Evaluation of Two Injection Techniques in Combination with the Local Anesthetics Lidocaine and Mepivacaine for Piglets Undergoing Surgical Castration

SIMPLE SUMMARY: Pain elimination during the surgical castration of male piglets is an important animal welfare issue. Bans on castration without pain relief are pending in most pork-producing European countries. Surgical castration of suckling piglets without anesthesia has been prohibited in Germany since January 2021. Boar fattening, gonadotropin-releasing hormone (GnRH) vaccination and surgical castration under isoflurane or injection anesthesia are permitted as legal alternatives. These alternatives have advantages and disadvantages, and a feasible and simple method is still needed. The legal basis for another alternative, i.e., the application of a local anesthetic by the farmer prior to castration, has been created in Germany, but evidence of its efficacy remains controversial. The present study developed a feasible and effective method for local anesthesia in piglet castration. Two different injection techniques in combination with the local anesthetics lidocaine and mepivacaine were investigated using nociceptive parameters, injection pressure and tissue distribution. Both injection techniques significantly reduced the nociceptive parameters regardless of the local anesthetic used and achieved similar distribution patterns and injection pressures. One method performed slightly better in the experimental setup, based on injection duration and handling. ABSTRACT: The present study evaluated the effects of two injection techniques in combination with lidocaine or mepivacaine for piglets undergoing castration. To improve ease of use, a cannula with side holes (one-step fenestrated (F)) was invented to simultaneously deliver a local anesthetic into the testis and scrotum and was compared to a two-step injection technique. The distribution of a lidocaine/contrast agent mixture using the two methods was examined using computed tomography. Piglets were randomly divided into treatment groups: handling, castration without pain relief and castration after lidocaine or mepivacaine injection using the one-step F or two-step method. Acute physiological responses to noxious stimuli were evaluated by measuring the mean arterial blood pressure (MAP), heart rate (HR) and nocifensive movements. Fos protein expression in the spinal dorsal cord was semi-quantitatively analyzed. Both injection techniques achieved similar distribution patterns. The one-step F method was faster and easier. Injection was not associated with significant changes in MAP or HR, but Mepi1 and NaCl elicited significantly increased nocifensive movements. Both techniques significantly reduced MAP and nocifensive movements when the spermatic cords were cut, regardless of the local anesthetic type. Compared to NaCl, only the lidocaine treatments significantly reduced HR during skin incision. Lido2 significantly reduced Fos protein expression.