Plasma Creatine Kinase and Aspartate Aminotransferase Activities Before, During, and After 5 Days of Intramuscular Penicillin Administration in Sedentary Horses.
Authors: Foreman-Hesterberg Catherine R, Foreman Jonathan H
Journal: Journal of equine veterinary science
Summary
# Editorial Summary Intramuscular penicillin injections are routine in equine practice, yet their effects on muscle integrity markers have not been formally documented. Foreman-Hesterberg and Foreman investigated plasma creatine kinase (CK) and aspartate aminotransferase (AST) activity in nine healthy sedentary horses receiving twice-daily procaine penicillin G (22,000 IU/kg) for five days, with blood sampling conducted daily for seven days before treatment, throughout the injection period, and for 32 days afterwards. Plasma CK elevated above reference range within 24 hours of the first injection, peaking at 2,046 U/L on day three before normalising by day nine; AST showed a more prolonged response, peaking at 703 U/L the day after final injection and remaining elevated in some individuals for up to 29 days post-treatment. These findings have important implications for practitioners interpreting bloodwork in treated horses—elevations in muscle enzymes should not automatically be attributed to exertional myopathy or systemic disease within approximately two weeks of intramuscular antibiotic therapy, particularly for AST which demonstrates considerably longer elevation than CK. Clinicians should consider injection timing relative to diagnostic blood sampling and be aware that exercise during the treatment window may compound these enzyme elevations, making communication with owners about temporary activity restriction advisable during antibiotic courses.
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Practical Takeaways
- •Intramuscular penicillin injections cause measurable muscle damage indicated by elevated plasma enzymes; clinicians should be aware that elevated CK and AST after IM injections do not necessarily indicate systemic disease
- •AST elevation persists much longer (28 days) than CK elevation (8 days) after IM penicillin treatment, so post-treatment enzyme monitoring should account for this extended timeline
- •Consider alternative administration routes or injection site management strategies if frequent IM injections are needed, as repeated muscle damage from injections can compromise muscle integrity
Key Findings
- •Plasma creatine kinase (CK) peaked at 2,046 ± 627 U/L after 3 days of intramuscular penicillin and remained elevated for 8 days after treatment onset
- •Plasma aspartate aminotransferase (AST) peaked at 703 ± 135 U/L one day after the last injection and remained elevated for 28 days after treatment onset
- •CK returned to reference range 9 days after treatment began, while AST took 29 days to normalize in all individual horses
- •Both CK and AST elevations were statistically significant compared to control period (P < 0.001 to P = 0.03)