Efficacy of a subunit vaccine against <i>Actinobacillus pleuropneumoniae</i> in an endemically infected swine herd

Objective: To evaluate lung lesions at slaughter after three-dose vaccination with a subunit Actinobacillus pleuropneumoniae vaccine containing ApxI, ApxII, ApxIII, and an outer membrane protein. Materials and methods: A total of 430 newborn piglets in a herd endemically infected with A pleuropneumoniae were assigned to control and treatment groups by block randomization at litter level. Pigs vaccinated at 6, 10, and 14 weeks of age and unvaccinated controls were housed in three rooms containing 12 pens each, with treated and control groups mixed in pens. Individual pig data included average daily gain (ADG), number of antimicrobial treatments, and clinical disease. Sera of vaccinated (n = 5) and control pigs (n = 6) collected at 6, 10, 14, 18, and 23 weeks of age were tested for A pleuropneumoniae antibodies using the complement fixation test and ELISAs for Apx toxins and outer membrane protein. At slaughter, lungs were examined for gross and microscopic lesions and cultured for A pleuropneumoniae. Differences in ADG and occurrence of A pleuropneumoniae-related lesions were compared between treatment groups using multilevel mixed models with room and pen as random effects. Results: Actinobacillus pleuropneumoniae-related lesions and ADG did not differ between control and treatment groups. Maternal immunity against A pleuropneumoniae was detected until 10 weeks of age. Moderate vaccine titres were observed after the third vaccination. Implications: Maternal antibody may interfere with the response to subunit A pleuropneumoniae vaccines. Postponing vaccination until 10 to 14 weeks of age might be advisable in herds with high maternal immunity