How much does it take? Antimicrobials and dosing strategies in food animals (Proceedings)
There are well thought out and clinically confirmed label regimens for label indications. For example, the antimicrobials labeled for individual animal treatment of respiratory disease in cattle and swine have label regimens which most likely give you the majority of the clinical results you will get. When we get into enteric disease, labels are very few, and we often end up looking for efficacy in extra-label use. The problem is that confirmed antimicrobial efficacy in food animal enteric disease is very rare, and we end up looking at in-vitro minimal inhibitory concentrations for which there are no approved veterinary breakpoints.
Combination antimicrobial therapyThere are some in-vitro studies showing synergism for food animal pathogens, but they are few and far between. I have been unable to find clinical efficacy studies demonstrating improved clinical response to a combination of antimicrobials in food animals. It appears that this practice is still relatively common, in cattle anyway, with acceptance and promotion based on anecdotal reports. I am not a believer in antimicrobial combinations for diseases such as bovine respiratory disease. This presentation will focus on individual antimicrobial regimens; if you just want to believe and go ahead with antimicrobial combination therapy, you are on your own as far as supporting data.
Going off label
When we are off label, our best bet is to understand the pharmacokinetics /pharmacodynamics (PK/PD) of the antimicrobials we are using and adjust regimens accordingly. This is not a perfect approach, and is always trumped by clinical data. However, the PK/PD approach is worthy in that we can rule out some completely inappropriate clinical strategies.
There are some drugs where we likely won't be increasing dosing regimens due to cost, and some where we cannot legally use them off-label (e.g., fluoroquinolones in food animals, sulfas in lactating dairy cattle, any antimicrobial in feed). In contrast, there are some classes where properties of the drugs (cost, withdrawal time, safety, spectrum) make them ideal candidates for extra-label regimens. We will focus on two, oxytetracycline, and ceftiofur.