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..
The question is particularly relevant when you consider the large number of single-injection antimicrobials available for
• Ceftiofur crystalline free acid (Excede®, Pfizer)
• Enrofloxacin (Baytril®, Bayer)
• Florfenicol (Nuflor®, Schering-Plough)
• Tilmicosin (Micotil®, Elanco)
• Tulathromycin (Draxxin®, Pfizer)
• Oxytetracycline 200 mg/ml (Liquamycin LA-200®, Pfizer, and generics)
• Oxytetracycline 300 mg/ml (Tetradure®, Merial)
In addition, there are antimicrobials with cattle labels indicating administration 48 hours apart as either one of the label
regimens or the sole label regimen.
• Ceftiofur hydrochloride (Excenel® RTU, Pfizer)
• Danofloxacin (A-180®, Pfizer)
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.
Table 1: Suggested post-treatment intervals for antimicrobials used in cattle
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).