Relationship between the amounts of antibodies to Actinobacillus pleuropneumoniae serotype 2 detected in blood serum and in fluids collected from muscles of pigs.

An indirect ELISA method, previously used to detect antibodies to Actinobacillus pleuropneumoniae serotype 2 in serum of pigs, was further developed aiming to measure antibodies to the microbe in muscle fluids. Serum and muscle fluid were collected from Specific Pathogen Free (SPF) pigs as well as from SPF pigs challenged with A. pleuropneumoniae which were either treated with effective antibiotics or left as infected controls. The antibody responses measured in serum correlated well to the clinical signs of respiratory disease observed and to pathological lesions found at necropsy performed 17 days post-infection. The amounts of antibodies monitored in serum and in muscle fluid collected from the diaphragm and the thigh, respectively, were compared. Higher concentrations of antibodies were assessed in serum than in diaphragm fluid, which in turn contained more antibodies per ml than fluid collected from the thigh. The amount of antibodies to A. pleuropneumoniae measured in fluid from the diaphragm diluted 1/50 correlated well with the quantity measured in serum diluted 1/1000 (r2 = 0.87; P < 0.001). When validated by using serum antibody responses as a standard, the specificity of the ELISA employed in fluid from the diaphragm was found to be 100%. The sensitivity was determined to be 88% when calculated on seropositive pigs (A450 = 0.3 in serum diluted 1/1000). That figure increased to 97% if calculated on pigs expressing pronounced amounts of serum antibodies (A450 > or = 0.5).