What do BRD bacterial susceptibility tests really tell us? It's not as straightforward as it sounds.

When you are having problems with bovine respiratory disease (BRD), you want answers fast. You might’ve sent in a bacterial susceptibility test from a case and you may even have seen resistant bacteria and switched treatment products, but did changing antibiotics improve your BRD treatment response? Maybe yes or maybe no.

The fact is that we don’t have sufficient evidence to prove bacterial culture and susceptibility results relate closely to clinical outcomes. Lab results are easy to obtain, but real-world outcomes are what you need to keep your cattle healthy and your operation running smoothly.

During my 27 years of general practice, I was often frustrated while trying to interpret and apply BRD bacterial culture and sensitivity results: What if it showed no growth? What if it was a false positive? Was I wasting my client’s time and money on tests that didn’t tell me the whole story or, worse yet, led me to choose an antibiotic that resulted in poorer treatment response?

With a better understanding of evidenced-based medicine, I realized there is other evidence to use to choose the most effective antibiotic for treatment or control of BRD. Here are three reasons bacterial culture and antimicrobial susceptibility testing can be misleading

1.   Susceptibility test results are specific to the one colony chosen in the lab to represent the infection. But BRD is a disease complex. Many bacterial and viral pathogens can cause similar clinical signs, so a culture sample that shows the susceptibility of one type of bacteria to antibiotics is not giving us the whole picture. In fact, susceptibility tests do not account for impacts of viruses, an important part of the disease complex. A common bacterial pathogen of BRD is Mycoplasma bovis, and there are no testing standards for antibiotic susceptibility testing of M. bovis for BRD. Inability to account for impacts of M. bovis  and viral pathogens is a significant shortcoming of antimicrobial susceptibility testing for a complex disease like BRD.

2.    Small sample sizes. Samples for BRD bacterial susceptibility tests are often taken from a small sample of the pen or herd — and most likely that sample is taken postmortem. Then we try to apply those findings to the larger living group. Sampling a small portion of a population may not give us the whole picture or, worse, give us a wrong answer. Postmortem lung samples are most likely from cattle previously treated with antibiotics and are obviously treatment failures, but how well does this group represent newly identified and untreated sick cattle? It is important to understand antimicrobial sensitivity is only a measurement of how well that bacteria grow on an agar plate or in a broth tube in the presence of an antibiotic. So it is important to not fall into the trap of assuming all BRD is caused by that particular bacteria and antibiotic treatment response of all cattle will be the same as antimicrobial susceptibility results from the specific bacterial colony isolated from a single animal with BRD.

3.      Sampling from the wrong location in the respiratory tract. Most samples for susceptibility analysis in live animals are taken from the upper respiratory system. However, the lower respiratory system is where BRD bacterial pathogens grow, causing pneumonia. BRD studies comparing culture results from the upper respiratory tract with samples obtained from the lower respiratory tract show inconsistent conclusions.

So what can you do?

My advice is to evaluate treatment response of antibiotics in your cattle, analyze evidence from published randomized controlled clinical trials, and concentrate on controlling or managing risk factors for BRD rather than chasing sensitivities to antibiotics.

1.     Look at your overall disease pressure as a whole and the risk level of the cattle to determine your treatment or metaphylactic protocols. If you see a change in response to your protocols, first consider the outside factors that changed. For example: If you switched from buying medium-risk cattle to buying high-risk cattle or doubled the number of cattle you bought during that time, you might be seeing pressures from long hauls, commingling, labor shortages or stress factors like temperature changes, which can be the cause of poorer treatment response rather than “the antibiotic quit working” or the bacteria became resistant.

2.    Look at the data. I have more confidence in data from a well-designed controlled clinical trial for BRD or meta-analyses. This data is more helpful for making treatment decisions because there is less potential for bias compared with historical comparisons of different treatments or lab results from culture and sensitivities.

3.     Vaccinate against major BRD-causing viruses. It is better to prevent disease rather than treat disease. Remember: BRD is a disease complex involving bacterial and viral pathogens. When determining vaccination protocols, consider not only which viral and bacterial pathogens are the most important but also how they interact. Antibiotics aren’t effective against viruses. So if the majority of the sick cattle have respiratory viral infections, treatment response will be poor no matter which antibiotic is used. This emphasizes the value of implementing effective vaccine protocols in addition to effective treatment protocols.

It is critical to think twice about the issues of trying to apply susceptibility test results to achieve better clinical outcomes for BRD. Changing the antibiotic should never be the only thing you do when you have a BRD issue. BRD is a complex disease involving many bacterial and viral pathogens, in addition to various environmental and management risk factors, which all determine the outcome of the disease. Don’t get tunnel vision and concentrate on only one of them.

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