Significance of different types and levels of antigen-specific immunity to Actinobacillus pleuropneumoniaeinfection in piglets

The aim of the study was to verify under experimental conditions the hypothesis that a reliable protection of piglets against Actinobacillus pleuropneumoniae (App) infection can be obtained by colostrum-derived antibodies and/or a low-dose infection from sows naturally infected with App. Twenty-eight piglets were allocated into four groups. Piglets of groups 1A and 1B were the progeny of App-seronegative sows, piglets of groups 2A and 2B originated from App-seropositive sows. At the age of 4 weeks, piglets in groups 1A and 2A were exposed to a low infection dose with the pathogenic strain of App serotype 9 (4 × 104 CFU/ml). At the age of 8 weeks, all four groups of piglets were exposed to the App serotype 9 challenge (2.5 × 107 CFU/ml). We compared the differences in clinical, radiological, pathological changes and cytological findings in blood and bronchoalveolar lavage samples between groups with different levels of antigen-specific immunity after challenge. Piglets that obtained colostrum-derived antibodies and experienced a low-dose App infection were best protected against App challenge. More apparent clinical and pathological changes were observed in groups protected with either only colostrum-derived antibodies or active immunity induced by a preceding low-dose infection. However, none of the types of protection prevented developing of the disease and characteristic changes in the lungs. Cell counts changes in blood and lavage gave evidence of only bacterial infection in progress; however, between-group differences were not marked. Antibodies (IgG, IgA) present in the respiratory tract at the time of challenge played a major role in protection against App infection.