Forty sows were utilized in a study to determine the plasma concentration profile and tissue residues of procaine penicillin G after injection by two methods (20 sows each method). A target dose of 33,000 IU/kg (5 ml/100 lbs) of a 300,000 IU/ml commercially available procaine penicillin G suspension was injected by either conventional intramuscular injection with a needle (CI) or injection by needle-free device (NF) in the hip.

For the plasma concentration study, 15 plasma samples were taken over 24 hours from 8 pigs in each of the two treatment groups. Anaylsis of the samples showed that the penicillin G was absorbed more slowly after CI as compared with NF, with mean times to maximum concentration of 10.1 ± 6.3 hrs and 2.4 ± 0.9 hours respectively. The mean maximum plasma concentrations and elimination half-lives also differed between the groups with values of 107.7 ± 39.0 ng/ml and 38.3 ± 17.1 hours for CI and 430.8 ± 83.6 ng/ml and 6.1 ± 0.9 hrs for NF respectively. These results suggest that the frequency of administration for the two routes may need to be different, but this would depend on several characteristics of the target pathogen and should be discussed with a veterinarian.

In the tissue residue study, samples of kidney, liver, fat, muscle, and injection site were collected at 2, 4, 6, and 8 days post-injection. The analysis method resulted in varying levels of detection and quantification in different tissues. In swine tissues, the accepted tolerance is no detectable concentration. Only in fat were no concentrations detected for NF at any sample time, and in all samples on days 4, 6, and 8 for CI. For kidney, liver, muscle, and injection site, at least one animal in both treatment groups had detectable concentrations of penicillin G on day 8. While it would be preferable to have had all animals negative at the final sampling time, statistical analysis of the data suggests a tissue slaughter withdrawal time of 28 days for 95% confidence that 99% of animals injected at the dose in this study would have no detectable residues in kidney using the test reported here. The kidney was reasonably representative of the injection sites for NF tested in this study, however, the CI injection sites displayed much higher concentrations than the kidney. For this reason, the data derived for CI injection sites in this study were not suitable for calculating when the injection site would be free of detectable residue. There were no statistically significant differences in the injection sites by microscopic examination.

Injection of procaine penicillin G by conventional intramuscular needle injection and by needle-free injection resulted in different plasma concentration profiles but similar slaughter withdrawal estimates using kidney as the target tissue. Injection site concentrations after injection of up to 10 ml/site with a conventional needle technique suggest that injection site residues will persist for significantly longer than residues in the kidney.