Bovine respiratory disease complex includes bacterial components, which cause the classic clinical signs of lethargy, depression,
and fever, with variable nasal discharge, cough, or other signs. This bacterial component of BRD (most commonly Mannheimia
haemolytica, Pasteurella multocida, Histophilus somni, and Mycoplasma bovis) may be treated with antimicrobial drugs designed
to kill or inhibit the growth of the pathogenic bacteria. Veterinarians may use antimicrobial drug characteristics, such as
physiochemical properties (e.g., lipid solubility), mechanisms of action, and effects on in vitro bacterial growth (e.g.,
bacteriostatic, bactericidal, peak-dependent, time-dependent), to help make a decision about which drug to use and how to
use it (regimen: dose, route, frequency, duration, withdrawal time). While these characteristics may add to the discussion
and provide pieces of evidence, the ultimate and most convincing evidence is clinical efficacy. For the treatment of bovine
respiratory disease, there is considerable good quality published evidence for the efficacy of a number of antimicrobials,
as exemplified in the table below. These drugs, all approved in the U.S. for BRD, span the spectrum of antimicrobials, including
lipid-soluble and water-soluble, peak-dependent and time-dependent, and bacteriostatic and bactericidal. Therefore, using
drug characteristics to make drug decisions for respiratory disease does not address the real questions related to how well
the drug works. With this many drugs approved for the treatment and control of respiratory disease, how does one make a decision
about which drug to use, and equally important, how to use it?
Drugs can be selected by using the principles of evidence-based veterinary medicine:
1. Based on a need for information (a single animal or herd outbreak of respiratory disease), ask a clinically relevant
question, including Patient, Intervention, Comparison, and Outcome (PICO).
• The purpose of the clinical question is to focus your information search and to identify what you are really interested
in answering. A vague clinical question will not result in good clinical decision-making.
• Some examples of relevant clinical questions associated with the therapy of respiratory disease include: In high
risk recently weaned calves that have just arrived at a feedlot, which drug product has been shown to decrease mortality?
In pre-conditioned, recently-weaned calves headed for backgrounding, has metaphylaxis been shown to reduce costs of disease,
including morbidity, mortality, and cost of gain? In dairy cows with signs of respiratory disease, has a particular drug product
been show to be more effective at return to production?
• Clinical questions can be asked about a current problem, or they can be anticipated: a plan to wean calves in
the fall suggests a need for information on treating respiratory disease in these calves prior to the fall season.
2. Using the clinical question, search for all the available published evidence to answer the question.
• Typically, this step is searching in the published biomedical and animal science literature. While this may be
a prolific source of data, in some cases, evidence may be lacking in the published literature and must be found elsewhere.
Technical reports from animal health companies on the subject of bovine respiratory disease therapy abound. In addition, well-designed
animal record systems with good acquisition of high quality data may also be a source of evidence, although the emphasis needs
to be on "high quality data". Bad data is usually worse than no data at all. The use of anecdotal data or clinical impression
is always fraught with opportunities for misinterpretation and should generally be avoided. Our minds are not designed to
retain the best evidence but rather to retain the interesting, unusual, and exciting evidence. This is not to argue that we
do not recognize patterns – that is the basis of many clinical diagnoses. However, patient outcomes are best analyzed via
written record rather than recollection of clinical cases.
3. Once the available evidence is found, it must be reviewed for relevance and quality.
• Many more thorough pieces have been written on how to evaluate evidence, but here are a few of the important things
to look for when reviewing a published paper, a technical report, or your own clinical data related to pharmacotherapeutics.
• Were animals randomized to treatment group?
• Were the outcomes (like days with fever) similar to what you expect from successful therapy?
• Are the animals in your patient population the same as or similar to those in the paper?
• What did the investigators not mention that you wanted to know about?
• Are the conclusions appropriately specific, or do they over-generalize?
• Who paid for the study? Is it possible that information was left out of the report?
• If the authors find no difference between treatment groups, did their study actual have enough statistical power
to find a difference (usually related to number of animals)?
4. Making the decision about which drug to use and how to use it is the last step. An informed clinical decision come
from high quality evidence. Low quality evidence leads us to ask another clinical question, continue our search for data,
or search for alternative therapies.
Because of the number of drugs approved for the treatment of respiratory disease in cattle, it would be very difficult to
justify any extralabel use of antimicrobial drugs. The AMDUCA hierarchy for extralabel use demands that there are no drugs
approved for the condition that are clinically effective for extralabel use to be considered. A veterinarian would have to
be convinced that none of the drugs in the table below are clinically effective, which would be challenging.