Assessing the efficacy and side effects of lidocaine used as an analgesic on rainbow trout (Oncorhynchus mykiss)

It has been said that “in many animals, the most simple and elegant way to control pain perioperatively is to perform a local or regional anesthetic block” (1). The experiments outlined in my thesis aimed to show whether or not fish can be counted among those animals. Fish are proving to be a useful animal for many types of research. Unfortunately, their increased popularity in recent years has not been accompanied by an important improvement of our understanding of their welfare. There is ongoing debate about whether or not fish can consciously feel pain even though it is agreed that they can perceive and respond to noxious stimuli. They are regularly exposed to such stimuli through various procedures related to research, yet analgesics are rarely used(2). This is why I evaluated lidocaine, a local anesthetic widely used in humans and other species, as a potential addition to current anesthetic protocols for fish. I started by looking at potential side effects of the infiltration of a high dose of lidocaine (20 mg/kg) on each side of the dorsal fin of young rainbow trout (15 fish per group) as compared to infiltration with an equal volume of saline in the same locations or anesthesia only. The fish were housed individually for the trial during which they were observed and recorded for 4h after infiltration and fed at 6 different time points (15 min, 30 min, 1h, 2h, 3h, 4h). Their behavior was then analyzed using tracking software and potential tissue lesions were assessed by histopathology of the infiltration site. Ventilation rate was recorded using the remote function of GoPro cameras placed on the side of each trial tank. Another experiment aiming to establish a beneficial dose range for the infiltration of lidocaine repeated this procedure while adding a noxious stimulus in the form a small metal spring on the dorsal fin and testing doses of 2 mg/kg, 6 mg/kg, 20 mg/kg and 60 mg/kg. My results show some potentially beneficial effects of lidocaine at a dose of 20 mg/kg when used as a local anesthetic for rainbow trout, such as increased food consumption and ventilation rate closer to usual reported range compared with the other groups. There was degeneration and inflammation in the muscle at the site of lidocaine infiltration for the first 15 days post-infiltration but the infiltration sites were mostly healed at 30 days post-infiltration. This study introduced a new model to test local anesthetics and potentially other analgesics. This model appears promising to test local anesthetics and potentially other analgesics, but should be refined before being used in the future. Further studies are needed to find an optimal analgesic dose, as no clear conclusions could be reached based on my data