Development of an indirect ELISA based on the major outer membrane protein for detection of antibodies against Chlamydia felis.

Chlamydia felis is a primary cause of feline conjunctivitis and upper respiratory tract disease. Detection of C. felis largely relies on molecular tests; however, these tests cannot assess prior exposure or herd immunity. In China, no standardized commercial serologic assay for C. felis is available. We developed an indirect ELISA based on the major outer membrane protein (MOMP) of a locally isolated C. felis strain (GXNN36). A truncated ompA gene (encoding MOMP, amino acids 22-392, which retains the major antigenic domains, including the 4 variable domains VD1-4) was expressed in Escherichia coli, and the recombinant protein was purified. After optimizing reaction conditions, the iELISA had high specificity (no cross-reactivity with feline calicivirus, panleukopenia virus, or feline coronavirus), high sensitivity (detection limit up to 1:20,480 dilution), and good reproducibility (intra- and inter-assay CVs <10%). The cutoff optical density (OD450) was 0.630. Assessment against western blotting using 20 clinical sera had 95% concordance. Testing of 144 feline serum samples from Guangxi, China revealed a seroprevalence of 20.1%. Our MOMP-based iELISA using a Chinese C. felis isolate offers a practical and regionally relevant tool for serosurveillance.