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.

The research paper evaluates a test for diagnosing Rhodococcus equi pneumonia in foals using a specific peptide from a protein associated with the disease. Results showed the test demonstrated a higher rate of accurately identifying the disease when the VapA-specific IgGb antibodies were detected instead of general IgG antibodies.

Research Methodology and Sample Testing

• Employing 227 samples taken from naturally exposed foals aged between 3 weeks and 6 months, the research aimed to evaluate the effectiveness of a peptide-based enzyme-linked immunosorbent assay (ELISA) for diagnosing Rhodococcus equi infection in foals.
• The team used a biotinylated peptide dubbed PN11-14, which was derived from the virulence-associated protein A (VapA) protein of R. equi, a pathogen affecting horses.
• The specific peptide used corresponded to the N-terminal B-cell epitope TSLNLQKDEPNGRASDTAGQ of the VapA protein.

Original Assay Accuracy

• Using a serum immunoglobulin (Ig)G titre of 512 as a cut-off value to label a case as an infection of R. equi, the peptide-ELISA showed an overall sensitivity of 47.62%, a specificity of 69.67% and a detection accuracy of 59.47%.
• Its predictive strength for true R. equi pneumonia was 57.47%.

Improved Assay Using VapA-specific IgGb Antibodies

• On identifying the need for a more accurate detection method, the researchers enhanced the assay’s sensitivity and specificity by detecting VapA-specific IgGb antibodies, rather than IgG antibodies.
• This tweak was warranted because IgGb antibodies are known to be more disease-specific, as they are part of the secondary immune response and get produced when the body is revisited by an infection.
• The VapA-IgGb ELISA assay depicted an overall sensitivity of 70.47%, specificity of 72.13%, and a detection accuracy of 71.36%, with a positive predictive value of 68.52% for the R. equi infection.

Evaluation of the Two Tests

• Ideally, these tests should depict a high specificity and sensitivity. Sensitivity refers to the test’s ability to correctly identify patients with the disease, while specificity is about correctly identifying patients without the disease.
• When the tests were applied to diagnose R. equi disease in 6-week-old foals, the VapA-IgGb ELISA demonstrated an increasing trend (P=0.0572) in sensitivity — 82.4% in comparison to the VapA-IgG ELISA’s 58.8% sensitivity.
• However, the specificity of the two tests didn’t show statistically significant differences (P=0.357).
• Therefore, the researchers concluded that detection of VapA-specific IgGb antibodies might be a better predictor for diagnosing R. equi disease in foals.