Development and evaluation of the internal-controlled real-time PCR assay for Rhodococcus equi detection in various clinical specimens

Rhodococcus equi is the causative agent of rhodococcosis in horses, resulting in significant morbidity and mortality in foals. This bacterium has also been isolated from a variety of animals and is being increasingly reported as a cause of infection in humans, mainly in immunosuppressed individuals. Laboratory diagnostics of R. equi infections based only on conventional microbiological methods shows low accuracy and can lead to misidentification. The objective of the study was to develop and evaluate a real-time PCR assay for direct detection of R. equi in various clinical specimens, including tissue samples. The species-specific region of the gene encoding R. equi cholesterol oxidase, choE, was used as a qPCR-target. The diagnostic applicability of the assay was confirmed by testing various tissue specimens obtained from horses with clinical signs of rhodoccocal infection and swine submaxillary lymph nodes. The rate of R. equi detection in clinical specimens by the developed assay was higher in comparison to the culture method (90% vs. 60.0% of positive samples) and conventional PCR (90.0% vs. 20.0% of positive samples). In case of 13 samples that were negative in the culture-based method, R. equi was detected by the developed assay. Only in one case, it gave negative result for culture-positive sample. The assay may provide a simple and rapid tool to complement the classical methods of R. equi detection based on culture and phenotypic identification of isolates, as the performed evaluation indicated a high specificity and accuracy of the results.