Validation of a Commercially Available Monoclonal Antibody-Based Competitive-Inhibition Enzyme-Linked Immunosorbent Assay for Detection of Serum Antibodies toNeospora caninumin Cattle

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Abstract
A previously described monoclonal antibody (MAb)-based competitive-inhibition enzyme-linked immunosorbent assay (cELISA) was modified to optimize performance, and the assay was validated in various defined cattle populations for detection of serum antibody toNeospora caninum, a major cause of bovine abortion. Modifications to the cELISA included capturing nativeN. caninumantigen with a parasite-specific MAb (MAb 5B6-25) and directly conjugating the competitor MAb (MAb 4A4-2), with both MAbs binding different epitopes of a conserved, immunodominant 65-kDa tachyzoite surface antigen. The assay was validated using three serum sets, a “gold standard” set of 184 cow sera defined by fetal histopathology andN. caninumimmunohistochemistry and by maternalN. caninumindirect fluorescence assay (IFA) at a 1:200 serum dilution, a relative standard set of 330 cow sera defined by IFA alone, and a set of 4,323 cow sera of unknownN. caninumstatus. A test cutoff of 30% inhibition was identified. The diagnostic sensitivity was 97.6%, and diagnostic specificity was 98.6% for the gold standard abortion-defined sera. The diagnostic sensitivity was 96.4%, and diagnostic specificity was 96.8% for the relative standard IFA-defined sera. Testing of the 4,323 bovine sera of unknownN. caninumstatus revealed a distinct bimodal distribution and steep sigmoid frequency curve with only 1.8% of samples within 5% of the test cutoff, indicating a sharp discrimination between test-positive and test-negative samples. In summary, the modifiedN. caninumcELISA provided a simple, rapid, and versatile method to accurately identifyN. caninuminfection status in cattle using a single cutoff value.