resistance in Mannheimia haemolytica isolated from high-risk stocker cattle at arrival and two weeks after processing. J Anim Sci. 95.

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