Clinical application of a polymerase chain reaction assay in the diagnosisof pneumonia caused by Rhodococcus equi in a horse

A 6-week-old Quarter Horse filly was evaluated because of severe pneumonia that developed after residing at a breeding farm for 4 weeks. A virulent st rain of Rhodococcus equi was identified by use of a polymerase chain reacti on assay, which was performed within 24 hours of collection of a fluid samp le obtained by transtracheal aspiration. Several days later, results of bac teriologic culturing confirmed the presence of R equi in the sample. Becaus e there was no history of R equi at the farm of origin, the owners were con cerned that the breeding farm was the source of infection in the foal. Feca l samples were collected from the farm of origin and the breeding farm. One fecal sample from a horse at the farm of origin yielded positive results f or the virulent strain of R equi. The owners accepted this as evidence that the foal may have become infected prior to transport to the breeding farm. Pneumonia in the foal resolved after 2 months of treatment with erythromyci n and rifampin. Polymerase chain reaction assays may be used for rapid and specific diagnosis of R equi infections in foals and during epidemiologic i nvestigations of patterns of shedding of the virulent strain of R equi in f eces.