Clinical signs and etiology of adverse reactions to procaine benzylpenicillin and sodium/potassium benzylpenicillin in horses.

This study investigates adverse reactions experienced by horses after treatment with either procaine benzylpenicillin or sodium or potassium benzylpenicillin. Through 59 reported cases, it suggests that the reactions may be due to allergies in some instances but are largely attributed to the toxic effects of procaine, particularly manifesting as locomotor and behavioral changes. Contributing factors such as repeated injections and lower plasma esterase activity, which increases the likelihood for procaine toxicity, are also identified.

Study Methodology and Data Collection

• The study centered on case reports from horse owners in Sweden, collected between 2003 and 2005. These reports documented instances where horses exhibited adverse reactions after receiving procaine benzylpenicillin or sodium or potassium benzylpenicillin treatment.
• Various parameters were considered in assessing these reports, including the time it took for reactions to occur, any information on previous penicillin treatments, the clinical signs observed in the reacting horses, and the interventions made following the reactions.

Findings and Analysis

• The data indicated that two horses had received sodium or potassium benzylpenicillin intravenously, while the remaining 57 horses were treated with procaine benzylpenicillin via intramuscular injection.
• Results pointed to the likelihood of allergies being the cause of adverse reactions in horses treated with sodium and potassium benzylpenicillin, and in a few horses administered procaine benzylpenicillin.
• However, the majority of reactions in horses treated with procaine benzylpenicillin were determined to be possibly due to the toxic effects of procaine, particularly signified by changes in locomotion and behavior.

Risk Factors and Toxicity

• The study also identified common risk factors for procaine-induced reactions. One such factor was the administration of repeated injections, which not only increased the odds of delivering the drug intravascularly, but also heightened the sensitivity to procaine through a process called neuronal sensitization or kindling.
• Procaine’s quick hydrolyzation by plasma esterases into non-toxic metabolites was another factor explored. The study found that if large amounts of procaine entered the circulatory system, the body’s capacity to hydrolyze the substance might be overwhelmed, leading to toxicity.
• This was further supported by an analysis of plasma esterases in reacting horses, which showed lower activity levels than those in non-reacting control horses. This indicated that lower esterase activity could increase a horse’s susceptibility to procaine toxicity and contribute to the occurrence of adverse reactions.