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 o bscure. In the present study, transtracheal aspirates were collected f rom thirty-one 1- to 6-month-old foals, which showed clinical signs of respiratory tract infection, and R. equi isolates were analyzed for t he presence of virulence plasmids and virulence-associated antigens. M oreover, this method was compared with a serodiagnosis by an enzyme-li nked immunosorbent assay (ELISA) to evaluate the sensitivity of the EL ISA. Of the 31 foals, 21 revealed positive cultures for R. equi. Of th e 21 foals, 20 (95%) had an ELISA OD value of 0.3 (positive limit of t his test) or higher at the initial medical examination. All of the iso lates from the aspirates were virulent R. equi, which contained virule nce plasmids and expressed virulence-associated antigens. In the remai ning 10 foals showing a negative culture for R. equi, 3 foals had posi tive ELISA titers. Six foals died during the treatment, and necropsy r evealed that 5 of the 6 foals had R. equi infection characterized by l arge abscesses in the lungs, and 3 of the 5 foals also had intestinal lesions. All clinical isolates from the lesions of the foals were viru lent 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.