Background
Deep digital septic conditions represent some of the most refractory causes of severe lameness in cattle. The objective of this study was to determine the distribution of tulathromycin, gamithromycin and florfenicol into the synovial fluid of the metatarsophalangeal (MTP) joint of cattle after single subcutaneous administration of drug to evaluate the potential usefulness of these single-dose, long-acting antimicrobials for treating bacterial infections of the joints in cattle.

Results
Twelve cross-bred beef cows were randomly assigned to one of the drugs. Following subcutaneous administration, arthrocentesis of the left metatarsophalangeal joint was performed at various time points up to 240 hours post-injection, and samples were analyzed for drug concentration. In synovial fluid, florfenicol pharmacokinetic parameters estimates were: mean Tmax 7 +/− 2 hours, mean t½ 64.9 +/− 20.1 hours and mean AUC0-inf 154.0 +/− 26.2 ug*h/mL. Gamithromycin synovial fluid pharmacokinetic parameters estimates were: mean Tmax 8 hours, mean t½ 77.9 +/− 30.0 hours, and AUC0-inf 6.5 +/− 2.9 ug*h/mL. Tulathromycin pharmacokinetic parameters estimates in synovial fluid were: Tmax 19 +/− 10 hours, t½ 109 +/− 53.9 hours, and AUC0-inf 57.6 +/− 28.2 ug h/mL.

Conclusions
In conclusion, synovial fluid concentrations of all three antimicrobials were higher for a longer duration than that of previously reported plasma values. Although clinical data are needed to confirm microbiological efficacy, florfenicol achieved a synovial fluid concentration greater than the MIC90 for F. necrophorum for at least 6 days.