Evaluating antibody isotype-specific ELISA, complement fixation, and Apx1 hemolysin neutralization tests to detect serum antibodies in pigs infected with Actinobacillus pleuropneumoniae serotype 1

JAM: Facultad de Estudios Superiores-Cuautitlan, UNAM, Coordinacion de Estudios de Posgrado, Apartado Postal 25, Cuautitlan Izcalli, Edo. de Mexico 54700, Mexico. BF, MR: College of Veterinary Medicine, Kansas State University, Manhattan, Kansas 66506. HH: College of Veterinary Medicine, Iowa State University, Ames, Iowa 50011. Summary: Eighteen 8-week-old disease-free pigs were infected with Actinobacillus pleuropneumoniae (APP) serotype 1 by intranasal inoculation. Serum antibody responses of 15 of these pigs to APP were measured before the trial and at 2, 4, 6, 9, 10, 12, 14, and 17 weeks post infection (PI) by antibody isotype-specific ELISA (IgG, IgM, and IgA), complement fixation test (CF), and an Apx1 hemolysin neutralization (HN) test. All pigs were serologically negative by all tests prior to being exposed to APP. ELISA IgG, IgA, and HN titers increased by 2 weeks PI and remained stable throughout the sampling period. ELISA IgM titers peaked 2 weeks PI, declined gradually up to week 12, and again increased during the last 5 weeks of the experimental period. The CF titers were erratic with some pigs remaining negative throughout the study. Others were positive soon after becoming infected only to turn negative at later sampling times. Only one pig was CF positive at all sampling times PI.