Actinobacillus (Haemophilus) Pleuropneumoniae: Use of Coagglutination and Complement Fixation to Determine the Relationship between Presence of Organism and Antibody Titer in Slaughterhouse Pigs

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Abstract
The conventional culture method was compared to coagglutination for detection of Actinobacillus (Haemophilus) pleuropneumoniae in 425 sets of pig lungs. Sera from the same animals were evaluated for antibodies to A. pleuropneumoniae by the complement fixation (CF) test. All samples were collected at 2 packing plants in Iowa. In 2 nonvaccinated herds with no history of respiratory disease, the difference between standard culture results and coagglutination was highly significant (P < 0.001). None of the 57 pigs in this group were positive for A. pleuropneumoniae by conventional culture, but 7 were positive by the coagglutination test. There were 15 animals with CF titers between 1:8 and 1:32. Animals from 6 herds vaccinated for A. pleuropneumoniae and without recent respiratory problems were evaluated. One out of 118 animals tested was positive for A. pleuropneumoniae by standard culture as compared to 9 positive by coagglutination. The difference in positive results between culture and coagglutination was highly significant (P < 0.001). Twenty-eight animals had CF titers to A. pleuropneumoniae (1:4 to ≥ 1: 128). Two hundred fifty lungs and sera samples were collected from 7 herds which had recently experienced varying degrees of respiratory disease. Thirty-nine lungs were positive for A. pleuropneumoniae by culture and 182 were positive by coagglutination. The number of positives detected by coagglutination was significantly different (P < 0.001) from the number positive by culture. There were 172 animals with antibody titers ranging from suspect to ≥ 1:128. There were significantly fewer positive animals detected by standard culture than with the CF test (P < 0.001). There was no significant difference between coagglutination results and CF titers when a titer of 1:4 was used as the positive threshold.