Estimating the Sensitivity and Specificity of Real-Time Quantitative PCR of Fecal Samples for Diagnosis of Rhodococcus equi Pneumonia in Foals.

Abstract

BACKGROUND: Real-time, quantitative PCR (qPCR) methods for detecting Rhodococcus equi in feces have been developed as a noninvasive, rapid diagnostic test for R.&#xa0;equi pneumonia, but have not been evaluated in a large population of foals. OBJECTIVE: The objective of this study was to evaluate the clinical utility of fecal PCR as a diagnostic test for R.&#xa0;equi pneumonia in foals using receiver operating characteristic (ROC) methods. ANIMALS: 186 foals born in 2011 at an R.&#xa0;equi-endemic ranch in Texas. METHODS: Fecal samples were collected at the time of onset of clinical signs for pneumonic foals (n&#xa0;=&#xa0;31). Foals with pneumonia were matched by age and birth date to healthy (n&#xa0;=&#xa0;31) and subclinical (n&#xa0;=&#xa0;124) control foals; fecal samples were collected from these controls. DNA was extracted from feces using commercial kits and concentration of virulent R.&#xa0;equi in feces was determined by qPCR. RESULTS: Concentration of R.&#xa0;equi in feces differed significantly (P&#xa0;<&#xa0;.05) among groups. The area under the ROC curve for fecal qPCR for diagnosis of R.&#xa0;equi pneumonia was 89% (95% CI, 83-99), with a sensitivity of 94% and specificity of 72%. CONCLUSIONS AND CLINICAL IMPORTANCE: qPCR of feces can be useful as an alternative to tracheobronchial aspiration for the diagnosis of R.&#xa0;equi in foals with clinical signs of pneumonia. Caution should be used in extrapolating results of this study to other populations because fecal concentration of R.&#xa0;equi might vary by geographic location or management practices.