Acute phase protein concentrations following serial procaine penicillin G injections in horses

Background: Acute phase protein (APP) measurement is used to detect inflammation. Intramuscular (IM) injections could cause tissue injury and induce an acute phase response (APR). Objectives: To evaluate the effects of IM procaine penicillin G (PPG) injections on APP concentrations in horses. Study design: Prospective longitudinal design. Methods: PPG was administered intramuscularly to six horses, twice daily, for 5 days. Plasma fibrinogen (FIB), serum amyloid A (SAA), haptoglobin (HAP), creatine kinase (CK), and aspartate aminotransferase (AST) were quantified daily for 5 days before the first injection, during the course of administration, and for 4 days after the final dose. Analytes were quantified every other day for the remaining 16 days. Data were compared using a parametric or non-parametric repeated measures ANOVA and a Tukey's or Mann–Whitney rank sum test, respectively. Significance was set at p < 0.05. Results: CK was increased over baseline (mean ± SD: 200 ± 74 IU/L) on Days 1–6 (p < 0.001 to p = 0.02, mean ± SD: 723–1177 ± 355–544 IU/L) and AST was increased above baseline (mean ± SD: 233 ± 58 IU/L) on Days 2–7 and 10 (p < 0.001 to p = 0.05, mean ± SD: 307–437 ± 79–146 IU/L). Increased FIB was noted over baseline (mean ± SD: 177 ± 30 mg/dl) on Days 6–8 and 10 (p = 0.02 to p = 0.03, mean ± SD: 234–252 ± 33–49 mg/dl). SAA was increased above baseline (mean ± SD: 4.7 ± 2.9) on Day 6 (p = 0.02, mean ± SD: 113 ± 186 μg/ml). There was no change in HAP. Main limitations: Healthy horses were used, small sample size, and a lack of a negative control group. Conclusions: Serial intramuscular procaine penicillin G (IM PPG) injections may result in increased positive APP concentrations in horses and this must be considered when these test results are interpreted.