Serological profile and pleurisy lesions associated with Actinobacillus pleuropneumoniae in pig farms in North Macedonia

Actinobacillus pleuropneumoniae (App) is one of the most important swine respiratory pathogens that causes porcine pleuropneumonia and massive financial losses in pig industry. The objective of this study was to investigate App infection in five pig herds from North Macedonia experiencing clinical respiratory infections by serological testing and a slaughterhouse pleurisy evaluation system (SPES). In total, 250 blood samples were taken from pigs aged 6, 10, 14, 18, and 22 weeks. Ten animals per age category from each farm were sampled and analyzed for presence of antibodies against App. At the slaughterhouse, 50 lungs per herd from slaughtered age pigs were scored by the SPES for the presence of lesions associated with chronic pleurisy. The overall seroprevalence to App was 65.6%, ranging from 34% to 98% at the farm level. The highest seroprevalence was found in 6-week-old pigs in all farms, while significant differences were observed between farms in the 10-, 14-, 18-, and 22-week-old pig groups. Pleurisy associated with App was found in 26.4% of all examined lungs, with the mean SPES score being 0.75 (0.14 - 1.10). The percentage of SPES scores of 0, 1, 2, 3, and 4 in all lungs were 73.4%, 1.6%, 8.8%, 8.4%, and 7.6% respectively. A significant difference in mean SPES score was obtained between two farms. High seroprevalences of App detected on the tested farms were probably due to constant natural infection. The highest seroprevalences, measured in the youngest pigs, could be due to maternally-derived antibodies. Higher seroprevalence against App and lower SPES scores in some farms suggests immunity resulting from infection by corresponding field serotypes.