Diagnostic evaluation of real‐time pcr in the detection of Rhodococcus equi in faeces and nasopharyngeal swabs from foals with pneumonia

Rhodococcus equi is a Gram-positive, facultative, intracellular bacterial pathogen and the most common cause of severe infectious pneumonia in foals of less than six months of age (Prescott 1991). The pathogen is ubiquitous in the environment, but becomes concentrated in breeding farm situations due to its ability to reproduce in the gastrointestinal tract of herbivores. A foal is suspected of having R equi pneumonia if it originates from an endemic farm, has signs of pneumonia, a severe inflammatory leukogram and a prominent alveolar and/or interstitial pattern on thoracic radiographs (Ainsworth 1999). Bacterial culture combined with cytological examination of a tracheobronchial aspirate is the only acceptable way of making a definitive diagnosis of R equi pneumonia. Although culture is the only means of identifying concurrent bacterial pathogens and testing in vitro antimicrobial susceptibility, it can yield false-negative results, possibly because of previous antimicrobial administration or overgrowth by multiple bacterial species (Sweeney and others 1987). Furthermore, culture does not allow differentiation between virulent and avirulent strains of R equi. In recent years the use of PCR amplification based on the VapA gene sequence has been shown to be a more sensitive means of identifying R equi in tracheal wash (TW) fluid samples than bacterial culture, especially if the foal sampled is being treated with antimicrobials at that time (Sellon and others 2001, Halbert and others 2005). Occasionally, the severity of clinical signs in foals suspected of having R equi pneumonia precludes the sampling of the lower airways. This short communication describes a study in which, during the search for alternative diagnostic specimens, it was hypothesised that PCR analysis of nasopharyngeal swabs (NS) and faeces collected from foals diagnosed with R equi pneumonia would be a sensitive alternative to standard culture techniques. Clinical samples were collected from 31 foals that had been referred to the Veterinary Medical Teaching Hospital, University of California, Davis, due to pneumonia. The foals ranged from four weeks to seven months of age (mean [sd] 3·3 [1·6] months). There were 17 female and 14 male foals, which included 15 thoroughbred, five quarter horse, five Arabian, two paint horse and one of each of Friesian, warmblood, standardbred and Percheron breeds. Before presentation, 19 of the 31 foals had been treated with a variety of antimicrobials (rifampin, azithromycin, erythromycin, doxycycline, penicillin procaine G, ampicillin, gentamycin, ceftiofur and trimethoprim-sulfamethoxazole) for one to 30 days (8·9 [7·7] days). All foals displayed clinical signs (cough, nasal discharge, tachyponea, fever and adventitial lung sounds) consistent Veterinary Record (2007) 161, 272-275