Isolation and pathogenicity of Actinobacillus pleuropneumoniae Serovar 19 in pigs.

Porcine contagious pleuropneumonia (PCP), caused by Actinobacillus pleuropneumoniae (APP), is a common and severe respiratory disease of pigs worldwide. To date, 19 serovars have been documented and serovar 19 has recently been detected in strains isolated by several researchers in China. However, its in vivo pathogenicity has not yet been reported. In this study, an APP serovar 19 strain, designated HN0314, was isolated from a pig with respiratory disease. Twenty-four 10-week-old pigs were intranasally infected with high, medium, or low doses of APP strain HN0314, with six pigs in each group, while PBS-inoculated pigs served as the mock group. The bacterial shedding dynamics, rectal temperature, clinical signs, body weight, and ApxIV antibody responses were monitored over 28 days post-infection (dpi). Seroconversion to ApxIV antibodies was first detected at 14 dpi. Bacteria with loads over 10⁷ CFU in nasal swabs and over 10⁶ CFU in tonsil swabs could be isolated within five days post-infection followed by a gradual reduction in bacterial loads, while qPCR confirmed prolonged detection of APP in nasal and tonsil swabs. Bacterial persistence was longer in the tonsils than in the nasal cavity and lungs. Necropsies were performed at 14, 21, and 28 dpi for histopathological examination and APP molecular testing. Infected pigs developed fever, anorexia, depression, coughing, and dyspnea, whereas no epistaxis or mortality was observed. Gross lesions were characterized by fibrinous pleuritis and interstitial pneumonia, with neutrophil infiltration persisting in infected pigs. The average daily gain of infected pigs decreased by 41%-66% in a dose-dependent manner. These findings indicate that serovar 19 HN0314 exhibits dose-dependent pathogenicity, resulting in considerable clinical and economic impacts. Furthermore, characterizing its colonization and shedding dynamics provides essential information for surveillance and the development of transmission control strategies against APP serovar 19 in pig farms.