Welfare benefits of single versus multiple injections for vaccinating gestating sows

Vaccination is essential for disease prevention in swine production; however, repeated intramuscular (IM) injections can compromise animal welfare. This study evaluated the impact of two vaccination protocols in gestating sows under commercial farm conditions: one involving a single IM injection (VAC1) and the other involving three separate IM injections (VAC3). Each vaccination protocol was administered twice (6 and 3 weeks before the expected farrowing day; dose 1 and dose 2, respectively). Sixty-four pregnant sows were randomly assigned to either VAC1 or VAC3. The main variables were the time required for vaccination and welfare assessments, including behavioural observations, vocalizations, and fear responses. Additionally, health parameters and salivary biomarkers (salivary haptoglobin, chromogranin A, oxytocin) were evaluated before and after vaccination. The VAC1 protocol reduced handling time per sow (13.3 ± 1.39 s/sow in VAC1 vs. 38.3 ± 2.23 s/sow in VAC3; p < 0.0001). Health parameters and farrowing performance did not show relevant differences between protocols under the conditions of this study. Both protocols led to decreased activity and altered resting behaviour, likely due to discomfort at the injection site. These changes were mild and consistent with normal post-vaccination responses. However, sows in the VAC3 treatment displayed more pronounced behavioural indicators of pain and fear during injection, including a higher incidence of rushing or retreat attempts (37.4% ± 9.41 in VAC1 vs. 62.1% ± 9.47 in VAC3; p = 0.0139), back arching (17.2% ± 5.11 in VAC1 vs. 39.6% ± 6.67 in VAC3; p = 0.0127), and high-pitched vocalizations. These findings support the use of a single-injection protocol (VAC1) to improve on-farm applicability and sow welfare. Further research is warranted to investigate the long-term effects of single versus multiple intramuscular injections on animal welfare, with particular attention to the human–animal relationship, chronic stress biomarkers, and productive performance.