Surgical castration in piglets: its impacts on pain and affective states

Abstract

Surgical castration of young male piglets is an extremely common husbandry procedure in the United States swine industry, with nearly 100% of males castrated. The procedure is performed primarily to prevent the occurrence of boar taint in the meat and to reduce aggression as the animals age. No research has investigated the impact of surgical castration on the long-term mood states – also known as the affective state – of piglets, but it has been proven to cause both acute and chronic pain that can last up to 4 days, modify piglet behavior, and reduce weight gain. Because of this, feasible approaches for pain alleviation are needed. A hinderance to this is that currently no analgesics are approved for piglets. In addition, we need validated, practical methods to recognize pain in piglets in order to help researchers, veterinarians, and caretakers to identify the need for analgesics. One useful tool for this could be the Piglet Grimace Scale, which has previously been validated in this context, but is not widely applied. The current lack of analgesic use during castration may be cause for concern for consumers, however little is known about the public's knowledge on the industry's practices. Therefore, the thesis' objectives are to (1) assess the impact of surgical castration on pain and affective states, (2) determine people's ability to recognize pain expression after surgical castration, and (3) get insights into public perception of swine industry practices. In Chapter 3, we assessed the impact of surgical castration and practical pain alleviation methods during and after castration on piglets' short and long-term affective states and activity levels. Piglets were surgically castrated, castrated with analgesics, or sham-handled at 3 days of age. Data on piglets' activity (using accelerometers), tails (affective states), and grimaces (pain expression) were collected for 1, 6, and 24 hours after castration treatments while piglets were in their home pens. An attention bias test was then performed in week 1 and 12 to assess anxiety (an affective state). Home pen activity after castration treatments suggested that there was a positive impact of using analgesics. The piglets' behavior (eating) and activity during the attention bias test at week 1 suggest a negative impact of castration on affective states. The differences observed in week 1 were no longer found in week 12, suggesting that any impact of surgical castration at 3 days of age on affective states are no longer detectable at 12 weeks of age. In Chapter 4, through an online survey we investigated the validity and reliability of Piglet Grimace Scale ratings applied by swine industry professionals and the general public, to assess its potential utility in non-research settings. The survey contained a training, followed by 9 piglet images showing facial expressions immediately after castration or sham-handling. Both response groups were able to recognize pain in castrated piglets. However, both response groups overestimated pain experience compared to trained experts, suggesting more training may be necessary. Nevertheless, overall, the scale can be widely applied by veterinarians, industry professionals and even members of the public. In Chapter 5, a survey provided initial insights into public knowledge and perceptions on castration and analgesia use and compared this to industry stakeholders. The results show that knowledge on industry practices was especially lacking for public respondents, but also for a minority of industry respondents, indicating opportunities for education and further research on the topic.