Chlamydia abortus (C. abortus) is a major cause of abortion in sheep and goats with pigs and cattle also being affected. As several infectious agents are known to cause late-term abortions in small ruminants, good detection methods to identify C. abortus infection are desired in order to develop a rapid course of action and prevention strategies. The objective of this study was to determine if an indirect ELISA assay utilizing the 65kD N-terminal fragment of the recombinant C. abortus Pmp18 (Pmp18-N) protein would function as an improved alternative to the complement fixation test (CFT) for detection of C. abortus-specific antibody in pigs. The test was assessed with serum samples from a panel of 58 experimentally infected and 24 uninfected specific pathogen-free (SPF) pigs as well as 495 clinical samples obtained from farm animals. Results were compared with those obtained by the CFT. Compared to the commercial CFT using MOMP antigen, the sensitivity and specificity of the test was 98.2 and 87.5%, respectively while the concordance was 91.7%. No cross reaction with sera positive for other abortion-associated pathogens was found, including classical swine fever virus, porcine reproductive and respiratory syndrome virus, porcine pseudorabies virus, Japanese encephalitis virus, Brucella suis and avian Chlamydia psittaci. Of the 495 clinical samples analyzed, 12.1% were positive with the Pmp18-N compared to 11.1% with the commercial CFT kit. Interestingly, the Pmp18-N based test detected 0.75, 15.3 and 20.0% positivity in boars, fattening pigs and sows, respectively. Taken together, serological diagnosis based on Pmp18-N was shown to be rapid, sensitive and specific in detecting C. abortus-specific antibody.

Key words: Antibody; pig, Chlamydia abortus, Indirect ELISA, N terminal of polymorphic Membrane protein 18