Comparison of tracheal aspiration with other tests for diagnosis of Rhodococcus equi pneumonia in foals.

The diagnostic value of tracheal aspiration was evaluated through comparison with other diagnostic methods using an experimental model of Rhodococcus equi (R. equi) pneumonia in foals. Pneumonia was induced by spraying of the virulent R. equi strain ATCC 33701 into the trachea of foals. All foals developed fever from 11 to 16 days after bacterial inoculation. One foal was euthanized on day 26 due to its poor prognosis, and other foals euthanized on day 43. During the experiment, some tests for diagnosis of Rhodococcus equi pneumonia such as tracheal aspiration, radiography, serodiagnosis and fecal culture were carried out. R. equi was continually isolated from tracheal aspirates collected via a silicone catheter inserted transnasally on day 8 to day 32 after bacterial inoculation. On the other hand, radiography, serodiagnosis and fecal culture were demonstrated to be valuable diagnostic methods, but to be limited compared with tracheal aspiration. Indirect fluorescent antibody technique (IFA) using a monoclonal antibody against the 15- to 17-kDa virulence-associated antigens (VapA) of R. equi and PCR targeting the structural gene of VapA detected bacteria in tracheal aspirates less sensitively than the isolation technique although they were more rapid. Therefore, we conclude that a combination of tracheal aspiration and bacterial isolation was the most valuable method for routine diagnosis of R. equi pneumonia in foals.