These proceedings present data related to the question of how long to wait after administering a single injection antimicrobial before applying success/failure criteria. More accurately, we will evaluate success/failure and mortality data based on administering a uniform regimen and then waiting different periods before applying success/failure criteria, and the animal subsequently being eligible for further therapy. This is quite different from administering the drug for differing periods of time and evaluating clinical outcome.

Data concerning the outcome of treatment following differing periods of application of the same repeated regimen are extremely rare. The current (and long-standing practice) of treating for an extended period beyond clinical cure to minimize relapses is being recognized as having a downside related to continued antimicrobial exposure of normal flora and any resistant pathogen subpopulation. In my opinion, the most critically needed antimicrobial use data are those that that will help us to balance minimization of treatment failures and relapses while at the same time minimizing resistance selection pressure.

Single injection, long duration antimicrobial therapy for veterinary use was first available for cattle; long-acting, 200 mg/ml oxytetracycline became available in the mid 1970s. Recently this type of therapeutic approach became available for dogs and cats with a cephalosporin demonstrating T ½ values of 133 hours in dogs and 166 hrs in cats (cefovecin, Convenia™, Pfizer Animal Health). Most recently, ceftiofur crystalline free acid (Excede™, Pfizer Animal Health) has been labeled for horses with two intramuscular injections administered 4 days apart, reported to maintain therapeutic concentrations against Streptococus equi ssp. Zooepidemicus for 10 days..

Post-treatment intervals for antimicrobials used in food animals

One method of looking at post-treatment intervals is to look at the individual drug pharmacokinetics along with susceptibility information (or susceptible breakpoints from susceptibility testing) and look at when serum concentrations (or tissue concentrations?) fall below the cutoff point. Table 1 gives suggested PTIs developed by the author using this method. Question marks indicate special cases where available clinical data merits additional discussion. In addition, the 3 studies following table 1 provide clinical trial data evaluating different post-treatment intervals for commonly used single-injection antimicrobials used in the therapy of bovine respiratory disease. These studies shed some light on the effects of waiting longer to determine success or failure in relation to respiratory disease.

Adapted, and modified from: Apley MD. Bovine Respiratory Disease: Pathogenesis, Clinical Signs, and Treatment in Lightweight Calves. in The Veterinary Clinics of North America, Food Animal Practice, Stocker Cattle Management, W. B. Saunders, Philadelphia, (22:2). 2006