Evaluation of an IgG Enzyme-Linked Immunosorbent Assay as a Serological Assay for Detection of Mycoplasma bovis Infection in Feedlot Cattle

Abstract

Mycoplasma bovis is a pathogen of emerging significance in cattle throughout the world that is causing a range of diseases, including mastitis, arthritis, and pneumonia. The limited availability and efficacy of current diagnostic and prophylactic tools for its control and its increasing antimicrobial resistance are contributing to its increasing importance in beef and dairy cattle. We have developed an indirect IgG enzyme-linked immunosorbent assay (ELISA) based on a recombinant fragment of the MilA protein and have shown its potential as an effective diagnostic tool. To more comprehensively estimate the diagnostic sensitivity and specificity of this IgG ELISA for detection of infection with M. bovis in cattle and to define a suitable cutoff for use in the field, we further assessed its performance in experimentally infected calves in a closed beef herd and by applying Bayesian latent class modeling to laboratory testing results from 7,448 cattle entering Australian feedlots. The most effective cutoff points were estimated to be 68.6 antibody units (AU) for experimentally infected calves and to be 58.7 AU for a closed adult herd. Under field conditions, in feedlot cattle the globally optimal cutoff was estimated to be 105 AU. At this cutoff, the diagnostic sensitivity was 94.3% (95% probability interval [PI], 89.9% to 99.6%) with a diagnostic specificity of 94.4% (95% PI, 90.3% to 99.6%). Applying this 105 AU cutoff, 13.1% of cattle were seropositive for infection with M. bovis on entry into feedlots, and 73.5% were seropositive when followed up approximately 6 weeks later suggesting a high risk of infection shortly after entry into feedlots.

FIG 1

Distributions of anti-M. bovis IgG ELISA concentrations (in antibody units) for 7,206 cattle at induction to feedlots (A) and 7,289 cattle at follow-up approximately 6 weeks later (B) by bovine respiratory disease (BRD) case status.

FIG 2

Diagnostic sensitivity and specificity of the M. bovis MilA IgG ELISA at different antibody concentration cutoffs (in antibody units) estimated using Bayesian latent class analysis for 7,206 cattle at induction to feedlots and 7,289 at follow-up. Shading represents 95% probability intervals, and the dotted reference line at 105 AU indicates the cutoff with the highest Youden index (best combined sensitivity and specificity).