Clinical evaluation of a peptide-ELISA based upon N-terminal B-cell epitope of the VapA protein for diagnosis of Rhodococcus equi pneumonia in foals

Abstract

<jats:title>Summary</jats:title><jats:p>A total of 227 field samples from naturally exposed foals aged between 3 weeks and 6 months were used in an evaluation of a peptide‐based enzyme‐linked immunosorbent assay (ELISA) for diagnosis of<jats:italic>Rhodococcus equi</jats:italic>infection. A biotinylated peptide derived from the virulence‐associated protein A (VapA) of<jats:italic>R. equi</jats:italic>, a horse pathogen, was synthesized and designated as PN11‐14. The peptide corresponds to the N‐terminal B‐cell epitope TSLNLQKDEPNGRASDTAGQ of the VapA protein. Based upon a serum immunoglobulin (Ig)G titre of 512 as a positive cut‐off value for the<jats:italic>R. equi</jats:italic>infection, the ELISA provided the overall sensitivity of 47.62%, specificity of 69.67% and an accuracy of 59.47% with a positive predictive value of 57.47% for true<jats:italic>R. equi</jats:italic>pneumonia. The assay was improved by detecting VapA‐specific IgGb antibodies against N‐terminal B‐cell epitope of the VapA protein rather than IgG antibodies. The VapA‐IgGb ELISA showed the overall sensitivity of 70.47%, specificity of 72.13% and accuracy of 71.36% with a positive predictive value of 68.52%. Diagnosis of<jats:italic>R. equi</jats:italic>disease in 6‐week‐old foals showed that the VapA‐IgGb ELISA provided an increasing trend (<jats:italic>P</jats:italic> = 0.0572) in sensitivity of 82.4% in comparison with the VapA‐IgG ELISA which showed the sensitivity of 58.8%. However, differences in specificity of both tests were statistically insignificant (<jats:italic>P</jats:italic> = 0.357) as analysed by the McNemar test. These results indicated that detection of VapA‐specific IgGb antibodies may be a better predictor of<jats:italic>R. equi</jats:italic>disease in foals.</jats:p>