ARS | Publication request: Nitrous oxide by itself is insufficient in relieving piglet castration-induced pain

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 1, 2011
Publication Date: October 1, 2011
Citation: Rault, J.Y., Lay Jr, D.C. 2011. Nitrous oxide by itself is insufficient in relieving piglet castration-induced pain. Journal of Animal Science. 89(10):3318-3325.

Interpretive Summary: Surgical castration is performed on all male pigs in the United States. However, castration is painful. Finding means to alleviate this pain will improve animal well-being. Administering gases which relieve pain during castration could represent an option. Inhalant gases act quickly and have only short-term effects, with a quick recovery once the gas is removed. Nitrous oxide, commonly known as the “laughing gas”, has been widely used in human surgery and dental offices to relieve pain or distress. Yet, nitrous oxide has not been investigated for use in farm animals. We hypothesized that nitrous oxide may reduce the pain induced by castration in piglets. We used 24, 3-day old, piglets from 12 litters, one piglet receiving nitrous oxide and a littermate receiving air as a control. After 150 seconds of exposure to the gas, castration was performed. Scores to quantify agitation of the piglet and total vocalization length were recorded during castration. Behavioral observations were continued for 3 days post-castration for 2 hour periods in the morning and afternoon of each day, to the expection of a 4 hour period immediately after castration. Weight gain was measured after 3 days and at weaning. Nitrous oxide successfully put asleep all nitrous oxide treated piglets, as the nitrous oxide piglets were unresponsive after less than 150 seconds under the gas. The nitrous oxide piglets vocalized less for the 150 seconds under the gas, but similarly to control piglets during castration itself as nitrous piglets awoke when castration started. The nitrous piglets showed less agitation, both in frequency and intensity, than C piglets during the whole procedure. For 2 hours following castration, nitrous oxide piglets displayed less huddling behavior, a possible indicator of pain, than control piglets. Over the 3 days, nitrous oxide piglets performed more tail wagging and tended to show less sleep spasms than control piglets, but the interpretation of those behaviors remain unclear. Piglets given nitrous oxide tended to have a lower growth rate than control piglets at 3 days post-castration and at weaning. We concluded that nitrous oxide was effective in putting young piglets asleep. Nonetheless, it appeared insufficient in fully preventing the pain induced by castration.

Technical Abstract: Surgical castration is performed on all male pigs in the United States. However, castration is painful and analgesics have been considered to relieve pain. Inhalant gases with analgesic properties allow for a fast induction, short-term and reversible effects, and are a needle-free option. Nitrous oxide (N2O), “laughing gas”, has been widely used in human surgery and dental offices as an analgesic, sedative and anxiolytic drug. Yet, N2O has not been thoroughly investigated for use in farm animals. We hypothesized that the analgesic effect of N2O may reduce the pain induced by castration. We used 24, 3-d old, piglets from 12 litters, one piglet receiving N2O (N) and a littermate receiving air as a control (C). After 150 s of exposure to the gas, castration was performed while the piglet remained exposed to the gas. Agitation behavior scores and total vocalization length were recorded during castration. Behavioral observations were continued for 3 d post-castration using a 5-min scan-sampling method for 4 h the first morning and for 2 h periods in the morning and afternoon of each day thereafter. Weight gain was measured after 3 d and at weaning. Data were analyzed using a mixed model in SAS. Nitrous oxide successfully induced anesthesia in all N piglets, as validated by a skin pinch test and the loss of palpebral reflex. Total vocalization length was shorter in N piglets during the induction phase (P < 0.01) but not different during castration itself as N piglets awoke and vocalized as much as C piglets (P > 0.1). Agitation scores during the whole procedure were reduced in N piglets, in both frequency (P < 0.01) and intensity (P < 0.05). For 2 h following castration, N piglets displayed less huddling behavior than C piglets (P < 0.01). Over the 3 d, N piglets performed more tail wagging (P < 0.05) and tended to show less sleep spasms (P < 0.1) than C piglets. Piglets given N2O tended to have a lower growth rate than C piglets at 3 d post-castration and at weaning (P < 0.1). Nitrous oxide was effective in inducing anesthesia in neonatal pigs. Nonetheless, its anesthetic effects appeared insufficient in preventing castration-induced pain.