Chase answers that by saying there has been work with some antibiotics that shows that macrophages will concentrate some of these drugs. “You just can’t use in vitro testing alone. In vitro testing can be very misleading.”
McVey notes that with sensitivity testing, it very much depends on bug-drug interactions. “I find those kinds of measurements more useful in an epidemiological sense as opposed to a therapeutic sense.”
“The whole concept of sensitivity testing and using that as more than just one piece of information is contrary to what we’re talking about in this multi-factorial setting,” Hunsaker adds. “We have a number of things going on and now we’ve isolated one bug in a lab environment in a Petri dish or a blood agar plate and then making an assessment of that bug in that environment versus the multi-factorial environment of the lung. It is tough to make that leap accurately.”
Another issue is actually getting the antibiotics to the bacteria. “Various bacteria are very good at creating barriers,” Griebel says. “We’ve talked about how Pasteurella multocida can create an abscess, and if there’s no blood flow, then there’s no antibiotic delivery.”
With the antibiotics available, Lukasiewicz says that some people may have the impression if they use an antibiotic that lasts seven to 10 days, they can ignore that animal for seven to 10 days. “It goes back to post-treatment care and animal husbandry, observing those animals on a daily basis,” he says. “Sometimes you need to recognize that you might have to bring them back in the following day and re-drench them or provide an anti-inflammatory — not necessarily giving them another antibiotic, but giving them something to help them out and feel better.”
Some people think every animal in the population is the same and they’ll all respond, but they’re all in a different place on that morbidity curve or in the incubation phase,” adds Hunsaker. “To consider they all ought to be treated the same is probably misleading.”
Antimicrobials may reduce the number of bacteria, but the animal has to heal itself. Post-treatment care is probably the most important — clean water, a clean pen, adequate feed, and a palatable feed source. Lukasiewicz believes the best medicine of all is free-choice prairie hay. “You want your hospital to be clean. Sometimes they have a better chance of surviving by going back to their home pen instead of staying in the hospital.” Lukasiewicz only wants to leave extremely ill or special-needs cattle in the hospital pen if he can.