Evaluation of a competitive enzyme immunoassay for detection of Coxiella burnetii antibody in animal sera

A competitive enzyme immunoassay (CEIA) was established and compared with other serological techniques for detecting Coxiella burnetii antibody in camels, goats, and sheep. This technique was evaluated because a conjugated anti-camel immunoglobulin was not available to serve as a direct signal for the demonstration of antigen-antibody reaction. A C. burnetii antibody-positive human serum and a peroxidase-conjugated anti-human immunoglobulin G were used as an indicator system competing against antibody in animal serum or as an indicator of the absence of antibody. Sera were considered antibody positive when the A414 of the test sera plus the competing positive antibody was less than or equal to 50% of the A414 of the negative-control serum plus the competing antibody. Antibody to C. burnetii was repeatedly demonstrated in 66% of camel serum samples (n = 200) by the CEIA. Among 48 camel serum samples, 71% were positive for antibody by CEIA versus 65% by EIA using peroxidase-labeled protein A. The CEIA detected C. burnetii antibody in 63% of sheep serum samples (n = 40) and in 50% of goat serum samples (n = 96), while the indirect fluorescent-antibody technique detected antibody in 38% of sheep and 34% of goat serum samples and the EIA detected antibody in 50% of sheep and 35% of goat serum samples. These data indicate that the CEIA is a reliable and sensitive technique for demonstrating C. burnetii antibody in camels, sheep, and goats.