BRD can rob a calf of healthy lung tissue each day it goes untreated. Actions taken or not taken during the treatment window can often be the deciding factor between an animal that lives and one that dies.
Understanding the different antibiotic options available can help producers make informed decisions about their operations with the help of their veterinarians. Below are some comparisons of different antibiotic types and categories commonly used in the treatment of BRD.
Bactericidal vs. bacteriostatic
Antibiotics are designed to destroy or inhibit the bacteria that can cause infectious diseases, especially those the animal’s body can’t quite fight on its own.
Antibiotics can be classified as either bactericidal or bacteriostatic, based on in vitro results. Bactericidal antibiotics actually kill the bacteria directly and prevent bacterial replication, while bacteriostatic antibiotics inhibit or slow the growth of bacteria, then require the immune system to take over and fight the bacteria.
- Bactericidal antibiotics: Penicillin, cephalosporins, trimethoprims and fluoroquinolones
- Bacteriostatic antibiotics: Sulfonamides, tetracyclines, macrolides, phenicols
Concentration-dependent vs. time-dependent
Another important way to classify antibiotics is whether they are concentration-dependent or time-dependent.
To understand this, one must first know a bit about the term minimum inhibitory concentration (MIC), which is the lowest concentration of antibiotic that will inhibit the visible growth of a bacteria after overnight incubation in the laboratory. It is the standard way to estimate relative efficacy in the laboratory and is specific for every drug-bug combination.
In simple terms, the MIC value is sometimes considered the “line of susceptibility,” and as a general concept, for a good chance of effectiveness by a certain antibiotic against a bacteria, one would prefer to select an antibiotic that achieves a level in the animal above the MIC rather than below.
• Concentration-dependent antibiotics tend to achieve a level that is quite high compared to the MIC value, and the amount of time the drug is above that MIC level is comparatively less important.
• Time-dependent antibiotics are those that depend upon being above the MIC for some extended period of time to achieve their effect, and the actual concentration achieved is comparatively less important.
Neither of these characteristics are automatically “better” than the other, but having some knowledge of this can help avoid misconceptions. For example, the question of “how long does the antibiotic last” is much less relevant to a concentration-dependent antibiotic than with one that is time-dependent.
And increasing the dose of a time-dependent drug is much less likely to improve treatment response than with a concentration-dependent drug.
Nearly all antibiotics approved for BRD in the U.S. are time-dependent compounds. Some are considered bactericidal, and some are considered bacteriostatic. Only the fluoroquinolones are both concentration-dependent and bactericidal.
Other antibiotic characteristics
There are several other characteristics of antibiotics that are also quite important and should be considered. One of these is antimicrobial resistance and how that resistance tends to develop. That discussion and learning is ongoing, complicated – and a full discussion is beyond the scope of this article.
Suffice to say, however, if antibiotics are not administered in adequate drug concentrations, resistant bacteria may survive while more susceptible bacteria are killed. This “selection” may result in treatment failure, spread of disease to penmates, harder-to-treat relapse infections or death.
Some other important antibiotic characteristics include spectrum of activity, protein binding, tissue distribution, immune status of the patient, speed of absorption, potential pre-existing antimicrobial resistance, withdrawal time required, site of action on the bacteria, etc.
Aim to be right the first time
When should you use which type of antibiotic? That answer depends on your operation, your BRD protocol and, most importantly, your veterinarian.
Antibiotics are an important tool to help reduce animal suffering and death. They do this by helping the animal fight the bacterial infection that is part of or often the primary challenge for calves sick with BRD.
It is extremely important to remember antibiotics cannot repair lung damage or reduce the underlying stresses and exposures that may lead to BRD in the first place. In other words, antibiotics can’t completely compensate for the factors that lead to BRD.
Reducing stress, improving immune function and identifying any sick calves early in the course of disease are three crucial factors that must be addressed from a management perspective if any antibiotic is to be effective.
Working with your veterinarian should entail addressing these factors – even before antibiotic selection.
Treating cattle with the right antibiotic, the right dosage and at the right time can go a long way in maximizing BRD treatment effectiveness. One study showed calves treated two or more times after arrival to a feedlot had lower average daily gain, a poorer marbling score and lost money on average. 
PHOTO: Management decisions that reduce stress, improve immune function and identify sick calves early in the operation, are all key steps to making sure an antibiotic works properly. Photo courtesy of Bayer Animal Health.
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References omitted but are available upon request. Click here to email an editor.
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Jim Sears
- Technical Services Veterinarian
- Bayer Animal Health
