Isolation of virulent Rhodococcus equi from transtracheal aspirates of foals serodiagnosed by enzyme-linked immunosorbent assay.

Although isolation of Rhodococcus equi from tracheobronchial aspirates is thought to be a definitive diagnosis of R. equi pneumonia in foals, virulence of isolates from the aspirates of infected foals remains obscure. In the present study, transtracheal aspirates were collected from thirty-one 1- to 6-month-old foals, which showed clinical signs of respiratory tract infection, and R. equi isolates were analyzed for the presence of virulence plasmids and virulence-associated antigens. Moreover, this method was compared with a serodiagnosis by an enzyme-linked immunosorbent assay (ELISA) to evaluate the sensitivity of the ELISA. Of the 31 foals, 21 revealed positive cultures for R. equi. Of the 21 foals, 20 (95%) had an ELISA OD value of 0.3 (positive limit of this test) or higher at the initial medical examination. All of the isolates from the aspirates were virulent R. equi, which contained virulence plasmids and expressed virulence-associated antigens. In the remaining 10 foals showing a negative culture for R. equi, 3 foals had positive ELISA titers. Six foals died during the treatment, and necropsy revealed that 5 of the 6 foals had R. equi infection characterized by large abscesses in the lungs, and 3 of the 5 foals also had intestinal lesions. All clinical isolates from the lesions of the foals were virulent R. equi. These results support the assumption that isolates from the transtracheal aspirates of infected foals are virulent R. equi and the sensitivity of ELISA might demonstrate a serodiagnostic value for early diagnosis of R. equi infection in foals.