Pain and behaviour - the acute response
In the report delivered by EFSA (2004) it is stated that since the testes and the scrotal skin are innervated with nociceptors, it is highly likely that castration induces pain and is, therefore, both a painful and a stressful event when it is performed without anaesthesia and post-operative analgesia.
A nociceptor is a sensory receptor that responds to potentially damaging stimuli by sending nerve signals to the spinal cord and brain. This process, called nociception, usually causes the perception of pain.
Application of nociceptive stimuli generally stimulates the adrenal and sympathetic axes which, in turn, induce numerous reactions such as acceleration of heart rate, release of energetic nutrients and glucocorticoid hormones. These reactions can be used as indices of pain (EFSA, 2004).
In humans, pain is described as a subjective perception that is very difficult to communicate and evaluate (EFSA, 2004), in animals it is even more difficult to identify and measure, as their communication capacities are restricted.
Several experiments carried out in pigs clearly indicate that surgical castration induces endocrine and behavioural responses, which are accepted as indicators of pain (EFSA, 2004).
Piglets react to castration with high frequency calls (> 1000 Hz) (Weary et al., 1998; Taylor and Weary, 2000) and Marx et al. (2003) have identified three call types during the castration of pigs: grunts, squeals and screams. The number of screams per animal was almost doubled in piglets that were castrated without local anaesthesia compared with piglets castrated with anaesthesia. These calls are accompanied by physical resistance movements and an activation of the sympathetic system, as demonstrated by an increase in heart rate (White et al., 1995).
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Conventional castration of piglet with simultaneous measurement of high frequency calls.
Comparison between methods of restraining (piglets held on a flat bench vs. piglets suspended by the legs vs. piglets restrained in a v-trough) and severing the cord (pulling and tearing vs. cutting) does not show any difference in the calls (Weary et al., 1998; Taylor and Weary, 2000).
It has also been found that castrated pigs spend less time at the mammary glands (McGlone and Hellman, 1988; McGlone et al., 1993; Hay et al., 2003). Castrated piglets remain even more inactive while awake and they show more pain related behaviours (prostration, stiffness, trembling) and tail wagging.
Finally, they frequently seek solitude and their behaviour is more frequently desynchronized compared with the remaining piglets in the litter (Hay et al., 2003).