Septic shock in pigs infected, vaccinated and challenged with Actinobacillus pleuropneumoniae: A clinicopathological study.

Porcine pleuropneumonia (PP), caused by Actinobacillus pleuropneumoniae, is a highly impactful respiratory disease in swine production. This study evaluated the efficacy and pathological outcomes of a polyvalent bacterin vaccine in both noninfected (ANI) and previously infected (AI) pigs. Eighty pigs were divided into four groups (n = 20): A, vaccinated ANI; B, nonvaccinated ANI; C, vaccinated AI; and D, nonvaccinated AI. Vaccinated animals received two doses over two weeks. All the groups were challenged via aerosols containing a combination of A. pleuropneumoniae serotypes. Each group was further divided into four subgroups (n = 5) exposed to different serotype combinations. Mortality occurred rapidly postchallenge: 10 % within 12-24 h, 22.5 % by 36 h, and 45 % by 60 h. Necropsy revealed extensive necrotic-hemorrhagic pulmonary lesions, affecting up to 77.5 % of the lung surface during early death, which was consistent with septic shock. Lesion severity varied by group: group A (10.1 %), C (11.5 %), D (13.5 %), and B (18.8 %). These results suggest that severe pulmonary vascular injury and a generalized septic response contribute to per-acute mortality in vaccinated and nonvaccinated pigs. These findings underscore the complex interplay between infection status, vaccination, and systemic inflammatory responses in PP pathogenesis. Early deaths in vaccinated, previously infected pigs were consistent with septic shock driven by endotoxin and RTX (Apx) toxin-mediated pulmonary vascular injury under overwhelming aerosol challenge; vaccination did not induce this phenomenon. These findings support screening for subclinical infection and pairing vaccination with strengthened biosecurity when herd status is uncertain.