99 problems...part 2

So, there you are having to decide how to treat before you have all the information you need. A bit of local knowledge and a close look at the horse will get you in the ballpark (see 99 problems...part 1). How do you take that first step?

Begin appropriate empiric treatment before going to narrow spectrum

It may take between 24 and 72 hours (plus transportation time and non-working days because some people have weekends) to obtain positive identification of the pathogen. While you are waiting, broad-spectrum therapy can begin. In horses, the combination of a beta-lactam like penicillin or ampicillin with an aminoglycoside, whether it be amikacin in foals or gentamicin in adults, is widely practiced with good reason. This will likely cover most common bacterial infectious agents. Alternatively, other broad spectrum antibacterials can be used. Knowledge of regional sensitivity and resistance patterns, gained from previous culture results, is invaluable to minimize chances of delaying effective therapy.

Once identification and sensitivity patterns (which antimicrobials are effective in laboratory testing) of the organism are known, narrow spectrum antimicrobials, when appropriate, can be initiated. Consideration should be given to cost, the potential for toxicity in the current situation, and potential to promote resistance in the population. Indiscriminate antimicrobial usage has painted all medicine into a corner. This is nature at work: bacterial resistance is promoted, and they will survive.

Have realistic expectations of the chosen agent

Treatment is more likely to succeed in an otherwise healthy horse versus an immunocompromised horse or neonate. Antimicrobial treatment is no substitute for considering the horse as a whole and providing the supportive care necessary. If the horse can’t breathe or is dying from dehydration it is time to address what will kill the patient first.

Some bacteria are inherently resistant to certain antimicrobials making them ineffective. Identification of the organism allows it to be categorized as shown. A particular antimicrobial may not be effective in all categories, but this may not matter once the bacteria is identified. If infection is mixed combination therapy may allow complete coverage.

Even if likely effective, the antimicrobial must be present at the site of infection at a sufficient concentration for a sufficient time to be successful.  Can the antimicrobial penetrate to the site of infection, and will it work in the conditions that exist? If fluid is present in the pleural space (pleuritis) there is a high probability that a very low oxygen environment exists within that fluid. This presents two problems: not all antimicrobials will be able to penetrate fully into the fluid, and conditions may not be favorable for their action even if they do. For example, aminoglycosides require oxygen to be effective, so they should be paired with a beta-lactam that is more likely to work in that environment as this will increase the range of organisms covered.

In a few weeks I’ll talk about how a laboratory can guide you through all of the above and help you make better treatment decisions.

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